Final mmMAGIC system concept · 2017. 7. 25. · access procedure can be supported by the low-band...

Document Number: H2020-ICT-671650-mmMAGIC/D6.6

Project Name:

Millimetre-Wave Based Mobile Radio Access Network for Fifth Generation Integrated Communications (mmMAGIC)

Deliverable D6.6

Final mmMAGIC system concept Date of delivery: 12/07/2017 Version: 145 Start date of Project: 01/07/2015 Duration: 24 months

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Deliverable D6.6 Final mmMAGIC system concept

Project Number: ICT-671650

Project Name: Millimetre-Wave Based Mobile Radio Access Network for Fifth Generation Integrated Communications.

Document Number: H2020-ICT-671650-mmMAGIC/D6.6

Document Title: Final mmMAGIC system concept.

Responsible: Ericsson.

Editor(s): Miurel Tercero (EAB), Miltiadis Filippou (Intel), Yue Wang (SRUK), Jian Luo (HWDU), Claudio Fiandrino (IMDEA), Danilo de Donno (IMDEA), Arnesh Vijay (Nokia), Hardy Halbauer (ALUD), Yaning Zou (TUD), Raffaele D'Errico (CEA), Michael Peter (HHI).

Contributors: Ali Zaidi (AB), Andreas Wolfgang (Qamcom), Arnesh Vijay (NOK-PL), Avelina Vega (TID), Camila Priale (INTEL), Claudio Fiandrino (IMDEA), Danilo De Donno (IMDEA), Dinh-Thuy Phan Huy (Orange), Domenico Giustiniano (IMDEA), Hardy Halbauer (Alcatel-Lucent), Honglei Miao (INTEL), Hua Wang (Keysight), Jaakko Vihriälä (NOK-FL), Javier Lorca (TID), Jian Luo (HWDU), Joerg Widmer (IMDEA), Jonas Medbo (EAB), Katsuyuki Haneda (AALTO), Michael Peter (HHI), Kilian Roth (INTEL), Maciej Soszka (TUD), Marie-Helene Hamon (Orange), Mario Castaneda (HWDU), Miltiadis Filippou (INTEL), Miurel Tercero (EAB), Mythri Hunukumbure (SRUK), Shangbin Wu (SRUK), Patrick Rugeland (EAB), Per Zetterberg (Qamcom), Raffaele D'Errico (CEA), Richard Tano (EAB), Tommy Svensson (Chalmers), Ulf Gustavsson (EAB), Valerio Frascolla (INTEL), Wenfang Yuan (UniBris), Xiaoming Chen (Qamcom), Yaning Zou (TUD), Yue Wang (SRUK).

Dissemination Level: PU

Contractual Date of Delivery: 12/07/2017

Security: Public

Status: Final

Version: 145

File Name: D6_6_Final mmMAGIC system concept_145.

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Abstract

The mmMAGIC project aims at the design and development of a system concept for a mobile radio access technology (RAT) operating in the frequency range of 6-100 GHz. This deliverable presents the 5G system concept which consists of 23 components and 43 component solutions (CoS). The system is designed considering the realistic mm-wave environment such as hardware impairments and channel characteristics, and key performance indicators (KPIs) obtained through extensive measurement and modelling in the project. Some components define the radio access architecture (higher layers) such as: the logical network architecture, interfaces and deployment, multi-connectivity, tight interworking, multiple service support and spectrum sharing. The system components that involve both physical and upper layer list improvements in the protocol stack, schemes to manage multi-nodes, initial access, multiple access, re-transmission schemes, active and inactive mobility solutions, and the use of self-backhauling. Finally, the components closely related to the physical layer design recommend solutions for waveform, numerology, channel coding, frame structure, the proposal for the transceiver architecture, several schemes for beam management, the MIMO scheme, and the definition of needed reference signals. This deliverable lists the components solutions (CoS) which are assessed based on the improved KPIs and their relation or relevance to standards. The partners contributed to standards and regulations through 16 contributions to 3GPP and 6 to ITU-R from which at least 15 solutions have been adopted. Finally, the closure of the project has been successful in achieving impact to industry, academia, and society.

Keywords

System concept, components solutions, design principles, mm-wave, 5G New Radio, network integration, key performance indicator, deployments, architectural enablers, multi-connectivity, cell clustering, mobility state, self-backhauling, interference coordination, network slicing, multi-RAT, energy efficiency, mm-wave radio interface, millimetre wave, higher frequency, above 6 GHz, waveform, channel code, retransmission schemes, frame structure and numerology, multiple access and duplexing schemes, initial access schemes, antenna array, transceiver architecture, hybrid beamforming, hardware impairments, multi-node co-ordination, non-linearity, channel behaviour.

Acknowledgements

The project would like to acknowledge the following people for the valuable reviews to the deliverable: Prof. Mark Beach (University of Bristol), Dr. Meik Kottkamp (Rohde & Schwarz) and Dr. Göran Klang (Ericsson).

Document: H2020- ICT-671650-mmMAGIC/D6.6

Date: 12/07/2017 Security: Public

Status: Final Version: 145

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Executive summary This deliverable describes the final milestone of the mmMAGIC project. It outlines a system concept, consisting of 23 system components, defining a mobile radio access technology (RAT) targeting operation in the 6 - 100 GHz frequency range. Solutions and realizations are presented taking channel as well as hardware impairments into account. The majority of the discussed solutions, referred to as the component solutions (CoS), have been evaluated, assessed, and demonstrated by the project by means of theoretical analysis, simulations, and/or hardware implementations. In-depth details for all system components, as well as for some component complementary realizations, can be found in: [MMMAGIC-D2.2], [MMMAGIC-D3.2], [MMMAGIC-D4.2], [MMMAGIC-D5.2]. In summary, the work of the mmMAGIC project led to the following suggestions and recommendations:

The logical architecture, protocol stack and interfaces for 5G system should be based on the E-UTRAN system (LTE-A). This is to guarantee system integration between the two RATs. Other components that are essential for integration of the system concept with other RATS are multi-connectivity, tight interworking with LTE, and Multi-service support based on network slicing. It is important that spectrum sharing using the RAN or core network interface is considered early on when specifying the interfaces.

Based on whether the deployment type is standalone or non-standalone, the initial access procedure can be supported by the low-band deployment (non-standalone). However, since standalone mm-waves systems are the most challenging, the synchronization signals and important system information should be broadcasted by the access point using wide beams. The random access is suggested to be performed by access points within one cluster to reduce the random access time, given that the reliability of transmissions is improved. The proposal for beam management includes strategies for beam sweeping dependent on the transceiver design, a fast beam tracking mechanism without dedicated training slots, a beam indicator scheme to handle interface, UE beam measurements procedures and a codebook to produce variable beam widths.

SDMA should be exploited to enable efficient reuse of radio resource, especially for integrated access and backhaul (IAB). SDMA can be well implemented using hybrid-beamforming architecture.

Multi-node coordination schemes are essential in mm-waves to increase macro-diversity gains towards shadowing/blocking and improve the radio link reliability. The project investigated a spatial multi-flow joint transmission using hybrid beamforming, sequential hybrid beamforming as a flexible way to support adaptive dual/multi-connectivity, the potential use of hybrid RF-FSO links for backhauling and the decode-and-forward relaying scheme to improve cell border throughput.

Active mobility should be smartly handled in mm-wave through clustering of nodes This can be complemented with configurable CSI-RS short transmissions when needed and must be turned off when not required. The purpose of the proposal of a new RRC Inactive state is to enhance idle mode mobility allowing users to quickly go back to connected mode.

For transmission, a scalable OFDM waveform is proposed, together with a numerology based on the LTE numerology that should scalable according to 15 2 kHz. A variable frame structure architecture is suggested allowing for downlink only subframes, uplink only subframes, mixed subframes, as well as mini-slots. LDPC codes should be used for data transmission and Polar codes for control information, with low complexity decoders.




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Reference signals such as for, CSI acquisition, DMRS, and PTRS, should be beamformed. Always-ON reference signals should be avoided for energy efficiency reasons.

The solutions brought forward by mmMAGIC added to the different key KPIs for mm-wave systems, supporting use cases where data rate, mobility, latency and reliability are important. Through its work, mmMAGIC has had impact on ongoing standardization and regulation in the mm-wave field. 16 contributions with acknowledgement to mmMAGIC have been submitted to 3GPP and 6 to ITU-R.




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Contents

1 Introduction ......................................................................................................................... 1 2 The mmMAGIC system concept ......................................................................................... 3 3 Impact of hardware and channel properties ........................................................................ 5

3.1 Hardware Impairments ................................................................................................. 5 3.1.1 Phase noise .......................................................................................................... 5 3.1.2 Power amplifier non-linearity ................................................................................ 6 3.1.3 Antenna design ..................................................................................................... 6 3.1.4 I/Q imbalance ........................................................................................................ 8

3.2 Radio channel properties ............................................................................................. 8 3.2.1 Path loss ............................................................................................................... 8 3.2.2 Large-scale parameters ........................................................................................ 9 3.2.3 Ground reflection ................................................................................................ 10 3.2.4 Blockage ............................................................................................................. 11 3.2.5 Intra-cluster characteristics ................................................................................. 11

3.3 Summary .................................................................................................................... 12 4 Components of the radio access architecture ................................................................... 14

4.1 Logical network architecture ...................................................................................... 14 4.1.1 Description of the solution .................................................................................. 14 4.1.2 Benefits ............................................................................................................... 14 4.1.3 Interworking with the system concept ................................................................. 15 4.1.4 Novelty and relations to standards ..................................................................... 15

4.2 Interfaces ................................................................................................................... 15 4.2.1 Description of the solution .................................................................................. 15 4.2.2 Benefits ............................................................................................................... 16 4.2.3 Interworking with the system concept ................................................................. 16 4.2.4 Novelty and relations to standards ..................................................................... 16

4.3 Spectrum sharing ....................................................................................................... 16 4.3.1 Description of the solution .................................................................................. 16 4.3.2 Benefits ............................................................................................................... 17 4.3.3 Interworking with the system concept ................................................................. 18 4.3.4 Novelty and relations to standards ..................................................................... 18

4.4 Multi-connectivity ....................................................................................................... 18 4.4.1 Description of the solution .................................................................................. 18 4.4.2 Benefits ............................................................................................................... 19 4.4.3 Interworking with the system concept ................................................................. 19 4.4.4 Novelty and relations to standards ..................................................................... 19

4.5 Tight interworking with LTE-A .................................................................................... 20 4.5.1 Description of the solution .................................................................................. 20 4.5.2 Benefits ............................................................................................................... 20 4.5.3 Interworking with the system concept ................................................................. 20 4.5.4 Novelty and relations to standards ..................................................................... 21

4.6 Multi-service support .................................................................................................. 21 4.6.1 Description of the solution .................................................................................. 21 4.6.2 Benefits ............................................................................................................... 21 4.6.3 Interworking with the system concept ................................................................. 22 4.6.4 Novelty and relations to standards ..................................................................... 22

4.7 Deployment ................................................................................................................ 22 4.7.1 Description of the solution .................................................................................. 22 4.7.2 Benefits ............................................................................................................... 24 4.7.3 Interworking with the system concept ................................................................. 24 4.7.4 Novelty and relations to standards ..................................................................... 24

4.8 Summary .................................................................................................................... 25 5 Radio interface components (PHY and Upper layers) ...................................................... 26




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5.1 Protocol stack ............................................................................................................ 26 5.1.1 Description of the solution .................................................................................. 26 5.1.2 Benefits ............................................................................................................... 27 5.1.3 Interworking with the system concept ................................................................. 27 5.1.4 Novelty and relations to standards ..................................................................... 28

5.2 Multi-node coordination schemes .............................................................................. 28 5.2.1 Description of the solution .................................................................................. 28 5.2.2 Benefits ............................................................................................................... 33 5.2.3 Interworking with the system concept ................................................................. 33 5.2.4 Novelty and relations to standards ..................................................................... 33

5.3 Initial access .............................................................................................................. 33 5.3.1 Description of the solution .................................................................................. 36 5.3.2 Benefits ............................................................................................................... 41 5.3.3 Interworking with the system concept ................................................................. 42 5.3.4 Novelty and relations to standards ..................................................................... 42

5.4 Multiple access schemes ........................................................................................... 42 5.4.1 Description of the solution .................................................................................. 42 5.4.2 Benefits ............................................................................................................... 43 5.4.3 Interworking with the system concept ................................................................. 43 5.4.4 Novelty and relations to standards ..................................................................... 43

5.5 Active mobility ............................................................................................................ 43 5.5.1 Description of the solution .................................................................................. 44 5.5.2 Benefits ............................................................................................................... 45 5.5.3 Interworking with the system concept ................................................................. 45 5.5.4 Novelty and relations to standards ..................................................................... 45

5.6 Inactive mobility ......................................................................................................... 45 5.6.1 Description of the solution .................................................................................. 45 5.6.2 Benefits ............................................................................................................... 46 5.6.3 Interworking with the system concept ................................................................. 47 5.6.4 Novelty and relations to standards ..................................................................... 47

5.7 Self-backhauling and Integrated Access-Backhaul (IAB) .......................................... 47 5.7.1 Description of the solution .................................................................................. 47 5.7.2 Benefits ............................................................................................................... 48 5.7.3 Interworking with the system concept ................................................................. 48 5.7.4 Novelty and relations to standards ..................................................................... 48

5.8 Re-transmission schemes .......................................................................................... 48 5.8.1 Description of the solution .................................................................................. 49

Single-hop and multi-hop retransmission protocols ............................................ 49 5.8.2 Benefits ............................................................................................................... 50 5.8.3 Interworking with the system concept ................................................................. 51 5.8.4 Novelty and relations to standards ..................................................................... 51

5.9 Summary .................................................................................................................... 51 6 Radio interface components (PHY layer) .......................................................................... 52

6.1 Waveform ................................................................................................................... 52 6.1.1 Description of the solution .................................................................................. 52 6.1.2 Benefits ............................................................................................................... 52 6.1.3 Interworking with the system concept ................................................................. 52 6.1.4 Novelty and relations to standards ..................................................................... 53

6.2 Numerology ................................................................................................................ 53 6.2.1 Description of the solution .................................................................................. 53 6.2.2 Benefits ............................................................................................................... 55 6.2.3 Interworking with the system concept ................................................................. 55 6.2.4 Novelty and relations to standards ..................................................................... 55

6.3 Frame structure .......................................................................................................... 55 6.3.1 Description of the solution .................................................................................. 55




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6.3.2 Benefits ............................................................................................................... 57 6.3.3 Interworking with the system concept ................................................................. 57 6.3.4 Novelty and relations to standards ..................................................................... 57

6.4 Channel coding .......................................................................................................... 57 6.4.1 Description of the solution .................................................................................. 57 6.4.2 Benefits ............................................................................................................... 57 6.4.3 Interworking with the system concept ................................................................. 58 6.4.4 Novelty and relations to standards ..................................................................... 58

6.5 Transceiver architecture and multi-antenna schemes ............................................... 58 6.5.1 Description of the solution .................................................................................. 59 6.5.2 The Benefits........................................................................................................ 61 6.5.3 Interworking with the system concept ................................................................. 61 6.5.4 Novelty and relations to standards ..................................................................... 61

6.6 Beam management .................................................................................................... 62 6.6.1 Description of the solution .................................................................................. 62 6.6.2 Benefits ............................................................................................................... 68 6.6.3 Interworking with the system concept ................................................................. 69 6.6.4 Novelty and relations to standards ..................................................................... 69

6.7 M-MIMMO scheme for multi-antenna ........................................................................ 70 6.7.1 Description of the solution .................................................................................. 70 6.7.2 Benefits ............................................................................................................... 70 6.7.3 Interworking with the system concept ................................................................. 70 6.7.4 Novelty and relations to standards ..................................................................... 71

6.8 Reference signals ...................................................................................................... 71 6.8.1 Description of the solution .................................................................................. 71 6.8.2 Benefits of the Solution ....................................................................................... 73 6.8.3 Interworking with the system concept ................................................................. 74 6.8.4 Novelty and relations to standards ..................................................................... 74

6.9 Summary .................................................................................................................... 75 7 Assessment of the system concept ................................................................................... 76

7.1 Contribution to the key performance indicators (KPIs) .............................................. 76 8 The technology roadmap .................................................................................................. 83

8.1 5G timeline in standards, regulation and mmMAGIC ................................................. 83 8.2 mmMAGIC component solutions and relation to standards. ...................................... 87

9 Final summary and Recommendations ............................................................................. 91 10 References ........................................................................................................................ 94




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List of Figures Figure 1-1: mmMAGIC use cases and their most stringent KPIs. .............................................. 1 Figure 2-1: An overview of the mmMAGIC system concept. ...................................................... 4 Figure 3-1: Power spectral density of phase noise. ................................................................... 5 Figure 4-1: Overall 5G logical architecture. .............................................................................. 14 Figure 4-2: NAS* interface. ....................................................................................................... 16 Figure 4-3: Architectural solutions supporting spectrum pooling between two different network operators. (a): interface at the RAN (base station), (b): interface at the CN, (c): RAN sharing, (d): CN sharing, (e): via a spectrum broker, (f): uncoordinated. ............................................... 17 Figure 4-4: Multi-connectivity bearer option. ............................................................................ 18 Figure 4-5: Principle of RRC diversity. ..................................................................................... 19 Figure 4-6: Slice selection at initial attach when the UE provides slice ID to RAN. ................. 21 Figure 4-7: Standalone and non-standalone operation modes. ............................................... 22 Figure 4-8: Redundant coverage scenario. .............................................................................. 23 Figure 5-1: The 5G protocol stack. ........................................................................................... 26 Figure 5-2: TCP and RLC in the protocol stack ........................................................................ 27 Figure 5-3: Multi-Node Beam Sweeping (figure modified from original figure in [HKC16]). ..... 29 Figure 5-4: Parallel beam search frame structure for test beam pair f 11f 12,w 2 , cf. [MMMAGIC-D5.2, sec. 3.3] for further details. ......................................................................... 30 Figure 5-5: Illustration of an RF-FSO multi-hop network. ......................................................... 30 Figure 5-6: Blockage by high building. ..................................................................................... 31 Figure 5-7: Illustration of the considered two considered access methods. (a) Direct access vs (b) Relay-assisted access. ....................................................................................................... 31 Figure 5-8: Illustration of multi-node cooperation. .................................................................... 32 Figure 5-9 Overview of system component solutions making up the envisioned procedure for mm-wave initial access (standalone deployment). ................................................................... 34 Figure 5-10: Low frequency-assisted initial access to a heterogeneous network. ................... 35 Figure 5-11: Example of beam shapes designed using the Widener method described in [Int16] and the technique using amplitude tapering in [Qia16]. Plotted is also the DFT beam and the subelement pattern. ................................................................................................................. 37 Figure 5-12: SS-block structure (left) and multiplexing options for CSI-RS and SS-block (right). ................................................................................................................................................. 38 Figure 5-13: The new transceiver architecture and frame structure. ........................................ 39 Figure 5-14: System model for coordinated random access within a cluster of three APs. ..... 40 Figure 5-15. Average SINR variations over UE route at 15 GHz. ............................................ 44 Figure 5-16: Illustrates a proposal for the function split in the mm-wave cluster. ..................... 44 Figure 5-17. EMM State transition diagram for 5G systems. ................................................... 46 Figure 5-18: Illustration of a heterogeneous network with mm-wave wireless BH and access. ................................................................................................................................................. 47 Figure 5-19: Conflict graph construction (left) and considered frame structure (right). ............ 48 Figure 5-20: Single-hop (left) and multi-hop (right) retransmission protocols. ......................... 49 Figure 5-21: schematic illustration of a centralized deployment with early triggering of HARQ retransmissions at the distributed units. ................................................................................... 50 Figure 5-22: illustration of Fast HARQ operation over Finite Block Length Codes. ................. 50 Figure 6-1: Achievable maximum SIR as a function of SCS in the presence of phase noise. . 53 Figure 6-2: OFDM numerology design for wide range of carrier frequencies, deployment types, and application latency requirements. ...................................................................................... 54 Figure 6-3: Subframe structures. .............................................................................................. 55 Figure 6-4: DMRS/PTRS structure. .......................................................................................... 56 Figure 6-5: Illustration of a mini-slot. ........................................................................................ 56 Figure 6-6. Hybrid Beamforming with analogue beamformer. .................................................. 58 Figure 6-7: Fully connected and partially connected architecture (subarrays). ........................ 59




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Figure 6-8: An example of the relay selection process: (a) initial transmission scheduling, (b) blockage detection, (c) broadcasting help message, (d) relay candidate identification (e) relay selection metric and (f) transmission via the relay path. .......................................................... 62 Figure 6-9: Illustration of broadcasting beam pattern examples. ............................................. 63 Figure 6-10: General condition for the appearance of inter-cell interference with strong beamforming, in the presence of reflectors. ............................................................................. 65 Figure 6-11: (Top) P1 to enable UE measurement on different TRP Tx beams to support selection of TRP Tx beams/UE Rx beam(s) (Middle) P2 to enable UE measurements on different TRP Tx beams to possibly change/select inter/intra-TRP Tx beam(s), (Bottom) P3, to enable UE measurement on the same TRP Tx beam to change UE Rx beam in the case UE uses beamforming. ................................................................................................................... 66 Figure 6-12: (a) Heterogeneous beam space (codebook); (b) three UEs served by two heterogeneous beams of 15° and 60° beamwidths. .................................................................. 67 Figure 6-13: B-DFT-SM-MRT system parameters. .................................................................. 71 Figure 6-14: Simulated environments and antenna model. ...................................................... 71 Figure 6-15: Timing configuration of CSI-RS processes for the UE. ........................................ 72 Figure 6-16: Front-loaded DMRS pattern of 1 and 2 OFDM symbols for a resource block of 7 OFDM symbols......................................................................................................................... 72 Figure 6-17: Single layer DMRS pattern for broadcast channels. ............................................ 73 Figure 6-18 Illustration of PTRS in every OFDM symbol. ........................................................ 73 Figure 7-1: Mapping of the component solutions (CoS) to KPIs. ............................................. 82 Figure 8-1: Timeline for regulation and standards activities, running in parallel to mmMAGIC project. ..................................................................................................................................... 83

List of Tables Table 3-1 Antenna specifications for the mm-wave user equipment. ......................................... 7 Table 3-2 Antenna specifications for the mm-wave access point .............................................. 7 Table 3-3 Antenna specifications for the mm-wave backhauling/fronthauling ........................... 7 Table 3-4. Main hardware and channel impairments addressed in mmMAGIC. ...................... 12 Table 4-1. Classification of New Radio (NR) deployment ........................................................ 23 Table 6-1: Proposed numerology. ............................................................................................ 54 Table 7-1. Summary of the CoS and benefits to the system concept ...................................... 77 Table 8-1. The project contributions on mm-wave channel to standards and regulation. ........ 83 Table 8-2. List of mmMAGIC contributions adopted by standards or regulators ...................... 85 Table 8-3. The project’s contributions on 5G new RAT. ........................................................... 85 Table 8-4. Mapping investigated technologies in mmMAGIC to standards. ............................. 86 Table 8-5. The novelty of the system component solutions and relevance to standards. ........ 88




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Definitions of technical terms Term Definition

5G experience

Immersive multi-media experiences including 4k/8k UHD video, virtual reality experiences and real time mobile gaming.

Access link The wireless link connecting the access point and user equipment. Access point (AP)

The base station in mm-wave network.

Analog beamforming

Beamforming function is implemented in the analogue part.

Antenna element

A single antenna in an array with fixed (amplitude and phase) element combining prior to beam former.

Antenna gain The ratio of the power produced by the antenna from a far-field source on the antenna's beam axis to the power produced by a hypothetical lossless isotropic antenna, which is equally sensitive to signals from all directions.

Backhaul These are portions of the network comprising intermediate connections between the core networks and the sub-networks (i.e. radio access nodes) attached to the macro-cell, and connections in-between the radio access nodes.

Backhaul network

Serves as the transport medium for a mobile radio access network (RAN) and connects the access points to core network. In case of multi-hop backhaul, connections between access points are also part of the backhaul network.

Baseline use case

A use case which will be the focus of research and KPI extensions from this baseline can generate the conditions for other use cases.

Carrier aggregation

This is a data transmission technique, where the network utilizes two or more carrier frequencies to transmit and receive data to/from the UE in downlink and uplink

Control Plane The part of the network that carries control information (also known as signalling) to provide functionalities such as connectivity management, mobility management, radio resource control etc.

Deployment Characterization of the network layout (i.e., physical and logical locations). Also, RAN configuration like antennas, transmit power, frequency band, bandwidth, system features and supporting architectural solution.

Digital beamforming

Beamforming function is implemented in the BB part.

Enabler Solutions essentially needed to fulfil technical challenges and system requirements covered by different functionalities identified in the project.

Fronthaul Connections between a network architecture of centralized baseband controller and remote standalone radio heads at cell sites.

High GHz range

70-100 GHz have been defined as high GHZ range in this project [MMMAGIC-D1.1].

Hybrid beamforming

Architectures mixing digital and analogue beamforming techniques to provide a performance trade-off on flexibility, multiplexing, power consumption and cost.

Key performance indicator (KPI)

A quantifiable measurement, agreed beforehand, that reflects the critical success factors of a proposed solution; it reflects the goals captured by each use case. The KPIs are linked to the use case, to link the proposed solutions with the usage driven test cases.

Low GHz range

The range 6-30 GHz have been defined as low GHZ in this project [MMMAGIC-D1.1].

Massive MIMO

Massive MIMO (also known as Large-Scale Antenna Systems, Very Large MIMO, Hyper-MIMO, Full-Dimension MIMO and ARGOS) uses a very large number of service antennas (e.g., hundreds or thousands) that are operated fully coherently and adaptively.

Mid GHz range

30-50 GHz have been defined as high GHZ range in this project [MMMAGIC-D1.1].

mm-wave spectrum

General definition of mm-wave spectrum includes frequencies between 30 and 300 GHz. However, as studied in mmMAGIC, it refers strictly to 6-100 GHz excluding 50-70 GHz.

Multi-antenna

A number of service antennas are implemented and operate coherently and adaptively.




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Multi-Connectivity

This is a key technology to fulfil 5G requirements on data-rate, latency, reliability and availability. The term multi-connectivity can accommodate a broad range of techniques, but with one objective: for a given UE, radio resources in the network are configured from two or more different access points.

Multi-node With adaptive joint transmission/reception in coordinated multipoint, the antennas of several RAN or backhaul nodes are regarded as part of one distributed antenna array - in reception and transmission.

Network architecture

Network architecture defines several logical RAN and backhaul node and their functionalities, the way that they are grouped and the interfaces between end users, RAN nodes, and backhaul nodes; between the core and RAN node, between RAN nodes, and between backhaul nodes.

Network element

A facility or equipment used as a manageable logical entity uniting one or more physical devices. Network element is a system that can be managed, monitored, or controlled in a telecommunications network, that has one or more standard interfaces, and is identified by a unique management address.

Propagation environment

Defines the medium and propagation conditions between the access point (AP) and the User Equipment (UE).

Radio Access Network Architecture

RAN architecture defines several logical RAN node and their functionalities, the way that they are grouped and the interfaces between end users and RAN node; between the core and RAN node, and between access RAN nodes.

Radio Access Technology

Technology that is used to connect different user equipment’s and applications to telecommunication networks by using radio frequency signals.

Radio Interface

Interface between the mobile station (MS) and the radio equipment in the network, defined by functional characteristics, common radio (physical) interconnection characteristics, and other characteristics, as appropriate. Radio interface spans over PHY and MAC layers.

Requirement Each use case is characterized by different needs in terms of KPIs. The quantified needs are called requirements.

Self-backhauling

The radio access node autonomously establishes backhaul connectivity to the existing network infrastructure and start operation in a plug-and-play fashion.

Spectrum suitability

The applicability of a particular spectrum for one or one group of use cases and KPIs.

Standalone Network

A network which can operate independently of any other device or system

Sub-6 GHz Refer to frequencies that are lower than the 6-100 GHz frequencies investigated in mmMAGIC. This term is used in contrast to mm-wave network especially in the context of overlay network, where mm-wave network is assisted by sub-6 GHz network.

Transceiver architecture

Transceiver architecture defines a number of components, such as power amplifier, antenna, BB part, etc., and their functionalities, the way that they are grouped and the interfaces between them.

Use case General account of a situation or course of actions that may occur in the future. It is described from end-user perspective and illustrates fundamental challenges. It provides an example on how, when and where end users can utilize mobile communication for a particular service

User Equipment

Unified term for user equipment, user terminal, mobile devices etc.

User Plane The part of the network that mainly carries user traffic (also known as data plane) coupled with minimized control signalling such as multiple access information.




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List of Abbreviations

3GPP 3rd Generation Partnership Project

4G 4th Generation

5G 5th Generation

ACK Acknowledgement

AP Access Point

ARQ Automatic Repeat Request

AS Access Stratum

BH Backhaul

BP Backhaul

BS Base Station

BW Bandwidth

CA Carrier Aggregation

CD Code Division

CDF Cumulative Distribution Function

CG Conflict Graph

CN Core Network

CP Cyclic Prefix

CPE Common Phase Error

CPM Continuous Phase Modulation

CSI Channel State Information

CW Congestion Window

DC Dual Connectivity

DFT Discrete Fourier Transform

DFTS Discrete Fourier Transform Spread

DL Down Link

DMRS Demodulated Reference Signal

DRX Discontinuous Reception

ECM EPS Connection Management

EIRP Equivalent Isotropic Radiated Power

EMM EPS Mobility Management

ESM EPS Session Management

eMBB enhanced Mobile Broadband

eMB Evolved Node B

EPC Evolved Packet Core

EVM Error Vector Magnitude

FBMC Filter Bank Multi-Carrier

FD Frequency Division

FDD Frequency Division Duplex

FDM Frequency Division Multiplexing

FDMA Frequency Division Multiple Access

FEC Forward Error Correction

FFT Fast Fourier Transform

FR Fast Retransmission

GA Genetic Algorithm

GI Guard Intervals

gNB NR base station

GR Ground Reflection

H2020 Horizon 2020

HARQ Hybrid Automatic Repeat Request

IAB Integrated Access-Backhaul

ICI Inter Carrier Interference

ICT Information and Communication Technology

IEEE Institute of Electrical and Electronics Engineers

IQ In-phase/Quadrature

IR Incremental Redundancy

KPI Key Performance Indicator

LDPC Low-Density Parity-Check

LO Local Oscillator

LOS Line of Sight

LTE Long Term Evolution

MAC Medium Access Control

MC Multi-Connectivity

MEMS Microelectromechanical Systems

MIMO Multiple-input Multiple-output

MMIMMO Massive Multiple-Input Massive Multiple-Output

MIS Maximum Independent Set

AMF Access and Mobility Management Function

MMMAGIC

Millimetre-Wave Based Mobile Radio Access Network for Fifth Generation Integrated Communications

mMTC massive Machine-Type Communication

MN Master gNB

Document: H2020-ICT-671650-mmMAGIC/D6.6



mmMAGIC Public xv

MPC Multipath Component

MRT Maximum Ratio Transmission

MS Min-Sum

NACK No Acknowledgement

NB Node B

NG New Generation

NLOS Non-Line-of-Sight

NR New Radio

NS Non-Surjective

NSSAI Network Slice Selection Assistance Information

OFDM Orthogonal Frequency-Division Multiplexing

PA Power Amplifier

PAPR Peak-to-Average-Power-Ratio

NR-PBCH Physical Broadcast Channel

PDCP Packet Data Convergence Protocol

PDP Power Delay Profile

PDU Protocol Data Unit

PHY Physical layer

PL Path Loss

PLL Phase-Locked Loop

PN Phase Noise

PRB Physical Resource Block

NR-PSS Primary Synchronization Signal

PTRS Phase Tracking Reference Signal

QC Quasi-Cyclic

RA Random Access

RAN Radio access network

RAT Radio Access Technology

RF Radio Frequency

RI Rank Indicator

RLC Radio Link Control

RMS Root Mean Square

RRC Radio Resource Control

RRM Radio resource management

RRU Radio Resource Unit

RS Reference Signal

RTO Retentive Timer On

RTT Round-Trip Time

SC Single-Carrier

SCo System Component

SCoS System Component Solutions

SCS Subcarrier Spacing

SD Spatial Division

SDMA Space-division multiple access

SE

SF Shadow Fading

SFN Single Frequency Network

SI Study Item

SINR Signal-to-Interference-plus-Noise Ratio

SIR Signal-to-Interference Ratio

SM Spatial Multiplexing

SN Secondary Node

SNR Signal-to-Noise Ratio

SS Synchronization Signal

NR-SSS Secondary Synchronization Signal

SVD Singular Value Decomposition

TCP Transmission Control Protocol

TD Time Division

TDD Time Division Duplex

TDM Time Division Multiplexing

TDMA Time Division Multiple Access

TP Throughput

TR Technical Report

TRP Transmitter-Receiver Point

TS Technical Specification

TTI Transmission Time Interval

UE User Equipment

UF Universal Filtered

UHD Ultra-High Definition

UL Uplink

UMi Urban-microcell scenarios

UP User Plane

URLLC Ultra-Reliable Low Latency Communication

UTRAN Universal Terrestrial Radio Access Network

UW Unique Word

VCO Voltage Controlled Oscillator

WP Work Package




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1 Introduction The overall objective of the mmMAGIC project is to design and develop a system concept for a mobile radio access technology (RAT) operating in the frequency range 6-100 GHz. It integrates the system and radio interface concepts and solutions developed in the project, considering various challenging user requirements, the unique mm-wave channel characteristics, and hardware impairments. The integration of the resulting RAT design into the 5G multi-RAT ecosystem is also considered as one of the important aspects of the mmMAGIC RAT design.

mmMAGIC identified eight most compelling use cases for 5G systems and seven key performance indicators (KPIs) for the evaluation of the proposed solutions [MMMAGIC-D1.1], [MMMAGIC-D3.1]. The use cases are defined to extend one or more of the following most critical KPIs: user data rate, connection density, traffic density, mobility, reliability, availability, and latency. The use case “50+ Mbps everywhere” illustrated at the centre of Figure 1-1 serves as a baseline representing the requirements of enabling a mobile and connected society where broadband access is available everywhere. Seven additional use cases are further considered representing specific scenarios where mm-wave technologies are envisaged to have important roles:

Cloud services, immerse 5G early experience and dense urban society are the use cases that stretch data rate requirements.

Smart offices and media on demand require very high connection density. Tactile internet and robotic control are pushing the limits for latency and reliability. Finally moving hot spots demand a high data rate at a very high speed.

Figure 1-1: mmMAGIC use cases and their most stringent KPIs.

In general, the mm-wave frequency range offers the possibility to allocate wider contiguous bandwidth in 5G system. Unfavourable propagation characteristics lead to high frequency reuse factor due to the increased losses in wave propagation compared to the lower frequencies. Thus, the use of narrow-beams is needed which leads to improved cell isolation. Those




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properties are beneficial for 5G small cell deployment in dense urban areas for capacity boost as well as beneficial for backhauling and access deployment when static crowds are targeted (e.g., in a stadium). On the other hand, due to blockage caused by people and obstacles, the main disadvantage of using mm-wave is the limited availability, reliability, and throughput consistency in the seamless networking context (particularly if the mm-wave system is standalone). It can be very challenging to support medium velocity mobility access of a user device on high frequencies in urban conditions, e.g., when moving massive crowds (e.g., sport events like marathons) are gathered in small areas with high probability of blocking the link. Similarly, it can also be problematic to establish reliable backhauling for high speed vehicles, to achieve fast handovers in the access link and to provide outdoor-to-indoor coverage in mm-wave frequency bands. The mmMAGIC system concept presented in this deliverable will address some of these critical challenges by integrating innovative solutions from different aspects that make the system operational and robust.

Through the life time of the project the integration of the system components was ensure by:

Considering the hardware and channel impartment aspects in the design of the components solutions.

Down-selecting from proposed solutions. The solutions were evaluated based on simulations, analysis, or even hardware implementation, and were proposed to enhance a certain KPI.

Aligning proposed solutions with the earlier down-selected solutions (e.g. transceiver architecture, waveform, multi-connectivity, etc.). The solutions have also been used for input to standards, regulation and for discussion in 5G PPP groups such as: architecture, pre-standards, technology board, vision, use cases, KPIs and requirements, spectrum, and the METIS-II coordination work.

Refining alignment, and identifying the interworking of a certain component with the other components of the proposed system concept.

This deliverable presents an integrated system concept and summarize the main findings and recommendations from the project. An overview of the mmMAGIC system concept will be discussed in Chapter 2. Two fundamental challenges of designing mm-wave systems, the RF impairments and radio channel properties, will be analysed in Chapter 3. Next, mmMAGIC proposed solutions for addressing radio access architecture components, radio interface components both in the PHY and higher up layers will be presented in Chapters 4, 5 and 6 respectively. Every component details the interworking with other components, the benefits, and the novelty. In Chapter 7, the solutions are assessed based on the enhancement of the listed KPIs and other robustness to hardware or channel impairment. The mmMAGIC technology roadmap and assessment of the technology components with regards standards is discussed in Chapter 8. Finally, conclusions, recommendations and mmMAGIC views will be drawn in Chapter 9.




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2 The mmMAGIC system concept The mmMAGIC system concept is a radio system concept for mobile broadband communication targeting operation in wide-contiguous bands above 6 GHz. It addresses the mm-wave specific challenges and recommendations as outlined in Chapter 3. The system concept is scalable and flexible, and it meets the anticipated end-user and traffic demands of the networked society in the 2020 time-frame.

The following design principles were guiding in the design of the mmMAGIC system concept:

Flexibility and re-configurability: the targeted system concept had to have the property to support operation in different frequency ranges, use of different transceiver architectures, be deployed in different user scenarios, and support both access and backhaul. The system will need to be future proof in terms of extendibility, modularity, and adaptability. It must fit into a diverse set of deployment scenarios.

Tight integration with the multi-RAT 5G ecosystem: the targeted system concept had to be designed to support both standalone and non-standalone mm-wave system deployments; support network slicing; distinction between synchronous and asynchronous functions; device capabilities signalled from and to the core network; efficient design of control signalling to support system access and mobility.

Support of heterogeneity (multi-vendor networks): the targeted system concept had to leave room for vendor-specific innovation and product differentiation.

Robustness against various distortions and impairments: the targeted system concept had to include the challenges and recommendations in Section 3.1 such as: phase noise, power amplifier non-linearity, antenna design, in-phase and quadrature imbalance.

Energy efficiency: the targeted system concept had to save operational cost and benefit to the environment.

Ensure signal processing: the targeted system concept had to cope with the large bandwidth for very high throughput.

Guaranteed minimum performance for worst case scenarios, i.e., when considering link blockage, or cell edge users; meeting the most critical requirements of the use cases in the system.

Figure 2-1 gives an overview of the system concept. Twenty-three key system components have been proposed to fulfil the listed design principles in the system concept. The components on the left side present solutions for the radio access architecture or higher layers such as the description of the logical network architecture, interfaces and deployment, the selection of key enablers that will support the integration with multiple-RAT, multi-connectivity, tight interworking, multiple service support and spectrum sharing. The system components that both involve physical and upper layers are in the middle of Figure 2-1, starting with the improvements in the protocol stack, new schemes to manage multi-nodes, initial access, multiple access, re-transmission scheme, solutions to handle active and inactive mode mobility, and the use of self-backhauling. Finally, the components closely related to the physical layer design include solutions for waveform, numerology, channel coding, frame structure, the proposal for the transceiver architecture, several schemes for beam management, the MIMO scheme, and the definition of reference signals.

In Chapters 4, 5 and 6, detailed descriptions of the different system component solutions (CoS) are given. For each component, the following aspects are addressed:

A description of the technical solution(s) needed for a component. One or more solutions may be detailed. The outlined solutions may be complementary to each other in most cases.

Highlights of the benefits of the different solutions to the system concept, depending on the KPIs that are targeted for the evaluation or analysis of the solution and other robustness to the hardware or channel imperfections. The main KPIs derived from the




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use cases are: data rate, connection density, traffic density, mobility, reliability, availability, and latency.

Its interworking with the other components of the system concept. The system component solutions will affect directly or indirectly the other components and the decision on the design of their own solutions.

Specifying the novelty of the developed CoS, whether it is 5G-specific or an evolution of LTE-A. The status with regards to the current standards is also mentioned, given that mmMAGIC has been developing the CoS relevant for standardization.

Figure 2-1: An overview of the mmMAGIC system concept.

The proposed system concept is built upon the listed components developed in the mmMAGIC project, and it is based on the insights provided by the evaluation and modelling efforts in relation to the radio channel and hardware impairments (bottom box along Figure 2-1). As with any technology development, the co-design between all system components is needed, and better understanding of the channel properties, the antenna performance and the behaviour of the hardware components is crucial. This project devoted a lot of efforts on channel measurements in various radio environments and converted this dataset into suitable channel models [MMMAGIC-D2.2]. A significant contribution to the wider community in this regard is the development and integration of the ground reflection model component, among others. The project has also analysed the performance of antenna arrays for user equipment and access points in the key 26 GHz band through both detailed simulations and experiments with prototypes [MMMAGIC-D5.2]. This analysis has yielded valuable results of the radiation patterns, antenna gains and losses that can be expected in these compact arrays at mm-wave frequencies. Hardware impairments, including both phase noise and power amplifier non-linearity, have also been evaluated in the project [MMMAGIC-D5.2]. A detailed phase noise model has been developed, whose source code has been made available to the public. The results from the model have contributed to the 3GPP discussions.




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3 Impact of hardware and channel properties This chapter describes the key characteristics of hardware and radio channels that affect the design of the mmMAGIC system concept. The following sub-sections will briefly describe the hardware impairment and channel properties by referring to models and evaluation methods in already-published mmMAGIC deliverables and showing their possible impacts or recommendations on the system design.

3.1 Hardware Impairments

Due to less advanced hardware components, the mm-wave systems are affected to hardware impairment including phase noise, power amplifier non-linearity, antenna design, and I/Q imbalance. This section will discuss the problems caused by each of the mentioned hardware impairments and recommendations for the system concept design which affects mainly the physical layer.

3.1.1 Phase noise

Phase noise is typically caused by Local Oscillator (LO) instability, which means that the LO output spectrum is not an ideal Dirac impulse but exhibits a skirt-like shape as shown in Figure 3-1, the relation to bit error rate can be found in [mmMAGIC-D4.2]. As a result of phase noise, the received signal samples will have random time-varying phase errors. In a OFDM system, phase noise will cause common phase error (CPE) and inter carrier interference (ICI), resulting in degraded Error Vector Magnitude (EVM) performance. Phase noise also causes leakage between adjacent channels, which is detrimental in near-to-far scenarios of a UE constellation in a cell. Phase noise also has adverse effects on interference cancellation schemes. Generally, the phase-noise variance grows with the square of carrier frequency [HL98], while it is inversely proportional to the power consumption of the complete frequency generation (PLL, reference, etc.). Therefore, it is an important effect in mm-wave systems, especially those striving for low power consumption, as it may limit the throughput. Careful selection of carrier bandwidth and sub-carrier spacing is therefore crucial, as well as phase-tracking reference signal design.

Free-running oscillator and phase-locked loop (PLL) based oscillators are the most common assumptions in the literature [PRF07]. The phase noise of the PLL-based oscillator consists of three main noise sources, i.e., noises from the reference oscillator, the phase-frequency detector and the loop filter, and the voltage controlled oscillator (VCO). Each of these three noise sources includes both white noise (thermal noise) and coloured noise (flicker noise). Detailed modelling of phase noise in the mmMAGIC project can be found in [MMMAGIC-D5.1].

Figure 3-1: Power spectral density of phase noise.




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Recommendation on the design of the system concept

Various approaches have been proposed in [MMMAGIC-D4.2] for phase noise compensation. The CPE can easily be estimated (and corrected) in the frequency domain as the common phase rotation of the constellation by using scattered pilots. ICI may be modelled as additive noise, though not always Gaussian, and it is usually hard to compensate. It requires denser pilots for phase noise and channel tracking, and can be computationally intensive. Generally, the larger the phase noise bandwidth (compared to subcarrier spacing), the more severe the ICI. With relatively large subcarrier spacing, it may be sufficient to compensate the CPE only. And this is implicitly done if the estimated channel transfer function using the scattered pilots (which includes the common phase rotation) is used for equalization. In multi-antenna systems, phase noise may yield asynchronous carriers on different antenna streams which must be compensated for when performing spatial multiplexing or doing digital beamforming.

3.1.2 Power amplifier non-linearity

Due to physical constraints, it is generally considered that the practical power amplifier (PA) efficiency at mm-wave is much less than at cm-wave frequencies [J58]. As the bandwidth and the modulation order are increased to achieve high data rates at mm-wave bands, the Peak-to-Average-Power-Ratio (PAPR) of communication signals increases correspondingly. Therefore, it can be expected that mm-wave PAs may most likely work in nonlinear region during transmission. This may limit the power efficiency of the power amplifier even further, unless suitable PAPR-reduction techniques are deployed. The issue of PAPR is however not considerably worse than in the sub-6 GHz case, since the PAPR of OFDM-signals tends to grow logarithmically [ZC02]. When signals with a large dynamic range go through a nonlinear PA, the signals will suffer from nonlinearity effects, which result in both in-band distortion and spectral regrowth. While in-band distortion increases the EVM of the transmitter signal, spectral regrowth causes adjacent channel interference. The Volterra model, or any of its common reductions, is widely used in the literature in the modelling of nonlinear PAs [VRM01].


In order to have high PA efficiency, it is required that the input signals have low peak-to-average power ratio (PAPR). One may employ linearization techniques, e.g., pre-distortion to compensate for PA non-linearity. However, such a technique comes with major baseband complexity and may not be effective for large bandwidth signals and/or hybrid beamforming architectures. It remains an open question to what extent such a technique can be effective for large bandwidth signals (e.g., in mm-wave bands) and how much additional complexity in terms of mixed signal and baseband processing is required. Much of the current state-of-the-art requires fully digital arrays. On the other hand, one may also employ simple PAPR reduction techniques (e.g., clipping and companding) to reduce the PAPR of the transmitter signal, either per transmitter or in beam-domain depending on the transceiver architecture. This will however corrupt the link or beam quality, but some trade-offs may be done provided enough degrees of freedom.

3.1.3 Antenna design

The main constraints for user equipment antenna’s design in general are the size and cost, as well as mutual coupling between antenna elements, while keeping efficiency and bandwidth. However, the short wavelength at mm-wave and hence the required small inter-element distances make even the planar arrays possible within the user equipment.

For radio access point, high gain (> 20 dBi) antennas with analogue or hybrid beamforming capability are needed to manage both multi-user interference and rapid changes of the channel due to user mobility. In the case of backhaul or fronthaul link, a gain greater than 30 dBi and fixed beam or a limited scanning capability (10° on one plane) are desired. In fact, for these kind of links, the beam-steering could be used to implement self-alignment function. Other practical constraints for the access point antennas are affordable antenna size, cost and complexity.




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Often antenna arrays are considered ideally lossless in high-level system performance evaluation. In practice, according to the antenna efficiency and arrangement in the array, the number of antennas could be increased to reach the desired gain.


Antenna specifications in different mm-wave bands for user equipment, access and backhaul have been proposed by mmMAGIC in [MMMAGIC-D5.2] and reported here below. These guidelines are aligned with practical deployments in 5G networks at some reference frequency bands of interest.

Table 3-1 Antenna specifications for the mm-wave user equipment.

X-band K-, Ka-band V- or E-band

Array topology Linear array Linear array Linear/Square array

Typ. Frequency bands 14.4 – 15.35 GHz 24.25 – 27.5 GHz 71-76 or 81 – 86 GHz

Typ. Gain (dBi) 10 – 15 15 15 – 20

Typ. minimum size1 1x4 (11 dBi Gain) 1×8 (15 dBi Gain) 4×4 (17 dBi Gain)

Polarization Linear/Dual linear Linear/Dual linear Linear/Dual linear

Beam-steering 40° (on one plane) 40° (on one plane) 40° (on one plane)

Table 3-2 Antenna specifications for the mm-wave access point


Array topology Square array Square array Square array


Typ. Gain (dBi) 15 - 20 20 - 25 32

Typ. minimum size1 4×4 (17 dBi Gain) 6×6 (20 dBi Gain) 20×20 (31 dBi Gain)

Polarization Linear/Dual linear Linear/Dual linear Linear/Dual linear

Beam-steering 60° (2D window) 60° (2D window) 60° (2D window)

Table 3-3 Antenna specifications for the mm-wave backhauling/fronthauling


Array topology Square array Square array Square array


Typ. Gain (dBi) 25 30 - 35 > 38

Typ. minimum size1 10×10 (25 dBi Gain) 20×20 (31 dBi Gain) 40×40 (37 dBi Gain)

1 Ideal loss-less case




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Polarization Linear Linear Linear

Beam-steering 10° (on one plane) 10° (on one plane) 10° (on one plane)

It was shown how losses and reduced efficiency can reduce the antenna performance with respect to the one of the ideal lossless case and yield a higher number of antennas to reach the target antenna array performance. Based on realistic conditions of implementations such as losses and uncertainty in phase adjustment, a database of antenna models including the full scattering matrix for different steering directions and the polarimetric antenna radiation patterns covering the entire solid angle has been generated. In particular, the 24.25-27.5 GHz band patch and dual-polarized planar antenna arrays up to 8x8 elements have been considered for the user equipment. Radiation characteristics of dipole arrays have been considered for massive MIMO performance evaluation (see Section 6.5).

For the base station transmit array, technology solutions have also been investigated. Electronically reconfigurable transmit arrays allowing fine beam steering for radio access have been proposed, considering a planar array up to 40×40 elements reaching the requirements for backhaul applications. With this solution, the phase control is directly implemented on the unit cells, by employing p-i-n diode switches, instead of using phase shifters as classically done in phased arrays. For backhaul applications, fixed or switched beam solutions can be adopted. The performance of such arrays as a function of phase quantization (i.e., states of the unit cells) has been investigated. The advantages of this technology, with respect to classical phased array is loss reduction due to the feeding lines and the high efficiency of the array. These antennas are recommended for base stations.

3.1.4 I/Q imbalance

Due to imperfections in in-phase and quadrature (IQ) mixers used for mm-wave systems, the orthogonality between the in-phase and the quadrature phase is not preserved. The phase and gain imbalance increases typically with increasing carrier frequency. This because the analogue IQ mixer is converting from a higher frequency to baseband. At higher frequencies, it is more difficult to guarantee the 90deg phase-shift in the mixer for the analogue design. An important aspect to consider is that the magnitude of the error depends on the baseband signal and that it can be frequency selective for larger signal bandwidth (e.g. >250MHz).


To support higher order modulation in mm-wave systems IQ imbalance compensation techniques need to be employed [MMMAGIC-D5.2]. For relatively small gain and phase errors as they can be found in heterodyne transceivers, digital compensation techniques are typically sufficient [MMMAGIC-D5.2]. Homodyne transceivers might require analogue pre-compensation or (self-) calibration to save dynamic range in the data converters. Dynamic range, which is mostly limited by gain imbalance, becomes affected if the gain imbalance starts to exceed 3dB.

3.2 Radio channel properties

The high frequencies have different properties than the one for which previous generation of radio systems have been designed at the low frequencies. mmMAGIC measured and modelled relevant radio channel characteristic at mm-wave like: path-loss, large scale parameters, ground reflection, blockage, and intra-cluster. This section will introduce those characteristic and list recommendations for the design of the system concept.

3.2.1 Path loss

The most remarkable difference of propagation at mm-wave frequencies compared to traditional cellular bands below 6 GHz is related to the path loss (PL). It is well known that the free-space path loss increases with the square of the carrier frequency, corresponding to a 20 dB increase




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per frequency decade. Assuming free-space conditions, a signal at e.g. 40 GHz therefore undergoes a 26 dB higher attenuation on the way from the transmitter to the receiver than a signal at 2 GHz. For realistic multipath environments, the scaling slightly varies, but is of the same order of magnitude. Under LOS conditions, the PL exponent is close to two for urban-microcell scenarios (UMi). In indoor environments with strong multipath, it can be smaller, e.g. values down to 1.3 have been observed in an office and a conference room at 60 and 82 GHz [MMMAGIC-D2.2]. Typical values for NLOS are in the range 3–4, which is in line with values at frequencies below 6 GHz.

Penetration losses of many typical building materials significantly increase with frequency, and the ability of providing indoor coverage by an outside base station is heavily dependent on the material of the exterior walls and the windowed area. Massive concrete walls are practically impenetrable for mm-waves, whereas standard window panes cause very low losses [ITU-R-P.2040-1]. Infrared reflecting glass results in severe losses above 20 dB [KOK+10]. However, the latter even applies to frequencies below 6 GHz and scales only moderately with frequency. Evaluations in mmMAGIC have shown that the building entry loss is a function of the elevation angle and the spread also increases with frequency. Due to the higher penetration losses and the fact that diffracted power decreases with frequency, blockage effects can be very severe. They are treated separately in Section 3.2.4.

For mm-wave backhaul links over longer distances, atmospheric effects become relevant. In particular, the oxygen absorption peak at 60 GHz can cause additional losses of up to 15 dB/km under standard conditions [Lie89]. Similarly, rain attenuation with values up to 30 dB/km for very heavy rainfall must be considered. For UMi access scenarios, however, neither oxygen absorption nor rain is a major concern. For indoor links, any atmospheric effects can be neglected.


The high PL and building penetration losses are a big challenge for mm-wave communication. However, the losses can be compensated by an appropriate system design and are advantageous regarding interference reduction and frequency reuse. The following recommendations are deduced:

The higher losses compared to lower frequencies must be compensated by antenna gain to achieve a sufficient link budget. High-gain directional antennas are appropriate for fixed backhaul links, but electronically steerable antennas are needed to support mobile access.

O2I mm-wave coverage may not be feasible in all cases, especially for high-loss buildings. The integration of supplementary indoor APs should be considered.

A high frequency reuse may be feasible in structurally separated areas and in different rooms. This should be taken into account to maximize the network capacity.

3.2.2 Large-scale parameters

The findings mentioned above lead to the following implications considering radio systems:

Multipath dispersion exists at mm-wave bands in a similar extent as at legacy bands, leading to frequency-selective fading of the received signals and inter-symbol interference if omni-antennas are used. Fading mitigation and channel equalization is therefore essential. Beamforming may help to reduce the delay dispersion through limiting the “visible” multipath spatially, reducing the small-scale fading effects called channel hardening and suppressing long-delayed multipath.

Similar density of users can be served for above 6 GHz bands when using an antenna array with the same electrical size and hence the angular resolution. This is because inter-user decorrelation stays similar for an inter-user distance. However, the feasible density of users may be smaller at above 6 GHz than below 6 GHz because of the angular sparsity of multipath channels [NHJ+15]. Even though the angular spreads are comparable for the below- and above-6 GHz, the number of clusters tends to be more




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limited at above-6GHz. For example, 20 clusters are assumed in the SCM for legacy bands while they are at most 5 at millimetre-wave frequencies [MMMAGIC-D2.2, Section 3.3]. For a fixed physical aperture size, the higher frequency can serve more users because the angular resolution becomes finer.

The cross-polarization ratios of radio propagation paths are inversely proportional to the excess loss. This model is valid for above 6 GHz [MMMAGIC-D2.2, Appendix A.6].

Spatial multiplexing remains effective at above 6 GHz. This is because the antenna of the same physical size can steer narrower beams with greater gains, which even compensates for channel sparsity as observed in [HNK16].


The findings mentioned above lead to the following implications considering radio systems:

Multipath dispersion exists at mm-wave bands in a similar extent as at legacy bands and channel equalization is therefore essential. Beamforming may help to reduce the delay dispersion and result in channel hardening.

Similar density of users can be served for above 6 GHz bands when using an antenna array with the same electrical size and hence the angular resolution. This is because inter-user decorrelation stays similar for an inter-user distance. However, the feasible density of users may be smaller at above 6 GHz than below 6 GHz [NHJ+15]. For a fixed physical aperture size, the higher frequency can serve more users because the angular resolution becomes finer.

Polarized-MIMO is as effective in the higher frequency band as below 6 GHz. Spatial multiplexing remains effective at above 6 GHz. This is because the antenna of

the same physical size can steer narrower beams with greater gains, which even compensates for channel sparsity as observed in [HNK16].

3.2.3 Ground reflection

Ground reflection (GR) is a deterministic multipath component (MPC) that can be received by a mobile user equipment (UE) which is in direct line of sight (LOS) to the base station (BS). The electromagnetic properties of the ground and the near-grazing incidence of the wave to the ground usually lead to a significant part of the energy being reflected. Due to the different lengths of the LOS and ground-reflected paths, they interfere with each other, which results in a periodic fading pattern over the distance between BS and UE.

At frequencies below 6 GHz, this fading occurs only close to the BS and the distance between successive “fades” can be up to several dozens of meters. The current approach is to model this effect by a dual-slope path loss (PL) model [3GPP38.900]. At close distances between the BS and the UE, the PL is like the free-space loss and GR fading is approximated by shadow fading (SF). However, after a certain distance, the GR interferes destructively with the direct path, which leads to a higher PL exponent. The transition BS-UE distance at which the value of the PL exponent becomes significantly higher as compared to free-space propagation, the so-called break point (BP), depends on the BS and UE heights and on the carrier frequency. The higher the carrier frequency is, the further away is the BP.

At mm-wave frequencies, the BP distance is several hundred meters from the BS, meaning that urban-microcell (UMi) deployments with typical cell sizes below 200 m will have to cope with the fading effects caused by the GR. The distance between fades can be less than 1 m and the signal strength might suddenly drop by up to 20 dB. Ground reflection fading depends on the receiver position, wave polarizations and is therefore important when considering mobility.


In order to maximize the reliability and the user data rate of the radio systems against the GR effects, the system must be highly adaptive and cope with large fluctuations of the received power due to mobile user equipment and/or dynamic environment. The following recommendations are deduced:




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Sufficient fading margin considered in the link budget. High adaptability, e.g., rate adaption; powerful channel coding to maintain link if SNR is

low for a short time interval. Consideration of fading prediction methods; GR fading effects are quasi-deterministic

when the moving direction of the user equipment is known. Large transmission bandwidth and very narrow beams to leverage frequency and

antenna gains and to reduce fading dips; effective to a certain extent for small distances and/or urban macro scenarios with a larger BS height.

Frequency agility, including non-contiguous carrier aggregation across usable frequency band, to reduce frequency-flat GR fading, especially for static scenarios (temporarily static UE or fixed backhaul links).

Polarization diversity and consideration of circular polarization to mitigate GR effects

3.2.4 Blockage

Blockage of radio propagation pathways can be caused e.g., by vehicles, people, and vegetation. It results in a fast drop of the received power and is usually most severe when a strong path, such as the line-of-sight (LOS) path, is blocked.

Blockage becomes more important as the carrier frequency increases because the power that is diffracted around the object decreases. Particularly in the mm-wave frequency range, where high gain antennas and beamforming techniques will be used to counteract the decreasing aperture size of a constant-gain antenna, blockage losses may be severe (e.g., up to 30 dB). Nevertheless, measurements and analyses show that diffracted and multipath power can be exploited to maintain a link during a blockage event. It is crucial to model blockage effects realistically to evaluate and optimize the system behaviour.


Similarly, to the ground reflection, the system must be highly adaptive and cope with large fluctuations of the received power due to dynamic blockage. In particular, the following recommendations are deduced:

Beamforming capabilities and fast beam finding and tracking methods are crucial, so that reflected paths are actively exploited if they exceed the diffracted power.

Consideration of blockage prediction methods: blockage events may be predicted even before they become effective based on oscillations of the receiving signals characteristic to diffraction.

Large transmission bandwidth, including non-contiguous carrier aggregation across available frequency bands, to reduce fading maxima during blockage event.

Additional link margin due to the dynamic blockage in the link budget analysis. High adaptability, e.g., rate adaption; powerful channel coding to maintain link if signal-

to-noise ratio is low for a short time interval.

3.2.5 Intra-cluster characteristics

At frequencies below 6 GHz, channel bandwidths are relatively small and the temporal resolution is therefore limited. High spatial resolution is achieved only by massive MIMO systems, which involve very large dimensions of the antenna array. At mm-wave frequencies, however, both very large bandwidth and antennas with a large number of elements are typical. The temporal and spatial resolution is very high and intra-cluster characteristics including sub-path delay spread and sub-path angle spread become significant. System throughput can be boosted if the richness of the mm-wave channel is properly utilized. Many massive MIMO channel estimation algorithms depend on the sparsity of the channel. However, according to the findings in this project, the sparsity condition may not be fulfilled when massive MIMO is used. Also, the richness of the channel in time domain can cause lower frequency correlations.




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Considering intra-cluster characteristics in mm-wave channels, the following recommendations are presented:

The number of RF chains in massive MIMO systems with hybrid beamforming should be carefully chosen to reach a balance between implementation complexity and utilization of the richness of the mm-wave channel.

The frame structures of different numerologies should be adaptive, i.e., the length of (shortened) TTIs and subcarrier spacing should be designed according to the richness of the channel.

3.3 Summary

In mmMAGIC, the impairments and issues related to the channel and the hardware have been identified and modelled. Table 3-4 summarizes the main impairments presented and outlines how they affect the system performance and design.

Table 3-4. Main hardware and channel impairments addressed in mmMAGIC.

Impairment

Impact Recommendation or Solution

Spectrum Reuse & Spectral Efficiency

Beamforming Range Power Efficiency

Phase noise

Subcarrier orthogonality loss.

Subcarrier orthogonality

loss.

• Increased subcarrier, spacing. • Time/freq. domain phase noise tracking.

PA non-linearity

Spectral regrowth due to intermodulation products (Guard

carrier insertion to fulfil mask

requirements).

Spectral regrowth is

dependent on direction.

Output-power. Back-off impacts power

efficiency.

• Waveform design with PAPR reduction.

IQ Imbalanc

e

Residual error affects SE and has impact on

architecture.

Increased EVM.

• Digital compensation • Heterodyne transceiver

Antennas Antenna bandwidth and efficiency are strictly dependent.

Practical antenna size

impacts range. Losses in

feeding reduce antenna

efficiency.

Coupling and phase-shifter

affect resolution. Practical

antenna size impacts range.

Losses and practical

antenna size could

require higher power.

• Hybrid beamforming. • Spatial feeding solution at BS to reduce losses (e.g. transmit arrays), including design of new unit cells.

Channel: Path loss

The path loss for LOS and NLOS scenarios is much higher compared to lower frequency bands.

Penetration losses and building entry losses can be extremely high. They are strongly dependent on the material.

• Electronically steerable antennas with sufficient gain to compensate higher losses.

• Supplementary indoor APs.

Channel: large-scale

parameters

The omni-directional delay and angular spreads are almost constant across frequencies.

The auto-correlation distances of omni-directional large-scale parameters are fairly constant across frequencies.

Cross-polarization coupling is proportional to the excess loss.

•Channel equalization

•Antenna beamforming •Waveform design • Polarized MIMO, spatial multiplexing.

Channel: ground

reflection

Signal strength might drop by up to 20 dB. • Fading margin considered in the link budget. • Large transmission bandwidth and very narrow beams. • Frequency agility to reduce




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frequency-flat ground reflection fading.

• Consideration of polarization diversity and circular polarization.

Channel: blockage

Blockage losses may be severe (e.g. up to 30 dB). • Beamforming capabilities and fast beam finding and tracking method blockage prediction methods. • Additional link margin due to the dynamic blockage. • High adaptability, e.g. rate adaption.

Channel: clusters

Intra-cluster characteristics including sub-path delay spread and sub-path angle spread become significant due to high temporal and spatial

resolution of mm-wave systems.

• Number of RF chains in hybrid massive MIMO transceivers: trade-off between proper utilization of multipath richness and transceiver cost/complexity. • Adaptive frame structures.

A complete 6-100 GHz channel model has been developed because of mmMAGIC activities [MMMAGIC-D2.2]. This model includes new important features (e.g., enhanced blockage modelling, incorporation of ground reflection effects, improved cluster modelling, large-scale parameters based on a large amount of consolidated measurement and simulation data) and has been used for the performance evaluations on link and system level. In addition, hardware impairment models have been proposed to enhance beamforming and antenna techniques [MMMAGIC-D5.2], as well as to develop the Radio Interface Components (RICO) in [MMMAGIC-D4.2]. The validity of some radio interface components has been corroborated by Hardware-in-the-Loop (HIL) experiments, showing the effectiveness of the waveform design, along with the proposed enhancement techniques, under real mm-wave channel and hardware conditions [MMMAGIC-D4.2]. The inclusion of these impairments in the mmMAGIC solutions is considered in the following chapters.




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4 Components of the radio access architecture In this chapter, the components of the system concept related to the radio access architecture that are needed for the operation of the proposed 5G mm-wave RAN, are described in detail, followed by several design proposals based on the presented architecture.

4.1 Logical network architecture

The logical architecture of the 5G RAN basically follows the E-UTRAN system design [3GPP TS 23.401] consisting of network access nodes connected via the S1 interfaces to user data plane (UP) and control data plane (CP) units, which provide connection to the core network.

4.1.1 Description of the solution

The proposed 5G RAN entities comprise new 5G nodes (gNBs), which can operate at mm-wave bands or low frequency bands below 6 GHz. They provide CP and UP termination towards the UE and terminate the interfaces Xn (logical interface between eNB and gNB or between gNBs connected to 5G-CN), NG (New Generation), AS (Access Stratum) or the so called Uu interface and NAS (Non-Access Stratum), as shown in Figure 4-1. The new 5G RAN can further comprise LTE eNBs connected to the 5G Core (5G-CN). Both types of nodes can interwork with E-UTRAN and support features like tight interworking with LTE-A (Section 4.3) and multi-connectivity (Section 4.5) to serve UEs from different access points. The new logical RAN interfaces are: Xn between eNB and gNB or between gNBs, and NG between eNB or gNB and the 5G-CN (more specifically to the Access and Mobility Management Function-AMF by means of the N2 interface and to the User Plane Function-UPF by means of the N3 interface). Network slicing (Section 4.4) must be supported to enable multi-operator sharing of the infrastructure and various multi-service support.

Figure 4-1: Overall 5G logical architecture.

4.1.2 Benefits

The logical architecture is an evolution of the LTE-A architecture and, therefore, can be easily integrated into the existing RAN architecture when adding the required new interfaces. With the proposed architecture, both standalone and non-standalone system implementations (Section 4.7) can be operated, and all foreseen use cases can be supported, in terms of satisfying their performance requirements with respect to e.g., data rate, mobility, reliability, as well as latency. For non-standalone operation, the Xn interface provides the necessary interconnection between eNB and gNB. The NG interface allows connection of eNB and gNB to 5G-CN. This architectural concept enables multi-connectivity, which is the main feature to deal with mm-wave challenges related to the physical channel properties, like blocking, shadowing or high path loss.




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4.1.3 Interworking with the system concept

The proposed logical architecture serves as a basis for the system concept, enabling functionalities like multi-connectivity and tight interworking with LTE-A, multi-service support and the possibility to handle different deployments.

4.1.4 Novelty and relations to standards

The 5G logical architecture is an evolution of LTE-A Rel-12 including new features like multi-connectivity over different frequency bands and access points. The proposed 5G RAN entities are well in line with current progress in 3GPP.

4.2 Interfaces

In this section, the focus is on interfaces essential to the proposed system concept. The interfaces to be described, are the NG interface, the Xn interface, as well as the NAS* interface, as previously shown in Figure 4-1.


NG interface

The NG interface, as referred in Section 4.1.2, allows connection of eNB and gNB to the 5G-CN and its design should comply with a number of criteria. More concretely, the NG interface:

Must be open and must be capable of supporting exchange of signalling information between the gNB and the 5G-CN;

Must support control plane and user plane separation, and at the same time have separate radio network and transport layer specifications;

Is expected to be diverse and future proof to fulfil new requirements, and support new services and features;

Must be capable of carrying out interface management, UE context and mobility management functions;

Enhanced features to support the transportation of NAS messages, paging and PDU session management must be included.

Xn interface

Focusing on non-standalone deployments, the Xn interface provides the necessary interconnection between the eNB and the gNB. A set of criteria would need to be satisfied during design, hence, focusing on either standalone or non-standalone deployments, the Xn interface:

Must offer logical connectivity between eNB and gNB or between gNBs; Must support the exchange of signalling information between the endpoints, and the

data forwarding to the respective gNBs; Must support CP and UP separation, and shall have separate radio network and

transport network layer specifications; Is required to be future proof to fulfil diverse requirements, support new services,

features and functions.

NAS*

The NAS* protocol provides non-access stratum between a UE and it is controlling the Access and Mobility Management Function (AMF). NAS* can be separated into network access control and service control function. NAS* messages are transferred between a UE and the RAN by RRC messages.




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Figure 4-2: NAS* interface.

4.2.2 Benefits

The mentioned interfaces, when designed according to the described criteria, can support multiple services and features, CP and UP separation, and at the same time, comply with separate radio network and transport layer specifications. Such an incurred flexibility can enhance future proof-ness, with the aim of fulfilling diverse requirements, as well as supporting new services, features and functions.


The proposed interfaces shall enhance system design flexibility, as they are key components of the logical network architecture, which aims at facilitating the applicability of solutions, such as the ones described in the remainder of this section.


As the logical network architecture described in Section 4.1 constitutes an evolution of LTE-A Rel.12 agrees with current developments in 3GPP [3GPP TS 36.401], [3GPP TR 38.801]. The mentioned interfaces are also in line with the current discussions in standardization bodies.

4.3 Spectrum sharing

The spectrum pooling mechanisms and, specifically, the architectural solutions provided in this section, may impact the radio access network sharing mechanisms and deployment options of currently evolving networks operating in bands below 6 GHz. This is because future mm-wave networks will tightly interwork with networks deployed for operation below 6 GHz to boost the capacity of congested macro cells while supporting seamless mobility both locally and with the overlaid cellular network.


Interface at the RAN

This solution implies to enable distributed coordination using the “Xn” interface between APs belonging to different networks or to create a similar interface, as despite in Figure 4-2. From a logical architectural perspective. This alternative allows a fast information exchange between two different networks, and therefore near real-time spectrum pooling is possible. The identified functions to support spectrum sharing are: enhanced CSI, and distributed synchronization. A simple representation of this sharing architecture is presented in Figure 4-3 (a).

Interface at the core network (CN)

This solution refers to an architecture where the interface between the different networks is at the CN. Due to the latency involved, alternative in Figure 4-3 (b) does not enable real-time spectrum pooling. On the other hand, CN level coordination can handle a large number of cellular APs by exchanging a few protocol messages, since typically a large number of APs are associated with a few CN nodes. Enhanced CSI can be a required function.

UE 5G‐NB MME

PHY

MAC

NAS*

RLC*

RRC

PDCP*

PHY

MAC

RLC*

RRC

PDCP*

NAS*




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RAN sharing

When two or more network operators share the Aps, the single baseband unit can serve users associated to the different network operators in the sharing agreement, see the architecture in Figure 4-3 (c). As resource allocation and scheduling decisions are made by a single unit, alternative (c) is an effective way to implement real-time centralized coordination, as the coordination-related processing is handled by a single physical entity.

Core Network (CN) sharing:

Other form of sharing is when two or more network operators share the CN, Figure 4-3 (d). In the same way as alternative (c), alternative (d) allows centralized coordination. However, in this case real-time spectrum pooling is not possible for the same reasons discussed for alternative in Section 4.3.1.2.

Spectrum broker

The spectrum coordination can also be implemented by means of a spectrum broker. A spectrum broker is a central resource management entity that grants spectrum resources on an exclusive basis during some time window [IKS+13]. This alternative is illustrated in Figure 4-3 (e).

Uncoordinated spectrum pooling

Alternative in Figure 4-3 (f) refers to the case where the network operators do not coordinate. When the number of networks in the pool is not limited, uncoordinated spectrum pooling is reminiscent of a license-exempt regime. For example, in wireless LANs (for example, Wi-Fi) real-time spectrum pooling is realized through uncoordinated operation. In this case, no new interface or modification to the architecture is required.

Figure 4-3: Architectural solutions supporting spectrum pooling between two different network operators. (a): interface at the RAN (base station), (b): interface at the CN, (c): RAN sharing, (d):

CN sharing, (e): via a spectrum broker, (f): uncoordinated.

4.3.2 Benefits

Spectrum sharing will bring the benefits to increase spectrum utilization, and indirectly cost. The smart use of spectrum sharing can enhance the user data rates.




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Early consideration of spectrum sharing for 5G system may have an impact on the logical architecture proposed in Section 4.1, more specifically on the design of the interfaces in Section 4.3. Depending on the chosen architecture for spectrum pooling the deployment will need to be considered also.


The concept of including spectrum sharing enhancing on the design on the RAN or core network is novel, and relevant for future standardization especially at 3GPP RAN2 and RAN3.

4.4 Multi-connectivity

Multi-connectivity (MC) is a solution derived from the principle of Dual Connectivity (DC) as standardized in LTE Rel-12. The DC variants in Rel-12 use two data flows separated on PDCP level and transmitted via two access points. The solution for mmMAGIC extends this principle towards additional parallel links.


Dual connectivity with MCG/SCG-split bearer

The multi-connectivity solution proposed by mmMAGIC allows more flexible options of data flow split between Master gNB (MgNB) and Secondary gNB (SgNB). With this approach, a data flow can be transmitted either on MgNB or SgNB, or it can be split, also either in MgNB or SgNB and be transmitted from both NBs, see Figure 4-4. The Master gNB can be a RAT using low frequency band or mm-wave band. The same applies for the secondary node.

Figure 4-4: Multi-connectivity bearer option.

This concept can be extended beyond DC by adding additional SgNB cell groups towards multi-connectivity. A variant of MC, namely, Redundant Coverage, which will be described in detail in Section 4.6, uses preconfigured backhaul links to multiple Transmitter-Receiver points (TRPs), but transmits only from one TRP. This allows fast switching to another TRP in case of link blockage. A further application is RRC diversity to send RRC messages via multiple TRPs for increased reliability and reduced latency, which is described in what follows.

RRC diversity

RRC diversity exploits multi-connectivity on control plane (CP). It relies still on one RRC entity located in the MgNB, but RRC messages are routed via different paths, e.g., via low band connection and mm-wave connection, see Figure 4-5.




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Figure 4-5: Principle of RRC diversity.

4.4.2 Benefits


MC is the most important functionality to cope with the physical channel properties at mm-wave frequencies. Blockage of a link by obstacles and high path loss variations can be overcome when having multiple TRPs within reach of a UE and with the capability to use diverse transmission or fast switching between TRPs. With the proposed solution, an increased reliability especially for standalone deployments can be achieved.

RRC diversity

Having multiple RRC connections or transmission paths leads to less connection failures and therefore increases RRC packet transmission reliability. Also, service continuity can be assured through different CP MC options, to provide seamless mobility and reduced latency. Further, the RRC diversity can improve delivering measurement reports on time to handle dynamics of SgNB set configuration.



Interworking with the system requires the appropriate design of the involved interfaces. Especially the capacity of the TRP interconnections depends on the number of connected TRPs. The protocol stack for split of PDCP data flows must be adapted.

RRC diversity

Enabling of RRC diversity requires allowing the UE to establish an RRC connection via a second node (SgNB), i.e., while adding a SgNB, the RRC Connection reconfiguration ensures that further RRC messaging can be delivered either over MgNB or SgNB. In the case of UL signalling, RRC messages received at SgNB are forwarded to MgNB (where RRC entity resides) over the Xn interface.



MC is an extension of the DC principle. Novel aspects come from the fact that much higher data rates must be handled and the combination with systems with lower capacity must be seamlessly possible. Therefore, interfaces and protocols, as well as processing capabilities, must be adapted to the 5G data rate requirements. Optimization of the options in DC is discussed in 3GPP NR Study Item and are now part of [3GPP TR 38.801]. RRC diversity was studied for LTE Rel-12 [3GPP TR 36.842].




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RRC diversity

The novelty is to realize redundancy on the CP messaging. In LTE-A Rel-12/13 the RRC signalling can be realized only via one node, the master node. Functionalities allowing separate multi-connectivity on CP are required.

4.5 Tight interworking with LTE-A

Although the new mm-wave system is designed to be capable of stand-alone operation, most of the initial deployments will be in areas already covered by legacy systems, such as LTE-A. As the mm-wave links can be more outage prone than LTE-A links, it will be beneficial to leverage on incumbent installations to provide redundancy for reliability and capacity extension through aggregation.


In order to achieve a smooth and efficient end-user experience when utilizing resources from LTE-A and mm-wave NR, it is vital to enable a tight interworking between the RATs. When designing new feature for NR, it will be important to ensure compatibility to LTE-A to avoid a full reconfiguration when switching between the systems. To achieve this, NR is designed to enable dual- and multi-connectivity (see Section 4.3) between NR and LTE-A nodes interchangeably depending on which nodes have the best radio condition. The feature is modelled on LTE-A Rel12 Dual Connectivity, where the CN/RAN connection terminates in one RAN node, the master gNB, but the UP traffic is split in the packet data convergence protocol (PDCP) layer to both the master node and a secondary node.

However, the notion of tight interworking is not limited to dual-/multi-connectivity, but also entails fast switching between the RATs, requiring to:

Have a split-bearer, but only use one path at the time and can switch per TTI; Have mobility functions which can reselect between LTE-A and NR without interruption; One can reuse/translate the UE context in other RATs; RRC in each RAT should be able to be handed over to the other RAT.

Since the mm-wave spectrum provides unsurpassed bandwidths, NR should be utilized when beneficial.

4.5.2 Benefits

The main benefit of tight interworking between LTE-A and mm-wave NR is the possibility to provide seamless service to UEs, utilizing mm-wave frequencies when good coverage ensues while still offering service through LTE-A to UEs in poor mm-wave radio conditions.

Without tight interworking, UEs would suffer significant service interruption when switching between RATs as the UE would frequently have to be reconfigured. In addition, tight interworking between LTE-A and NR enables outdoor macro deployments, where UEs with LOS or near-LOS can fully utilize the mm-wave NR capacity, whereas the non-LOS and indoor UEs with poor radio coverage can be served by LTE-A or through supplementary indoor mm-wave picocells.


The tight interworking between LTE-A and NR assumes that all aspects of the system should be interoperable and compatible between the RATs. The most profound differences between the RATs can be found in the PHY layer, which can easily be isolated between the RATs and be controlled per node. For the synchronous functionalities of the lower layers, e.g., scheduling requires tight synchronization and may be prohibitively complex, especially if NR operates at shorter transmission time intervals (TTI). Instead, the tight interworking is achieved at the PDCP layer, which already supports asynchronous coordination of multiple nodes. In addition, the CP functions of RRC will need to be harmonized to enable cross-RAT configurations.




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Tight interworking between LTE-A and NR was proposed in mmMAGIC prior to the beginning of the standardization work on NR in 3GPP, it has since been identified in standardization as a fundamental enabler for initial deployments, where non-standalone operation will be prioritized in the first release of NR. The E-UTRAN to NG-RAN (LTE-A-NR) dual connectivity (DC) is standardized in [3GPP TS 37.340].

4.6 Multi-service support

It is envisioned that the 5G system will support a plethora of different services ranging from enhanced mobile broadband (eMBB), ultra-reliable low latency communication (URLLC), to massive machine-type communication (mMTC). As each of these services pose significantly different requirements, the system must be flexible enough to provide this.


The most straightforward way to support multiple services would be to deploy dedicated infrastructure and resources with customized settings, optimized for each service. However, this would likely be cost-prohibitive, and wasteful in terms of precious radio resources, which implies that the services would have to share some level of common infrastructure. To enable this, the concept of network slicing has been introduced, where different business operations can be provided with individual, separate logical networks on a shared infrastructure, including e.g., radio resources, transport, and processing as well as memory hardware.

In some networks, multiple network slices will be deployed offering the same service, e.g., focusing on the eMBB service type, whereas in other networks each slice will be dedicated to a specific service. Figure 4-6 shows the procedure required when the user equipment request to be served on a dedicated slice.

gNB CN Node1 CN Node2UE

RRC Connection Setup

Selected Slice ID = x

NG1 Setup Request

NG1 Setup Response (List of supported Slice IDs)

NG1 Setup Request

NG1 Setup Response (List of supported Slice IDs)

Identify Slice policiesIdentify CN Node supporting Slice ID

Initial UE Message (Slice ID x)

Identify Slice policiesIdentify CN Node supporting Slice ID

Figure 4-6: Slice selection at initial attach when the UE provides slice ID to RAN.

4.6.2 Benefits

Network slicing allows the network to be locally optimized to fulfil certain KPIs without detrimentally impacting other services. By pooling shared resources, while maintaining independent operations of each slice, market entry of new business operations is enabled,




22

providing management of specific slices without the need of maintaining the entire network system, optimizing cost.


As each network slice is intended to operate as an independent network, the interworking with the other components of the system should be minimized. To ensure slice isolation, where an issue, e.g., congestion, in one slice does not impair the performance of another slice it will be important to enable slice specific access control. To facilitate the network slice selection, a UE can be configured with a network slice selection assistance information (NSSAI) which it can provide to the network at initial access or connection resumption.


Although the concept of network slicing has previously been considered within the core network, it has not been considered for the RAN in legacy systems and is currently a prioritized topic within [3GPP TR 38.801].

4.7 Deployment


Standalone and non-standalone operation modes

According to the mmMAGIC system concept, the 5G network deployment and integration comprises two operation sets: standalone and non-standalone operation of mm-wave RANs, as shown in Figure 4-7.

Operation mode Work on solutions for

Standalone

mm-wave access with multi-link and opportunistic serving

Deployment and relations optimization for overcoming spotty coverage and mobility issues.

Non-standalone

mm-wave access, low frequency band assisted

Utilize joint deployment of mm-wave nodes with nodes operating on lower frequencies.

Figure 4-7: Standalone and non-standalone operation modes.




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In standalone scenarios, the mm-wave RAT will use only mm-wave frequencies and will comprise mm-wave access points for deployment purposes, see Table 4-1. The deployment of one (5G) RAT, operating at both low-band and high-band carriers can be deployed, where the high-band carrier will need low-band carrier frequencies for operation. In standalone deployments, the AP coordination will be optimized to mitigate coverage and mobility issues related to the mm-wave propagation. For example: methods have been introduced to organize the mm-wave cells into dynamic cell clusters (5.5.1.1), where the APs included in the cluster may be updated during UE mobility.

In contrast, non-standalone scenarios will utilize joint deployment of mm-wave nodes with nodes operating at lower frequencies (i.e., in the form of multi-RAT APs or neighbouring APs of different RATs). In case of co-deployment of low and high-frequency RATs, tight interworking between the two RATs offers more optimisation opportunities for system operation. Among others, low frequency-assisted initial access is an illustrative example highlighting the benefits of a non-standalone deployment.

Table 4-1. Classification of New Radio (NR) deployment

New Radio

low-frequency mm-wave frequency

LTE-A at low-frequency Out of mmMAGIC scope Non-standalone

NR at mm-wave frequency

Non-standalone Stand-alone

Redundant coverage

One of the typical mm-wave specific PHY layer effects is the link blockage, which is caused by sudden obstruction of the LOS path by obstacles entering the signal path. This link blockage can lead to long interruption times, since connection re-establishment requires the initiation of the 5G gNB detection and connection procedure. To avoid such connection interruption, one idea is to exploit other signal paths from different, not yet blocked gNBs.

Figure 4-8: Redundant coverage scenario.

The coverage area must be covered by multiple gNBs simultaneously, see Figure 4-8, which requires a sufficiently high gNB density. Due to the narrow beams assumed at the mm-wave gNBs, the interference from the redundant gNBs can be kept low to provide sufficiently high SINR for serving the UE from the different gNBs.




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4.7.2 Benefits


The mm-wave system has been designed as a single logical architecture that can be addressed to either standalone or non-standalone deployments, where it can operate without any fundamental support from legacy systems. If the system is primarily designed to operate as non-standalone, it may preclude the operation in areas where there is no legacy radio technology, or Mobile Network Operators (MNOs) that do not possess any legacy cellular network. For non-standalone deployments, the latencies associated with first connecting to the low frequency RAT before accessing the high throughput mm-wave RAT might be too large to fulfil the service requirements. Nevertheless, the already established connectivity to a low-frequency AP shall contribute to the reduction of the UE power consumption and latency, which stem from the application of a beam alignment procedure at mm-wave frequencies.

Redundant coverage

In case of sudden link blockage, e.g., in case when vehicles or pedestrians are moving through the LOS direction, or even the body of the user, e.g., when moving or turning around, is blocking the link, the redundant coverage feature can maintain the connection without interruption time and without applying a new, time consuming random access procedure. So, redundant coverage improves mobility handling by simplifying the handover procedure.



In the absence of low frequency support, the mm-wave RAT will need to provide system access and connectivity, and the design of the logical architecture will be flexible enough to allow both deployments and the ability to utilize the benefits of co-deployment. Naturally, if the mm-wave coverage is reduced, the connectivity will be provided only within a limited area, with possibility to increase latencies associated with initial access, and mobility taken into account when deploying the overall system. The system needs to be able to accommodate a wide range of 5G scenarios to allow e.g., ubiquitous coverage with energy efficient connectivity to fulfil the identified requirements and KPIs of the different use cases. For instance, the stringent latency requirements will have to be reduced and more flexible Transmission Time Intervals (TTIs) would need to be introduced to achieve the short round-trip times.

Redundant coverage

To enable fast switching between different gNBs, the gNB cluster being able to serve the UE has to be identified and backhaul links must be prepared to take over the data traffic in case of a fast switching request. Therefore, multi-connectivity based features can be applied. Identification of suitable UE-specific clusters need to be done by dynamic measurements on PHY layer. Support from low band connection is possible. A straightforward approach is e.g., a low band eNB hosting the RRC functionality and acting as PDCP manager, whereas for mm-wave access multiple gNBs with reduced functionality could be used.



The non-standalone solution is an extension of DC towards MC among mm-wave APs as well as mm-wave and low band ones. Therefore, it affects standardization on UP and CP split at the PDCP layer.

Redundant coverage

Redundant coverage is a solution between classical handover and joint transmission and can be realized with similar network functionalities. To define gNB clusters, measurement procedures of mm-wave gNBs without current data transmission are in principle available, definition of concrete procedures might be required.




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4.8 Summary

In this chapter, design options related to the logical network architecture of the proposed 5G mm-wave RAN and the needed interfaces, have been presented, followed by a number of solutions, namely¨: tight interworking with LTE, multi-service support, multi-connectivity and deployment (standalone and non-standalone) aspects. The benefits of these solutions have been highlighted, and their roles, as parts of the overall proposed system concept, have been discussed, along with the relation of the design of each component to wireless communications standards. As it is shown from Figure 2-1, the radio access architecture is expected to impact CoS of the radio interface design with the modification of the different protocols layers that will allow the use of the proposed solutions. In the section that follows the solutions synthesising the radio interface design will be presented in detail.




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5 Radio interface components (PHY and Upper layers) This chapter will describe the second set of system components of the new radio interface, detailing the required modifications in the protocol stack, schemes for multi-node coordination, the procedure for mm-wave based initial access, management of active and inactive mobility, solutions for integrated access and backhaul and re-transmission schemes. For each of the components, the concept is firstly introduced, followed by the explanation of the technical details of the proposed solutions and relative performance evaluation, concluding with the description of the benefits, impact on the overall system design and relation to the standards.

5.1 Protocol stack

The design of the protocol stack of the mm-wave RAT will be based on the protocol stack of LTE, with extension and modifications only where needed. To facilitate a smooth network deployment, it is important to maintain a tight interworking with LTE-A. The set of protocols layers considered on the 5G-RAN can be seen in Figure 5-1.

Figure 5-1: The 5G protocol stack.


The main modifications to the protocol stack will be implemented at the physical (PHY) layer, to accommodate the mm-wave multi-antenna transceivers. More details can be found in Chapter 6 and its subsections. In addition, the New Radio (NR) will support multiple numerologies reducing the transmission time interval (TTI) and impacting both the MAC and the RLC protocol layer. The project identified modification to the protocol stack layers, such as:

The LTE packet data convergence protocol (PDCP) layer can already handle access to different RATs, namely LTE-WiFi interworking and aggregation. Thus, the NR PDCP layer will be designed so that both LTE PDCP connected to NR RLC, and NR PDCP connected to LTE RLC are supported. Furthermore, a new interface between PDCP and RLC called F1 has been recently identified in 3GPP, and it is introduced to support a centralized deployment of higher layers, with distributed lower layers. This solution is developed in the multi-connectivity Section 4.4.1.1.

RRC INACTIVE: the new RRC state has been introduced to reduce the C-Plane latency requirements when moving the UE from the battery efficient state to connected state (see Figure 5-17.). This concept can be made feasible by retaining the RAN and CN connections, with the UE context information stored in the localized RAN, so that the operation can be resumed quite smoothly. Although the need and benefits of the new




27

state has been widely acknowledged by researcher community, the technicalities with respect to the modifications required in the protocol stack is currently unknown and is being extensively discussed in 3GPP. It has been envisaged that modifications will be required starting from the MAC, going higher up until the PDCP layer and above. But the exact characterization of the functions, features and protocol flow are yet to be clearly defined.

RLC Optimization: although a considerable amount of research studies has been focused on physical (PHY) and medium access control (MAC) layers, the impact of transport protocols in mm-wave systems still requires investigation efforts. 3GPP defines the Radio Link Control (RLC) protocol to perform error recovery in the RAN and it aims to reduce the performance degradation that TCP suffers due to packet losses over wireless links, see Figure 5-2. Given the high variability of mm-wave links, proper setting of the RLC buffer dimension is crucial to optimize the desired objectives, such as throughput or latency. An example, at a given point in time, TCP exploits fully the link capacity because the UE is in LOS, the congestion window (CW) assumes high values and the RLC buffer, set by default at 1 MB, is not sufficiently large to hold all the incoming packets. Once the UE enters in a NLOS situation, given the large CW size, the number of unacknowledged packets makes it impossible for the fast retransmission (FR) phase to retransmit all the segments before the Retentive Timer On (RTO) timer (set at 1 s by default) expires. As a result, the CW resets to its initial value and TCP restarts from the slow start phase. As a result, the desired throughput will drop dramatically. To optimize the throughput with fast retransmission, one solution is to increase the size of the RLC buffer to accommodate a sufficient number of packets that is able to prevent TCP in performing and incur a timeout.

Figure 5-2: TCP and RLC in the protocol stack

5.1.2 Benefits

The design of the mm-wave protocol stack will allow a flexible and future-proof deployment of the 5G system, which will be capable of operating both non-standalone, with support from e.g. LTE, and stand alone at higher frequencies. The new recommended state RRC_inactive is to reduced latency at the control plane, and facilitate UE mobility while in that state while reducing energy, thus the number of connected UEs increase. The proper setting of the RLC buffer enhanced data rate and latency.


The design of the protocol stack must facilitate all the concepts introduced for the mm-wave system, both for the PHY layer improvements and the higher layer functions. To enable connectivity to the new 5G core network (5G-CN), and a mapping between the quality of service




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(QoS) flows and the data radio bearers (DRBs) used in the RAN, a new UP adaptation layer will be introduced above PDCP.


Although the overall design is inherited from LTE, each element has been re-evaluated and extended in relation to all new features. For instance, the PDCP layer is enhanced to enable multi-connectivity beyond two nodes to support DRBs split in the secondary node. Regarding deployment, the new F1 interface allows a split between a central unit, with PDCP and RRC, and distributed units, with RLC, MAC, and PHY. Optimization of RLC buffer size is a known solution [ACH06], which was extended by mmMAGIC and is in line with the specifications for UE layer 2 buffer sizes defined in [3GPP TS 36.306].

5.2 Multi-node coordination schemes

Due to the difficult propagation conditions and hardware constraints at mm-wave carrier frequencies, there is a need for highly directional transmission in mm-wave systems. For this reason, it is often argued that the interference for mm-wave cellular systems might not be so detrimental as compared to current deployments at lower frequencies. However, whether interference plays a significant role or not, depends on the deployment, i.e. on the BS density as well as on the capabilities of the antenna array at the users. In fact, due to the limited range of mm-wave communication, a high BS density might be required to achieve an acceptable coverage. For ultra-dense deployments, users might have a line-of-sight to several BSs, eventually leading to higher interference if a hardware-constrained beamforming approach is accepted (e.g. due to phase noise or limited DAC/ADC resolution).

Nevertheless, in contrast to the traditional multi-node cooperation at lower frequencies, due to the sparse multipath and high penetration loss, the main goal of multi-node cooperation and coordination at mm-waves seems to be to reduce signal outage due to sudden blockages and avoiding intermittent interference, instead of simply providing higher data rates. With peak-power limited mm-wave nodes, there will also be coverage gains due to increased aggregated power using multi-node joint transmission.

As have shown in [mmMAGIC-D5.2], [mmMAGIC-D5.1], cooperative and coordinated multi-node schemes can play a key role in dense mm-wave network deployment scenarios, since several cooperating nodes can help to obtain:

Macro-diversity gains towards shadowing/blocking, Power gains in cases when the system is peak power limited per node, Artificially increase multipath and thus (distributed) MIMO rank to better support

(distributed) spatial multiplexing and massive MIMO gains at sparse mm-wave channels, Support integrated multi-node/multi-beam soft handover, Efficient load balancing for energy efficient operations.


In [MMMAGIC-D5.1] the state of the art in multi-node coordination was reviewed and cooperation, and several coverage analysis studies were performed. Key findings were that the multiple line of sight (LOS) BSs available in a small cell range allows to exploit coordination among multiple BSs to improve the coverage for a typical UE, and that increasing the number of BSs efficiently reduces the outage probability. Especially for narrow streets and for users blocked by buildings or foliage, multi-node schemes would be crucial.

With those initial investigations as a base, the project developed the following seven multi-node cooperation and coordination techniques.

Beam Sweeping for Multi-Node Networks considering Interference

The best beam pair is usually determined at the UE based solely on the signal strength to its own BS. In the case of a network with multiple BSs, however, the transmission from the BSs in other cells needs to be considered for obtaining the best beam pair. For the UE to properly




29

distinguish the signal in its serving cell, the serving BS and the interfering BSs need to transmit orthogonal downlink pilot sequences as traditionally done in low dimensional multi-antenna systems at lower frequencies. The downlink pilot sequence assigned to each BS is transmitted with each trained transmit beam from the given BS, i.e. it is repeated throughout the beam sweeping procedure, Figure 5-3.

Figure 5-3: Multi-Node Beam Sweeping (figure modified from original figure in [HKC16]).

To avoid the overhead and delay associated with jointly determining the best beam pairs via coordination among the BSs, the proposed approach determines the best beam pair by taking the interference into account at the users. To this end, the beam sweeping algorithm could be based on e.g. the average interference per Rx beam, or the maximum interference from each interfering BS, resulting in an implicit coordination among the BSs. The results in [MMMAGIC-D5.2] show that the average and the maximum interference metrics perform similarly and they both show large gains compared to using only the signal to its connected BS for beam paring (i.e. ignoring the interference), in particular at high SNR situations, cf. [MMMAGIC-D5.2, sec. 3.2] for further details.

Sequential Hybrid Beamforming for Multi-Link mm-wave Communication

Multi-beam connectivity puts extra requirements on the UEs, in terms of, e.g., signalling overhead. To relax that requirement, a sequential hybrid beamforming design for multi-link mm-wave communication was proposed in [MMMAGIC-D5.2], in which a two-step precoding approach is used, as illustrated in Figure 5-4

As a starting point, a baseline data communication link is established via traditional analogue beamforming at both the BS (BS #1) and UE. If an extra RF chain is available at the UE, it can continue to probe the propagation environment at the same frequencies. In case the environment is favourable and system resources allow a secondary data communication link is established to enable multi-stream transmission. In principle, the secondary link could be served by the same BS (BS #1) or another BS (BS #2).

To initialize the secondary data communication link, a parallel beam search scheme is also proposed in [ZCS+17], which helps the UE/BS to find a suitable beam pair with given optimization criteria without interrupting the baseline data communication. As shown in [ZCS+17], with proper pilot signal design and by observing only the relations between signal energy and interference energy without carrying out actual data demodulation, full and accurate synchronization will not be required for this parallel beam search algorithm.




30

Figure 5-4: Parallel beam search frame structure for test beam pair , , cf. [MMMAGIC-D5.2, sec. 3.3] for further details.

With this approach hybrid beamforming becomes an add-on feature that can be easily switched on over an analogue beamforming enabled system without interrupting its operation whenever the system requires. The purpose of establishing the secondary data communication link is to obtain diversity gain or spatial multiplexing gain, depending on digital beamformer design at the UE and BS(s). It was shown that such a parallel beam training has little impact on the baseline data transmission in the SNR range of interest for mm-wave communication, cf. [MMMAGIC-D5.2, sec. 3.3] for further details.

mm-wave based RF-FSO Multi-hop Networks

In D5.1 [MMMAGIC-D5.1] the potential of RF-FSO systems in single-hop deployments was showed, and in [MMMAGIC-D5.2] the concept to multi-hop networks was generalized. In particular, the project investigated the performance of a mm-wave-based RF-FSO system for multi-hop networks using HARQ.

Coherent FSO systems provide fiber-like data rates through the atmosphere using lasers. Thus, FSO can be used for a wide range of applications such as last-mile access, fiber back-up, backhauling and multi-hop networks. In the radio frequency (RF) domain, on the other hand, millimetre wave (mm-wave) communication is the key enabler to obtain sufficiently large bandwidths so that it is possible to achieve data rates comparable to those in the FSO links. In this perspective, the combination of FSO and mm-wave based RF links was considered as a powerful candidate for high-rate reliable communication. The data transmission efficiency of multi-hop RF-FSO systems considering the mm-wave characteristics of the RF links and heterodyne detection technique in the FSO links was studied. The closed-form expressions for the system outage probability was derived and the effect of imperfect hardware on the system performance was evaluated.

Figure 5-5: Illustration of an RF-FSO multi-hop network.

For the illustrated dual-hop scenario in Figure 5-5 (one RF and one FSO hop), it was shown that the HARQ based dual-hop RF-FSO system improves the energy efficiency significantly. With a maximum of 2 and 3 HARQ retransmissions, the required average power is reduced by 13 and 17 dB, respectively. In addition, these kind of hybrid links have the potential to be deployed in mm-wave LOS coverage enhancements with coordinated high-rise access points, which is the topic of the next section. For further details see [MMMAGIC-D5.2, sec. 3.5].




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mm-wave LOS Coverage Enhancements with Coordinated High-Rise APs

In particular, LOS coverage is attractive in mm-wave networks, but since it is difficult to find enough positions to deploy high-rise access points in dense networks, the potential of a joint deployment of high-rise and low-rise Aps was investigated, as illustrated in Figure 5-6.

Figure 5-6: Blockage by high building.

Considering the model in Figure 5-6, the building A will block the LOS link as long as its height h is larger than hy. It can be easily seen that the height of the AP and cell density play crucial roles in determining the UE LOS association probability. When a lower AP is installed on the street furniture, e.g., lamp post, with height 3 m, the LOS association probability reduces rapidly with increased cell radius, i.e., reduced cell density. However, when an AP is installed in a high building with height 30 m, the LOS association probability decreases much slower. It means that the high-rise APs can always provide significantly high LOS coverage even with low cell density. Numerical results in [MMMAGIC-D5.2] showed that only very few high-rise APs are needed to enhance the LOS coverage when the blocking building height is low, whereas with increased blocking building height, more and more high-rise APs are needed to provide a targeted LOS coverage probability. For further details cf. [MMMAGIC-D5.2, sec. 3.6].

Relay-assisted Access at mm-waves

The merits of multi-hopping were further analysed in [MMMAGIC-D5.2] based on a system level evaluation at mm-waves. A relay-assisted system with decode-and-forward RSs was compared to a direct access system, as illustrated in Figure 5-7.

(a) (b)

Figure 5-7: Illustration of the considered two considered access methods. (a) Direct access vs (b) Relay-assisted access.




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Two main configurations were compared, as illustrated in Figure 5-7:

Direct access (DA): no RS is deployed and the BSs directly serve the UEs; the whole bandwidth is available, but low SINR are experienced by the UEs because of the high BS-UE distance and PL.

Relay-assisted access (RA): RSs are deployed, BSs provide the backhaul to decode-and-forward RSs, which, in turn, serve the UEs; bandwidth is split among access and backhaul, but higher SINR is observed because of the reduced RS-UE distance.

Naturally, the cell-edge users benefit from the relay-assistance, but it was also shown that RSs at mm-wave can be very beneficial in improving system performance. For the exemplary ISD=500 m, the relay-assisted scheme provides a gain of about 120% in the cell border throughput, and the same sector spectral efficiency as for the direct scheme. For further details see [MMMAGIC-D5.2, sec. 3.7].

Joint Hybrid Precoding for Energy-efficient mm-wave Networks

The project studied joint hybrid precoding for energy-efficient mm-wave networks, as illustrated in Figure 5-8.

Figure 5-8: Illustration of multi-node cooperation.

mm-wave hybrid beamforming design, maximizing spectral efficiency, has been shown to give performance close to the fully digital precoding scheme [RMG+16], [ARA+14]. However, only a few works have studied the energy efficiency of the precoder design. When joint transmission is allowed, a major research problem is to jointly design the analogue and the digital precoder such that the total power consumption is minimised and the quality of service of each user is satisfied.

A hybrid precoding structure allowing joint transmissions from multiple BSs to each user was adopted, and the minimum transmit power required by each BS to satisfy a spectral efficiency constraint for each user was analysed. Because strict synchronization among BSs is difficult to achieve, each user uses successive interference cancellation to sequentially detect multiple streams from different BSs, and due to the unit modulus constraint, the optimization problem for hybrid precoding is usually too complex to find a global optimal solution. For this reason, the algorithm starts by solving for the optimal fully digital precoder without the analogue constraint. Then, the analogue precoder is initialised as the element-wise normalisation of the digital precoder and conditioned on the analogue precoder, the digital precoder is obtained by solving the optimisation problem. It was shown that, by jointly designing hybrid precoders, the total transmit powers can be reduced while satisfying the spectral efficiency constraint for each user. Compared to a fully digital beamforming the average total transmit power is naturally larger for the more constrained hybrid precoders, but the overall power consumption is lower for a given targeted rate for each user due to the fewer RF chains. It was also shown that there are large gains in total energy efficiency with joint multi-node multi-stream transmission, and it increases with the number of cooperative BSs. In addition, the distribution of the power consumption over




33

the cooperative nodes becomes more even, which might be useful for meeting potential EIRP restrictions of mm-wave nodes. For further details see [MMMAGIC-D5.2, sec. 3.8].

5.2.2 Benefits

The benefits of the identified cooperative and coordinated multi-node scheme techniques in [MMMAGIC-D5.2], [MMMAGIC-D5.1] can be summarized as follows.

Multi-node cooperation and coordination is very useful for mm-wave networks. Thus, it should be an integrated part of efficient mm-wave networks.

Spatial multi-flow joint transmission using hybrid beamforming is efficient from both throughput, outage, energy efficiency, load balancing and peak power reduction point of view (EIRP).

Cooperative hybrid beamforming has a great potential to meet key performance targets at a lower cost and energy consumption than fully digital beamforming, and the gains are increasing with the number of cooperating nodes.

Sequential hybrid beamforming is a flexible way to support adaptive dual/multi-connectivity. In certain deployment scenarios, even a fully analogue beamforming approach is competitive.

Decode-and-forward relaying can be deployed to substantially improve the cell edge throughput without sacrificing the spectral efficiency.

Hybrid RF-FSO links have a large potential for backhauling, and multi-hop Hybrid RF-FSO connections in combination with a few high-rise APs have a great potential for efficient line of sight (LOS) coverage.

In interference limited scenarios, due to the sparse multi-path and high potential for LOS channels, coverage can be increased in the uplink by increasing the total number of receive antennas, and use an optimal fraction of the antennas to cancel the nearest few strongest interferers.

To support random access and mobility at mm-wave frequencies, a flexible toolbox for beam management is necessary.


The proposed solutions are compatible with the architectural approaches discussed in [MMMAGIC-D3.2], i.e. Multi-Connectivity, LTE-NR tight interworking, RRC diversity, Multi-band system integration, Cell clustering, Mobility state transition, Self-Backhauling, Access-integrated backhaul in fixed wireless access, Joint optimization of access and backhaul, and Interference coordination.


The evaluations of the presented solutions [mmMAGICD-5.1] have been focusing mainly on standalone mm-wave communications for access and self-backhauling. This is motivated by that the non-standalone solutions are more straight-forward and currently covered by studies in 3GPP. For the stand-alone case, the compatibility with the 3GPP standards is unclear, but most of the proposed techniques should be possible to be incorporated in 3GPP NR with suitable standards extensions.

5.3 Initial access

In this section the initial access procedure for standalone mm-waves is presented, complemented by the description of initial access for non-standalone deployments where the use of low frequencies is considered to support the initial access phase.

A. Initial access for standalone mm-waves: the highly directional nature of mm-wave communications makes the initial access particularly challenging (specially in standalone deployments) compared to equivalent procedures in sub-6 GHz networks, where omnidirectional antennas are used at both the AP and UE sides. In fact, a time-consuming beam




34

search procedure is required during initial access for AP and UE to determine suitable directions of transmission and reception. This incurs significant overhead and waste of network resources. The overall idea of the envisioned mm-wave initial access procedure is summarized in Figure 5-9.

Figure 5-9 Overview of system component solutions making up the envisioned procedure for mm-wave initial access (standalone deployment).

It is assumed that initial access cycles are periodically repeated and start with a cell discovery phase, followed by the random access phase, during which network resources are allocated to the user, and conclude with a beam refinement/tracking phase. All the ingredients making up this procedure share the objective of maximizing the data rate over the established mm-wave link, satisfying, at the same time, reduced latency overhead, power consumption, and hardware complexity requirements.

During the cell discovery phase, synchronization signals and important system information are broadcasted by the access point (AP). In this phase, either the AP or the user equipment (UE), or both, need to implement beam sweeping with directional antennas to achieve sufficient link margin. In fact, due to the severe path loss at mm-wave frequencies, significant coverage issues can be experienced if beam sweeping is not applied at all during cell discovery (e.g., if both AP and UE use omnidirectional antennas). Sweeping across different beams implies that a preliminary, coarse estimation of suitable directions of transmission/reception is already performed at this stage, at least at one the two involved devices.

Upon successful completion of cell discovery, a random access procedure is initiated by the UE, which randomly selects one preamble index and one contention-based slot over the shared random access channel and transmits a preamble to the serving AP. As for cell discovery, either the AP or the UE, or both, need to use directional antennas with beam sweeping for random access. However, the UE has now potentially more spatial information about the channel, due to signal strengths measurements and synchronization attempts over different spatial directions performed during cell discovery. The following main objectives of the random access procedure are identified:

to finalize the beam search/training procedure already initiated in the previous phase, converging to suitable antenna patterns to be used for data transmission;

to allocate/schedule resources (e.g., time, space, frequency channels, RF chains in case of hybrid beamforming, etc.) to meet UE's QoS requirements;

to uplink-timing measure and send a timing adjustment command to the UE; to identify new UEs in the cell and providing the UE with an identity.

The objectives are somewhat related since the beam search results directly affect the resource allocation problem, mainly in terms of spatial resources to be allocated.

Once connected to the network, the UE starts to exchange data with its serving AP. During this phase, beam refinement/tracking strategies can be considered to further optimize beam shapes and steering directions. Slight corrections could be in fact required in order to account for the different frequency channels and modulations used in the beam search/alignment phase compared to the data transmission phase. Furthermore, beam tracking algorithms can be included to handle user mobility, for example by updating the steering directions based on the link quality and, eventually, triggering more advanced and thorough beam search procedures. The maximum time that a network can take for beam sweeping (for synchronization signal) is




35

5ms. The UE measurements configuration is called Size-synchronization signal Measurement Timing Configuration - SMTC window. In the following sections, the system concept solutions required to build up the initial access procedure is described and envisioned in Figure 5-9.

Note that all proposed solution assumes the PHY channels as defined in 3GPP. More details of the proposed initial access solution can be found in D4.2 [MMMAGIC-D4.2].

B. Low frequency assisted initial access: the study on initial access discussed previously, focuses mainly on standalone mm-wave systems, whereas this concept is applicable to non-standalone deployments, especially in the scenario, where, low-frequency and mm-wave RATs are co-deployed. In case of co-deployment of low and high-frequency RATs, tight interworking between the two RATs offers more optimization opportunities for system operation. Among others, low frequency-assisted initial access is an illustrative example of tight inter-RAT inter-working.

Typically, the initial access process comprises the three tasks of downlink timing and frequency synchronization, system information acquisition and uplink timing synchronization. It should be noted that some of these tasks are also required during transitions from the RRC_IDLE to the RRC_CONNECTED state; therefore, the proposed concept can be also applied for such RRC state transition procedure, when applicable. During initial access, a UE should establish a RRC connection with the corresponding mm-wave AP. The performance of this procedure directly impacts the user experience. Therefore, on PHY layer, a beam alignment must be achieved within short time. Exploitation of the limited a-priori information on the preferred transmission direction at both ends of the link will support this. In a non-standalone deployment, i.e., a heterogeneous network, as the one illustrated in XX, where mm-wave small cells are located within the coverage area of a macro cell operating at low frequency, low frequency RAT assistance can improve initial access performance significantly. Especially UE power consumption and latency can be reduced.

Figure 5-10: Low frequency-assisted initial access to a heterogeneous network.

In the following, the three mentioned tasks of low frequency RAT assistance are highlighted.

Downlink synchronization: For downlink synchronization, the UE exploits synchronization signals transmitted by the AP. These are time-frequency resources with a certain periodicity, which allow acquisition of symbol, slot and sub-frame timing. After




36

achieving that, the UE is able to obtain the cell ID. If the UE is located in a low frequency RAT coverage area, the low frequency RAT can transmit information about frequency and cell IDs of mm-wave small cells within its coverage area, by e.g., dedicated signaling to the UE. With this signaling, the UE does not need to perform an exhaustive search over the entire small cell ID space, but it only tries to detect the signaled cell IDs. As a consequence, the UE power consumption for downlink synchronization is significantly reduced.

System information transmission: Another task of the initial access procedure is to acquire the system information, which provides all the essential information for accessing the network to the UE. The coverage of the system information determines the coverage of the cell. Some of the system information components, e.g., the system frame number, are changing fast on the basis of one or several mm-wave RAT frames. Other system information components vary relatively slowly, so information about system bandwidth, random access resources, paging resources and scheduling of other system information components is typically semi-static. For this reason, it can be energy efficient to convey some of the slowly varying system information by exploiting the existing low frequency RAT. The fast-changing system information components, however, need to be transmitted by the mm-wave RAT.

Uplink synchronization: It is important that efficient UL data transmission in the mm-wave RAT is supported as well, especially for “UL data traffic dominant” use cases, e.g., uploading content, such as high-resolution videos to social media during sports events, concerts etc. UL synchronization needs to be achieved prior to any UL packet transmission to ensure that all the co-scheduled UEs’ uplink signals are time-aligned at the eNB. A RACH procedure, similar to that standardized in LTE can be used. Based on the RACH preamble transmitted by the UE, the eNB can determine the timing advance value for the UE. The radio resources for the preamble transmission are typically part of the system information and such system information can be signaled by the low frequency RAT. This can be viewed as a basic assistance to the UL synchronization. To ensure a certain UL preamble coverage, if several preamble formats are supported by the system, the low frequency RAT can signal a particular preamble format to the UE in order to realize the network assisted preamble format selection. In case of contention free RACH, the low frequency RAT can signal the exact preamble sequence to be used by the UE. During the LTE-like RACH procedure, the RACH response signal can be also transmitted by the low frequency RAT, e.g., LTE. In addition to the above-mentioned options for UL synchronization assistance, the low frequency RAT may also offer assistance to the possible beam alignment operations during the initial UL synchronization procedure.


Cell discovery with wide beams and broadcasting

It is commonly assumed that large array’s is only capable of transmitting or receiving in terms of narrow beams, which poses an issue for random access and transmission of cell-specific reference signalling. A wide-beam approach which maximizes the power utilization of the array, which leads to less loss of array gain compared to conventional methods, is proposed. The main idea lies in applying a widener function , on a base DFT weight vector. For the -antenna uniform linear array (ULA)the following precoding vector is obtained:

⊙ e , , . . . , e , ,




37

where for 0, . . . , 1. The widener function is given by:

, 41

2 121

,

in which ⊙ denotes the Hadamard product. Parameters , allows for further optimization of the beam shape given an immediate objective. Simulation of the proposed scheme for a 128x1 ULA is shown in Figure 5-11 . The new scheme is compared with state-of-the-art solutions such as the Widener method [Int16] and the amplitude tapering [QQL16].

Figure 5-11: Example of beam shapes designed using the Widener method described in [Int16]

and the technique using amplitude tapering in [Qia16]. Plotted is also the DFT beam and the subelement pattern.

Synchronization and CSI reference signals

Wireless networks based on mm-wave technology are expected to operate using single-beam and multi-beam operation, in scenarios below and above 6 GHz, in licensed and (forward compatible) un-licensed mode. Now, when considering possible beam forming architectures, it can be envisioned that a cell may have one or multiple Transmitter-Receiver Points (TRPs) and each TRP may be having one or multiple Transceiver Units (TXRUs). For example, with hybrid analogue-digital beamforming, this means that a cell may form multiple analogue beams at a time. In the single-beam approach, the cell would transmit synchronization signals via sector wide beams, e.g., once in the given periodicity. Furthermore, it is considered that, especially in single-beam mode, coverage enhancement should be supported, meaning that the synchronization signals should be able to be transmitted in a repeated manner. In multi-beam operation, the cell, which may consist of multiple TRPs, transmits synchronization signals in a beam sweeping manner. In some multi-beam architectures, the Synchronization Signal block (SS-block), i.e., NR-PSS, NR-SSS and Physical Broadcast Channel (NR-PBCH) could be transmitted individually to each beam direction or could be transmitted in a Single Frequency Network (SFN) manner, i.e., the same signal is transmitted in superposition manner from parallel beams simultaneously in the cell. The preferred/feasible beamforming configuration will of course depend on the deployed frequency band. Hence the required time domain structure of the SS-block signals needs to support different scenarios.




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The time duration of the SS-block will have a direct impact to the overall duration and/or overhead of the beam sweeping procedure. The design of SS-blocks within an SS-burst and an SS-burst set needs to consider how many simultaneous beams to support, especially in the case where some beam specific reference signal is multiplexed within the SS block, and how many beams a SS burst set is to support in total. Furthermore, it is expected that vendors will build the systems with different array architectures depending on the target scenario, carrier frequency range, etc. It is quite evident that the higher in the carrier frequency, the more the array architectures go towards distributed Power Amplifier (PA) architectures where the target EIRP is achieved the more via array gain and high number of low-power PAs. Because of this, antenna pattern beam widths become narrower and narrower. Similarly, as the higher in carrier frequency, the available system bandwidths are expected to increase.

Figure 5-12: SS-block structure (left) and multiplexing options for CSI-RS and SS-block (right).

The illustration of the considered synchronization signal (SS)-block structure is depicted on the left of Figure 5-12. The length of the proposed SS-block structure is 4 symbols, where the length of NR-PSS and NR-SSS is one symbol each. The proposed bandwidth is dependent on the assumed sub-carrier spacing with the sequence length of approximately 127. On both sides of the NR-PSS and NR-SSS, a guard band is preserved, extending the bandwidth to 144 sub-carriers (Resource Elements - REs in Figure 5-12). The SS block time index is included in NR-PBCH. This NR-PBCH is time multiplexed with NR-PSS and NR-SSS, also covering 2 symbols. To enable short and efficient SS-burst structure, it is preferred to limit the length of NR-PBCH to be 2 symbols, resulting to wider frequency domain allocation while enabling one-shot performance at acceptable level. The time/frequency relationship between NR-PSS and NR-SSS resource position is independent of the used duplexing mode (or beam operation). Therefore, the SS-block pattern would need to be common for FDD and TDD deployments. Furthermore, NR-PSS, NR-SSS, and NR-PBCH should have fixed resource relationship. The NR-PBCH is always transmitted. Additionally, a single set of possible SS-block time locations is specified per frequency band and that SS-block size should be kept constant.

The periodicity of DL sweeping sub-frame, which contains DL beam management reference signals (RS), depends on each TRP capability to simultaneously use NP antenna ports for CSI-RS transmission per a sub-time unit. It is worth noting that, to enable robust measurements, orthogonal antenna ports are preferred for beam and mobility measurements. As a result of NP options, different DL beam sweeping periodicities need to be supported in NR. From practical implementation perspective of TRP, the set of values for NP, i.e., 2, 4, 6, 8, can be consider as a feasible set for NR systems.

NR‐PSS

NR‐SSS

Guard band

PBCH

144 RE’s

288 RE’s

Frequency

Time




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The following SS-block and CSI-RS multiplexing options can be considered:

Frequency division multiplexing (FDM) with CSI-RS and SS-block; Time division multiplexing (TDM) with CSI-RS and SS-block.

On the right of Figure 5-12, both FDM and TDM options for CSI-RS and SS-block are shown. Here, the number of OFDM symbols for SS-block can be assumed to be larger or equal to four. Furthermore, it can be assumed that the number of PRBs per symbol reserved for SS-block is 24. Therefore, for the FDM option, by considering PRB reservation for SS-block, the remaining part of system bandwidth can be leveraged for CSI-RS transmission of DL beam management. For the TDM option, instead, it can be assumed that CSI-RS symbols are multiplexed in a TDM manner independently with respect to the SS-block.

Finally, the proposed synchronization and CSI reference signals are robust against mm-wave channel challenges such as high path loss, large bandwidth, and blockage. Furthermore, in terms of robustness to hardware impairments, the proposed schemes have been designed to provide both low complexity and low power consumption and to deal with antenna design challenges.

Cell discovery assisted by an auxiliary transceiver

When hybrid beamforming transceiver architecture is applied, both transmitter and receiver need to switch beams via analogue circuits (e.g., the configuration of analogue phase shifters). With low cost and power efficient implementation (e.g., based on RF MEMS), analogue beam switching can be quite slow since it could require from several hundred nanoseconds (ns) up to 40 micro second (μs). This means that guard intervals (GI) must be reserved accordingly, between the transmissions of different beams. During such GI, no signal is transmitted and received. As a result, the system will have signalling overhead and high latency of user access (long user waiting time). Most of the conventional schemes, e.g., those used in 802.15.3c and IEEE 802.11ad, require reconfiguration of the analogue phase shifter network, which can lead to high signalling overhead and initial access latency, resulting in long user waiting time.

In this section, a new transceiver architecture is proposed, which consists of a hybrid wideband transceiver as the main transceiver and an auxiliary fully digital narrow-band transceiver, both operating at mm-wave frequencies. The signal bandwidth is divided into two parts, i) one wide-bandwidth part for general data transmission by the main transceiver under hybrid beamforming constraints and ii) one narrow-bandwidth part (e.g., 1 MHz) for initial access and other optional tasks, such as small packet transmission by the auxiliary transceiver with fully-digital MIMO capability. The new architecture and the corresponding frame structure are shown in Figure 5-13:

Figure 5-13: The new transceiver architecture and frame structure.

For initial access, the auxiliary transceiver performs fast Tx beam scanning as well as Rx beamforming using advanced digital array processing/MIMO techniques, e.g. Angle-of-Arrival

…

Antenna array, N elements

Hybrid widebandtransceiver

Auxiliaryfull digital narrowbandtransceiver

… …

……

M wideband digital signals

N narrowbanddigital signals

M < N

BEACON Data BEACON Data… …

time

Fre

q.

Hybrid Only0.1 ms subframe

Data… …

time

Fre

q.

Hybrid + Aux. (proposed)0.1 ms subframe

BEACON

Data Data Data




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(AoA) estimation. During initial access of some UEs, the data transmissions for other UEs do not need to be interrupted. After the auxiliary transceivers assist AP and UEs to find the best beam directions, the optimal beam forming coefficients are calculated for the hybrid transceiver according to such beam directions.

More details about the procedure, along with extensive performance evaluations via simulations, are provided in [MMMAGIC-D4.2]. Our simulation results show that the beacon overhead of the proposed scheme is much lower than that of the hybrid-only scheme and very close to that of the fully-digital scheme. Furthermore, the beacon overhead of the proposed scheme depend on the bandwidth of the auxiliary transceiver. With very small bandwidth of the auxiliary transceiver, the proposed scheme even allows lower beacon overhead than that of the fully digital scheme. Furthermore, with much lower beacon overhead, the proposed scheme has much lower beam alignment delay than the hybrid-only scheme. Moreover, the larger the bandwidth of the auxiliary transceiver, the smaller the beam alignment delay. With 1 MHz or 10 MHz auxiliary transceiver bandwidth, the beam alignment delay is very close to that of the fully digital case. Finally, it should be reminded that for hybrid-only schemes, the beacon duration usually equals the duration of data transmission interruption. In contrast, with the proposed scheme, data transmission is not interrupted during beacon, but just with slightly reduced data bandwidth. Thus, the proposed scheme also helps to reduce latency of data transmissions.

The proposed strategy is robust against mm-wave channel challenges such as high path loss, large bandwidth, and large scale parameters, and against antenna design challenges, especially the analogue phase-shifter latency.

Random access with coordinated access points

This solution proposes a coordinated random access scheme for clustered APs, which can be used for efficient and fast initial access in ultra-dense mm-wave networks without the support of the legacy, sub-6 GHz technology. Compared to full beam sweeping schemes, where a time-consuming exhaustive search is employed, the APs within one cluster will perform the random access procedure in a coordinated manner, based on power delay profile (PDP) measurement reports shared with each other via backhaul links.

Figure 5-14: System model for coordinated random access within a cluster of three APs.

The proposed solution involves at least three APs (see Figure 5-14) and can be divided into four phases: measurement, coordinated beam sweep reordering, initial access, and asymmetric multi-cell association. In the first phase, the involved APs randomly choose a beam direction, while the UE conducts full beam sweep transmitting periodically the random access preamble on each direction. The APs measure the power delay profile (PDP) of the received preamble and share the measurement reports via the backhaul links. In the beam sweep reordering phase, APs coarsely estimate the UE location and jointly determine the best receive beam for each AP. In the third phase, the APs receive through the previously determined beams, while




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the UE conduct a second random access attempt via transmit beam sweeping until an uplink connection is established. In the last phase, the existing uplink connection can be used to establish multiple uplink connections, which may or may not associate the UE to the same AP in the downlink.

More details about the proposed procedure for coordinated random access, along with performance evaluations via simulations, are provided in [MMMAGIC-D4.1] and [MMMAGIC-D4.2]. The simulation results show that coordination among three APs is already effective to reduce the random access time by up to 40% compared with the conventional, single-AP approach.

Random access based on FDD

An Initial Access scheme is studied in a standalone mm-wave network under the assumptions that an FDD band is used, digital beamforming is available at the gNB side, and partial RAN centralization is present to enable centralized processing.

The initial access scheme has two key components: a non-reciprocity based beamforming scheme, and a mechanism to exploit RF contextual information that may help other gNBs to speed up the random access process. Suitable beamforming vectors must be obtained by each node. Nodes with better channel conditions can assist others in poorer conditions by providing rough estimates of the corresponding angles of arrival, hence speeding up the random access process.

The proposed initial access approach comprises the following steps, skipping any eventual power-ramping phase at the UE side.

The UE sends an Initial Access preamble comprising pilots. The gNB estimates the UL channel matrix and obtains the UL beamforming vector. With the aid of the UL beamforming vector, the gNB acquires the UL Initial Access

preamble. The gNB estimates the DL channel covariance matrix and obtains the DL beamforming

vector. The gNB sends a Random Access Response (RAR) message to the UE containing

random access information to complete the process. In very dense deployments there may be multiple gNBs receiving an initial access signal from a given UE. The device can send an initial access signal and each of the base stations may perform estimation of the channel covariance matrix, with different reliabilities depending on the received SINR values. The result of such estimations may be communicated to a central node in charge of resolving which cell is the best candidate for initial access. The central node can also perform a rough estimation of the UE’s location through intersection of the beams that correspond to the beamforming weights obtained by the different cells. This UE location can then be forwarded to other cells for which accurate estimation of the channel was not feasible, thus speeding up the initial access procedure.

Link-level and Monte Carlo simulations showed that the proposed method achieves excellent performance in terms of both number of access attempts and initial access time, even with very low SNR levels. Finally, the proposed schemes are robust against mm-wave channel challenges such as high path loss, large bandwidth, and time/frequency selectivity. They are also robust against hardware impairments such as the non-reciprocity of the transceiver and antenna design challenges.

5.3.2 Benefits

As highlighted in D4.2 [MMMAGIC-D4.2], the proposed solution for initial access aims at maximizing the achievable data rate over the mm-wave link and to reduce power consumption and latency as well as lowering the hardware complexity requirements.




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The proposed solutions can be easily integrated in the overall mmMAGIC system concept. The interworking is achieved, for example, when multiple APs are simultaneously detected during the cell discovery. In this case, the UE could select the AP that provides better received signal quality as its serving AP, but also keep track of other potential serving cells, including traditional 4G cells, to exploit multi-connectivity (Section 4.3). In the coordinated random access, PDP measurements are shared among different APs, meaning that interworking with system concepts such as cell clustering (Section 5.5) and self-backhauling (Section 5.7) is required. Finally, beam refinement and tracking algorithms are not only part of initial access, but they can be also used to handle user mobility during the ongoing data transmission. In that respect, interworking with system concepts such as multi-antenna beam management (Section 6.6) and CSI is required (Section 6.8).


Most of the system concept solutions proposed for initial access are not only aligned with 3GPP specifications, but represent also complementary schemes to enhance the performance on ongoing 3GPP specifications for release 15. This is the case, for example, of the proposed beam sweeping strategies for broadcast signalling. Several other presented system concept solutions are extremely novel and can be considered as potential, future items in 3GPP. This is the case of auxiliary transceiver based initial access, coordinated random access, GA-based beam selection, and beam training/tracking through hybrid beamforming.

5.4 Multiple access schemes

According to mmMAGIC studies, the conventional multiple access e.g. TDMA, FDMA, CDMA, SDMA, can be further used. However, for mm-wave transmissions, multiple access in spatial domain needs to be more emphasized, due to extensive use of beamforming. Further, due to constraints of hybrid beamformers, the following restrictions exist: The number of links to be multiplexed via spatial domain is limited by TDMA also becomes suitable. Since no subcarrier dependent precoding is possible when applying analogue beamformer, TDMA is used on top to serve different SDMA groups (each group containing several links under SDMA). FDMA can be used further on top to multiplex users with the same/similar beam directions.


The following two advanced multiple access schemes have been proposed to further enhance the system performance under mm-wave specific constraints.

QoS-centric resource allocation SDMA-based multi-user access via hybrid beamforming:

The available RF chains are grouped according to QoS requirements and used for different services. In this way, the hardware resource is used more efficiently to match diverse QoS requirements. More specifically, given the described system setup, the aim is to optimally allocate the available AP hardware resources (i.e., the RF chains) for the beam training and data communication stages, in order to efficiently serve both UEs via hybrid analogue-digital beamforming. An optimization framework with the objective to maximize the expected rate of one UE was evaluated, for a given constraint on the training time set by the other UE. For a range of system scenarios, the optimal data rates are illustrated for different latency constraints and for different strategies of exploiting the full RF chain set at the AP side. As it will be observed in what follows, the proposed access schemes can significantly outperform the basic Time Division Multiple Access (TDMA) approach. The details of the proposed solution can be found in D4.2 [MMMAGIC-D4.2].




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CG-MIS based joint scheduling and resource allocation for IAB:

A flexible joint scheduling scheme for backhaul and access traffics, exploiting SDMA of links from multiple AP’s and dynamic TDD. This scheme generates both scheduling results indicating which links (multi-AP) are served in SDMA manner, as well as the dynamic DL/UL switching point of each AP. The main idea of this algorithm is to determine which link(s) to be transmitted in each SDMA group (a group of simultaneous links multiplexed in SDMA manner) according to UE transmission request and interference information acquired by for example initial access and interference sensing procedure. To simplify analysis of considered IAB network, the network is abstracted to a directed graph, referred to as “link graph” where nodes represent network elements (BS, APs and UEs), and edges represent transmission links among the elements. With the interference information, the link graph can be transferred to a new graph referred to as “CG”. In this graph, the nodes now represent the transmission links (edges in link graph), and the edges depict the “conflicts” among links. Specifically, links that are “connected” by an edge either cannot be scheduled simultaneously due to half duplex constraint, or will result in interference above threshold if simultaneously transmitted. After scheduling the SDMA concurrent transmissions, the time duration of each SDMA group is allocated, followed by a power allocation at each node with transmit power constraint. The details of the proposed solution can be found in D4.2 [MMMAGIC-D4.2] as well as Section 5.7.

5.4.2 Benefits

The proposed solution achieves significantly higher throughput than state-of-the-arts (including pure TDMA-based joint scheduling and the ICIC method in LTE, specified in D4.2 [MMMAGIC-D4.2]) and reduced latency. Due to better reuse of the radio resource (by exploiting SDMA), higher connection density and traffic density can be supported.


The proposed solution is related to initial access, self-backhauling, and beam management. The first proposed scheme allows efficient initial access for different services with different Quality of Service requirements. It involves beam management for different services. At initial access, the beams are aligned between transmitters and receivers. Based on the beam alignment, the second proposed scheme can measure the interference information and perform scheduling accordingly, for both backhaul and access (self-backhauling).


the scheme QoS-centric resource allocation SDMA-based has novelty in decoupling the hardware resources according to QoS requirements. The resource allocation is 3GPP compliant. Regarding CG-MIS based joint scheduling, the integration of access and backhaul will be started as a new study item in 3GPP [3GPP RP-170148]. The proposed solution considers DL/UL as well as both backhaul and access traffic and fully dynamic TDD. No state-of-the-art consider all such aspects.

5.5 Active mobility

As beamforming is needed in mm-wave systems, the reference signals for channel estimation at the UE need to be provided per beam. In case of active mobility, fast beam switching is required in either intra-cell or inter-cell entities. Active mode mobility must enable detection of adjacent beams and switching of the data flow between these beams to allow the seamless mobility of UEs through the cells. In contrast, the requirement for the idle mode mobility (see Section 5.6) is to provide means for accessing the network, which is much more latency tolerant than the active mode mobility.




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Figure 5-15. Average SINR variations over UE route at 15 GHz.


mm-wave clustering

For 5G mm-wave systems, the beam-based antenna patterns will result in dynamic variations in coverage, signal and channel quality, as minor UE movements and rotations will change the directionality of the beams. At the same time, signal blockage due to obstacles can greatly reduce the beam coverage. This results in frequent handovers between the different beams to provide sufficient coverage and connectivity. One way to resolve this issue is the deployment of access point clusters within which the UE mobility is transparent to the CN (RRC). Thus, within the cell cluster, the handover of UEs between different nodes or beams can be performed without any RRC signalling. The cluster is defined as a group of access points (APs) in the vicinity of the UE capable of serving the UE, such that:

the cluster is specific to each UE which is configured and may be reconfigured by the network and as the UE moves,

Toto coordinate the mobility within the cluster, one of the APs is designated as the cluster head which is connected to the CN through the NG interface and is also connected to all other APs in the cluster.

Depending on the topology of the network and the quality of the backhaul, the location and role of the cluster head may vary.

Figure 5-16: Illustrates a proposal for the function split in the mm-wave cluster.

Configurable CSI-RS

In the mm-wave cluster concept the synchronization signals (SS block) coming from all the access points is the same, however it is still needed a separate signal to identify the TRP (the access point) that the UE is listening to., and a configurable CSI-RS is proposed.

The active mode mobility related to requirements in NR pushes for the solutions wherein the reference signals are transmitted using beamformers that are audible to the UE in the entire




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coverage area of the cell with sufficient periodicities to maintain the radio link with the serving cell and also to perform RRM measurements related to mobility.’. However, the transmission of reference signals in narrow beams with frequent periodicity leads to large transmission overhead on the network side and also results in increased interference to the UE., the overhead and added interference would be prohibitive. Thus, the varying mobility requirements from stationary UEs up to 500 km/h need to be considered for the periodicity and the shape of the related reference signals (CSI-RS, e.g. from every 100 ms up to every 5 – 10 ms). For beam-switched mobility, the beam management can be resolved using Channel State Information - Reference Signals (CSI-RS), where the UE measures and reports specific signals configured by the network. The CSI-RS should be configurable to be transmitted with very short periodicities when needed and must be turned off when not required. It is noted that the reference signals in case of inactive mode is more narrowband, whilst the active mode reference signals span for a wider bandwidth and demands frequency diversity.

5.5.2 Benefits

The proposed solution for active mode mobility addresses the channel related issues of high path loss and is relevant for standalone and non-standalone deployments. It adapts reference signal overhead to the velocity and traffic requirements of the UEs to minimize signalling overhead, while enabling fast beam switching and maintaining seamless connection. Furthermore, the configurable reference signal design significantly contributes to energy efficiency.


To enable RRC driven mobility, the measurement procedures of the UE needs to be enhanced to report CSI-RS measurements from various beams within the cell and from adjacent cells. This could be supported by cell clustering procedures and by prediction schemes, to identify the most probable beams for switching and reducing the measurement options. Also, to include only a sub set of the beams within a cell i.e., CSI-RS resources of a cell can be grouped into multiple groups so that the network can configure the events specifically towards each of these groups.


Active mode mobility extends the mobility features known from legacy (LTE) systems towards faster and more flexible handover schemes. In 3GPP, related beam management features and handover procedures are in discussion. However, the usage of CSI signalling for handover procedures is a new concept used in NR.

5.6 Inactive mobility

The mobility state transition is a fundamental concept for mm-wave RAN integration in 5G systems. The existing mobility states, RRC_IDLE and RRC_CONNECTED, are optimized for minimizing UE power consumption and network usage and for full active data transmission respectively. To cope with the diverse 5G system requirements and to better support new use cases, a new state has been introduced.


RRC_Inactive

The proposal of a new state to complement the existing states, i.e. RRC IDLE and RRC_CONNECTED was for the first time introduced in this project’s previous deliverables [MMMAGIC-D3.1] [WP_MMMAG16]. The new state introduced is referred to as RRC_INACTIVE, which allows a UE to benefit from several aspects of its original two states. Similar to RRC_IDLE, the UE in this state can perform cell-reselection based on the reference signals without providing the network with measurement reports. Additionally, when the network needs to reach the UE, e.g. when DL traffic arrives, the network pages the UE following a




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random access (RA) connection process. Likewise, when the UE needs to initiate UL traffic, it performs RA to the current cell to synchronize and connect to the network. The difference between the RRC_INACTIVE to that of RRC_IDLE, is that the UE and gNB maintains configurations obtained in RRC_CONNECTED, e.g. AS context, security, and radio bearers; facilitating the UE to resume its old configurations without much delay. In addition, the gNB can maintain the CN/RAN interface (NG-C and NG-U), thereby reducing the resumption latency.

RRC_CONNECTED

RRC_INACTIVE

RRC Connection Release

C

States in NAS Layer

Bearer Context Active

EMM Registered EMM Deregistered

ECM Connected ECM Idle

ESM State

ECM State

EMM State

Bearer Context Inactive

RRC_IDLE

RRC Connection Suspend

RRC Connection Resume

RRC Connection Setup

RRC Connection

Reject

Figure 5-17. EMM State transition diagram for 5G systems.

Since the connections are maintained in RRC_INACTIVE, the CN will assume that the UE is in ECM_CONNECTED. Whenever the network is required to reach the UE, e.g. when there is DL data available, the network will page the UE, as the RRC connection is suspended. However, as the core network assumes that the UE is in connected mode, the paging can be initiated by the RAN network. To facilitate a more efficient paging scheme, the RAN can assign a limited area, covering one or more cells, within which the UE can be tracked and paged. When the UE moves within the RAN tracking area, it does not need to notify the network of its location, but if the UE moves outside the RAN area, then it must signal the network about its new location.

Fast switching to RRC_connected

As the gNB maintains the RAN-CN interfaces, there is a significant reduction in the connection latency when switching back to RRC_Connected. When resuming to fully connected state, the new cell must extract the UE context information stored, from the localized RAN/old cell. If the context fetch fails, then the network will instruct the UE to follow a RRC Connection Setup procedure similar to that performed for RRC_IDLE.

5.6.2 Benefits

With the introduction of the new state, the control plane latency is greatly reduced as the RAN and CN connections are already established. Further ahead, the UE power consumption is reduced with the deployment of suitable DRX mechanism. Another advantage of the RRC_Inactive is the key harmonization of solutions for the UE states in wide range of frequency bands which can be tailored to number of diverse use cases.




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As this is case of incorporating a new state into the existing two state system, this in-turn will include the design of MAC and higher up layer designs to reflect the new mobility state transition mechanism.


The new state has been strongly promoted and after several rounds of discussions, this has been finally agreed for standardization and included in [3GPP TR 38.804].

5.7 Self-backhauling and Integrated Access-Backhaul (IAB)

IAB operation is very important for mm-wave network deployment, since the backhaul demand and cost can be greatly relaxed, and the deployment flexibility can be greatly enhanced. This section proposes a scheme to dynamically multiplex the access and BH traffics in both UL and DL directions, taking into account fully dynamic TDD. It is assumed that BH and access links share the same air interface, and all network elements (including BS, APs and UEs) are equipped with directional steerable antennas and can direct their beams in specific directions. Figure 5-18 shows an example of considered IAB network.

Figure 5-18: Illustration of a heterogeneous network with mm-wave wireless BH and access.


In the context of maximizing network throughput of the IAB network, it becomes quite challenging to schedule transmission links and to allocate radio resource to both BH and access links, for downlink and uplink transmissions, when same radio resource and air interface are shared between mm-Wave BH and access links as well as TDD mode is assumed. Scheduling is considered, as many concurrent transmission links simultaneously as possible to fully exploit spatial multiplexing, and time/power resource allocation on the simultaneous scheduled links relies on the result of concurrent transmission scheduling. To solve the optimization problem efficiently with low complexity, a heuristic scheduling scheme based on the Conflict Graph (CG) theory and the Maximum Independent Set (MIS) algorithm is proposed, together with transmission duration and power allocation algorithms, which is described in the following.

The main idea of this algorithm is to determine which link(s) to be transmitted in each SDMA group according to UE transmission request and interference information acquired by for example initial access and interference sensing procedure. To simplify analysis of considered IAB network, the network is abstracted to a directed graph, referred to as “link graph” in Figure 5-19, left side, where nodes represent network elements (BS, APs and UEs), and edges represent transmission links among the elements. With the interference information, the link graph can be transferred to a new graph referred to as “CG”. In this graph, the nodes now represent the transmission links (edges in link graph), and the edges depict the “conflicts”




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among links. Specifically, links that are “connected” by an edge either cannot be scheduled simultaneously due to half duplex constraint, or will result in interference above threshold if simultaneously transmitted. An example of CG construction and the considered frame structure are illustrated in Figure 5-19, respectively on the left side and on the right side.

Having the CG, an MIS based scheduling algorithm is proposed to distribute links into different SDMA groups, where the maximum number of nodes in the CG will be found so that no edge exists between any chosen nodes. In other words, the MIS based scheduling algorithm finds maximum number of links that can be transmitted simultaneously without violating half duplex constraints/causing strong interference. The algorithm iteratively schedules concurrent transmission links for each SDMA group by obtaining MIS of the CG until all links are scheduled. The details of the proposed solution can be found in D4.2 [MMMAGIC-D4.2].

Figure 5-19: Conflict graph construction (left) and considered frame structure (right).

After concurrent transmission scheduling, the transmission duration and power allocation follow the “proportionally fair” and “water filling” principles, respectively.

5.7.2 Benefits

By exploiting the beamforming and the spatial domain multiplexing of links, the proposed schemes achieves much higher user throughput and provides significant improvement compared to the benchmark schemes. Finally, the proposed solution exhibits low-complexity baseband processing.


It is related to frame structure and multiple access schemes. It helps to determine the DL/UL ratio in dynamic TDD (for each AP) and also help to determine how the traffics are multiplexed in different domains (e.g. spatial, time).


IAB will be started as a new SI in 3GPP. The proposed solution considers DL/UL as well as both backhaul and access traffic and fully dynamic TDD. No state-of-the-art consider all such aspects.

5.8 Re-transmission schemes

Retransmissions are generally based on a combination of two nested schemes: so-called Hybrid Automatic Repeat Request (HARQ) and Automatic Repeat Request (ARQ). The former operates at a lower level than the latter, providing a two-step correction mechanism whereby packets not correctly received are first retransmitted through HARQ (at MAC level) and, in case this does not yield a correct packet, further retransmitted through ARQ at RLC level.

Both HARQ and ARQ are intended to evolve in 5G so as to provide more flexibility in retransmissions. To this end, three complementary solutions are proposed in the project.




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Single-hop and multi-hop retransmission protocols

The project proposes the protocol for NR based on LTE protocols, with three components: “Super-Fast HARQ” feedback, “Scheduled HARQ” feedback, and RLC ARQ layer (as in LTE). The utilization of these components depends on the scenario. This is schematically depicted in Figure 5-20 for single-hop and multi-hop scenarios. “Super-Fast HARQ” feedback provides feedback for one or a few downlink transmissions. This feedback can comprise of a single bit (as in LTE) or multiple bits. A single bit ACK/NACK can be sent after decoding (or decoding failure) based on a received downlink assignment. A feedback can also be sent before complete decoding and may include a soft quality measure, for example, an indicator for the likelihood of decoding which would comprise of more than one bit. “Scheduled HARQ” feedback is a multi-bit HARQ feedback scheduled on the uplink data channel. It is suitable for dynamic TDD, where it is required that the protocols can handle dynamic and possibly timing varying relations. This feedback can be extensive, comprising of many bits. Robustness can be achieved by means of Cyclic Redundancy Check (CRC) and by including built-in error mitigation techniques. Compared to the HARQ acknowledgements, the RLC status reports are transmitted relatively infrequently, but with higher reliability, as in LTE. Further details can be found in [MMMAGIC-D4.1].

Figure 5-20: Single-hop (left) and multi-hop (right) retransmission protocols.

Early detection of packet errors without full FEC decoding

It is focused on estimating the presence of errors after a reduced number of FEC decoding iterations, hence enabling early triggering of HARQ retransmissions. Results demonstrate the feasibility of two iterations in rate-1/3 turbo-encoded packets, as described in [MMMAGIC-D4.1], hence easing centralized deployment where FEC decoding takes place at a central entity (Figure 5-21).




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Figure 5-21: schematic illustration of a centralized deployment with early triggering of HARQ retransmissions at the distributed units.

Fast HARQ over Finite Block-length Codes

It is based on deriving sufficient conditions on feedback and decoding delays such that HARQ increases end-to-end throughput, compared to open-loop communication. In order to save on end-to-end delay, ACK/NACK feedback signals are omitted and the receiver attempts to decode the received messages only if it estimates high successful decoding probability (Figure 5-22).

Figure 5-22: illustration of Fast HARQ operation over Finite Block Length Codes.

5.8.2 Benefits

Single-hop HARQ protocol is fast, reliable and presents low overhead without any fixed timing. Multi-hop/self-backhaul scenarios can achieve very good delay performance. Both schemes are robust to hardware impairments and channel variations as long as there is some correlation between successive channel realizations.

Early detection of errors enables centralized coordination without tight constraints for HARQ RTT. User data rates and deployment related challenges in standalone mode can benefit from them. The solution is robust to large channel bandwidth issues, being the error detection on a per-codeblock basis irrespective of whether the system BW is large or not, and to hardware impairments as long as the resulting SNR falls within the expected range for HARQ operation.

Fast HARQ over finite block length codes allows substantial end-to-end throughput increments at low/moderate signal to noise ratios. The scheme is robust to channel variations, as long as there is some correlation between successive channel realizations, and to hardware impairments, being the impact of PA inefficiency on system performance already studied.




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Retransmission mechanism spans PHY, MAC, and RLC layers. Retransmissions are handled by HARQ in MAC layer and ARQ in RLC layer. Generation of different redundancy versions, soft combining at the receiver, and partial/full FEC decoding are part of physical layer. Moreover, retransmissions also have relationship with frame structure, e.g., the earliest possible timing for ACK/NACK and retransmission that a given frame structure allows.


The above proposals are an extension of the retransmission protocols already used in LTE, hence fully compatible with LTE layered protocol stack.

3GPP has already agreed to adopt adaptive and asynchronous HARQ, fully aligned with the first proposal. The others might be applicable for future alternative HARQ schemes with more relaxed HARQ RTT or with further delay constraints.

5.9 Summary

The chapter presented the system components of the new radio interface, described the proposed solutions, and highlighted the benefits and the relation to standardization activities. The system components presented in this chapter involve the different layers of the protocol stack and, as highlighted in Figure 2-1 of Section 2, these components represent the connecting link between the components of radio access architecture and components of the radio interface described in Chapter 4 and Chapter 6 respectively.




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6 Radio interface components (PHY layer) The PHY layer components related to the radio interface include: waveform, numerology, frame structure, channel coding, reference signals, and several multi-antenna schemes. A subset of the components, including waveform, numerology, frame structure, and channel coding have been tested and verified under real hardware and channel conditions in two Hardware-In-the-Loop experimental platforms [MMMAGIC-D4.2].

6.1 Waveform

mmMAGIC has assessed ten multi-carrier and single carrier waveforms (OFDM, W-OFDM, P-OFDM, DFTS-OFDM, UW-OFDM, UF-OFDM, FBMC-QAM, FBMC-OQAM, CPM-SC-FMDA, ceCPM-SC) [mmMAGIC-D4.1]. The evaluations are performed under common simulation assumptions and are based on several performance indicators that are important for mm-wave mobile communications. The project concluded that OFDM is most suitable waveform for mm-wave mobile radio communications.


The solution is based on a scalable OFDM waveform for all transmissions (data, control, initial access, synchronization) in UL and DL, with the possibility of additional processing on top of OFDM that is receiver agnostic. The scalability refers to the scalable choice of subcarrier

spacing according to 15 2 kHz and cyclic-prefix according to .μ , where is an integer.

The additional receiver agnostic processing aims at PAPR reduction (clipping, companding, constrained amplitude clipping schemes) and spectral confinement (windowing, pulse shaping, filtering). For spectral confinement, low complexity windowing/pulse shaping/filtering is preferred. The waveform solution includes the possibility of using a known sequence as prefix (unique word) instead of a cyclic-prefix (CP) for channel estimation. It can also be used for phase noise compensation. Although PAPR reduced OFDM is preferred, DFTS-OFDM is not excluded from the solution. The details of the proposed solution can be found in D4.2 [MMMAGIC-D4.2].

6.1.2 Benefits

The waveform solution addresses all KPIs of the use cases. In particular, the waveform solution offers the following benefits:

High spectral efficiency, to support extreme requirements on data rate, connection, and traffic densities;

MIMO compatibility, to enable straightforward usage of MIMO techniques, which is the driving technology for mm-wave communication;

Low Peak-to-Average-Power-Ratio (PAPR), to compensate for power amplifier’s (PA’s) inefficiency at high frequencies. (This is achieved by low complexity PAPR reduction schemes for OFDM.);

Robustness to channel time-selectivity and frequency selectivity; Robustness to hardware impairments – phase noise and power amplifier nonlinearities; Low transceiver baseband complexity; Time localization, to enable efficient TDD duplexing and support low latency

applications; Flexibility and scalability, to enable diverse use cases and deployment scenarios; Frequency localization, to support potential co-existence of different services by

multiplexing different waveform numerologies in the frequency domain.


Hardware impairment (PA and PN) models and channel models have been used as inputs for the development of the waveform solution. The waveform solution impacts design choices related to numerology, frame structure, reference signals, MIMO schemes, initial access, and multiple access. All these blocks are developed for scalable OFDM waveform solution.




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mmMAGIC performed a thorough evaluation of several state-of-the art waveform solutions for important waveform-specific KPIs, under common evaluation assumptions on channel, hardware impairments, carrier frequencies, and numerologies [MMMAGIC-D4.1]. Most waveforms are also implemented in two mmMAGIC common simulators. These comparisons have strong relationship to NR standardization work in 3GPP, which agreed scalable CP-OFDM for up to 40/50 GHz, which is aligned with mmMAGIC evaluation campaigns. Novelty within the solution includes: i) scalable OFDM considering various requirements (robustness against phase noise, mobility, service latency, deployment, carrier frequency), ii) low complexity windowing/pulse shaping designs, iii) advance prefix design, and iii) PN mitigation schemes.

6.2 Numerology

Numerology in communication systems defines the lengths of subframe, symbol and CP, and in multicarrier systems also the subcarrier spacing. These parameters should be jointly designed so that the overhead is kept at an acceptable level, while fulfilling the requirements. For mm-wave, phase noise and the channel propagation introduce the main challenges. A detailed description of numerology can be found in Chapter 4 of [MMMAGIC-D4.2].


The wide range of carrier frequencies requires scalable design of the numerology. Since the waveform is OFDM-based, phase noise should be considered (its detrimental effects tend to increase as carrier frequency increases). Generally, the free space path loss and penetration loss increase as a function of carrier frequency when beamforming is used.

Figure 6-1 shows the achievable maximum SINR as a function of subcarrier spacing (SCS) for different carrier frequencies fLO in the presence of phase noise. As SCS increases, the SINR increases (since symbol length decreases), and the SINR depends also on the carrier frequency. Accordingly, SCS should increase as carrier frequency increases. However, it should be taken into account that large SCS also increases CP overhead.

Figure 6-1: Achievable maximum SIR as a function of SCS in the presence of phase noise.

Figure 6-2 shows the principle of the OFDM numerology design for 5G. Mixed numerology is possible, where several different subcarrier spacing values coexist on a carrier, depending on the cell size, latency requirement, and carrier frequency.




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Figure 6-2: OFDM numerology design for wide range of carrier frequencies, deployment types,

and application latency requirements.

The other parameters to consider are the CP duration (which has to be larger than the RMS delay spread), the clock frequency (minimum requirement: integer multiple of LTE’s clock, with multiple of power of 2 preferred), as well as the FFT size (roughly constant for all frequencies, but up to 8k FFT may be required for very large bandwidths).

The proposed numerology is shown in Table 6-1. The chosen parameters are selected to take the following into account: achievable latency, overhead, implementation complexity and maximum SE. The set of parameters is a compromise, since for example very small latency and high SE are contradictory.

The details of the proposed solution can be found in D4.2 [MMMAGIC-D4.2].

Table 6-1: Proposed numerology.

(a) Carrier frequency fc

Up to ~10 GHz

Up to ~20 GHz

Up to ~40 GHz

Above ~40 GHz

Subcarrier spacing (SCS)

15 - 30 kHz

30 - 60 kHz 60 - 240 kHz

2 60 kHz

(b) Subcarrier spacing

15 kHz 30 kHz 60 kHz kHz

Symbol duration 66.77 µs 33.33 µs 16.67 µs 16.67/2 µs

CP duration 4.69 µs 2.35 µs 1.17 µs 1.17/2 µs

Clock frequency 30.72 MHz

61.44 MHz

122.88 MHz

2 122.88 MHz

FFT size 2048 2048 2048 2048




55

CP samples 144 144 144 144

CP samples (Extended)

512 512 512 512

6.2.2 Benefits

The proposed numerology is scalable for all carrier frequencies, with reasonable overhead, i.e., benefits the data rate, and enables low latency and large throughput.


Numerology forms a basis (together with waveform design) for the low level PHY design, since it defines the OFDM symbol length, onto which the rest of the frame structure has to be built on.


Many of the proposed numerology concepts have been accepted in 3GPP, for example the scaling by power of two of the LTE numerology, and the concept of keeping the FFT size roughly constant. For frequencies above 40 GHz, the numerology has not been decided in 3GPP TR 38.802.

6.3 Frame structure

Frame structure describes the time and frequency domain structures of the signals, including subframe structures for DL/UL, control and data, backhaul and access, time/frequency resource granularity, reference signal structure and others.


Frame structure is a hierarchical structure consisting of frame, subframe/slot, mini-slot, and symbol. In LTE, a radio frame has the length of 10 ms, and is further divided into smaller sections for NR-PBCH periodicity (5 ms for example). For mm-wave, the frame length is scaled down due to larger bandwidth. The frame is further divided into subframes/slots, which is the smallest resource allocation unit. A slot typically consists of optional DL control symbols(s), DMRS symbol(s) for channel estimation in the beginning of the slot, followed by data and optional UL control. A mini-slot is a shorter version of a slot, at minimum only one symbol long, and it is used for very small latency services and unlicensed operation.

There are five different subframe/slot structures defined in mmMAGIC, as shown in Figure 6-3.

Figure 6-3: Subframe structures.

The length and number of symbols per subframe depend on the subcarrier spacing (and therefore on the carrier frequency).

DL control and data – Downlink only subframe

UL control and data – Uplink only subframe

DL CTRL GP UL CTRL

DL CTRL UL CTRLGP

Subframe TSF

(a)

(b)

(c)

(d)

GP (e)

DL data

UL data

DL CTRL DL data UL CTRLUL data




56

The first two subframe structures are the DL and UL only subframes, and the three remaining are special subframes with opportunity for UL and DL control, and UL or DL data, depending on the traffic situation. The last one has bidirectional data, and it is used for self-backhauling.

As mentioned above, the DMRS symbol is in the beginning of the data part (“front loaded” structure). An example is shown in Figure 6-4 to illustrate front loaded DMRS for a rank 4 MIMO transmission. DMRS and PTRS designs are further discussed in Section 6.8.

Figure 6-4: DMRS/PTRS structure.

An important concept is the mini-slot, which is illustrated in Figure 6-5. The length of the mini-slot is one or more symbols, but shorter than a slot. It is used for very low latency applications (where the data has to be sent right away as it arrives from the upper layers), and for unlicensed operation (e.g., by applying a listen-before-talk scheme).

The details of the proposed solution can be found in [MMMAGIC-D4.2].

Figure 6-5: Illustration of a mini-slot.

DL CTRL GP UL CTRLDL data

1 2 3 4 5 6 7 8 9 10 11 12

1 layer 1

2 layer 2

3 layer 3

4 layer 4

5 data

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29 Subcarrier

OFDM symbol




57

6.3.2 Benefits

The proposed solutions generally benefit data rate and latency KPIs. In particular, they are scalable for all carrier frequencies and bandwidths (for the highest bandwidths, carrier aggregation is used.) The 5 subframe structures enable fully flexible TDD with adaptation to any traffic conditions with low overhead. Also, FDD is supported with UL/DL only subframes. Since DL control is in the beginning of the slot, the UE can go to micro sleep if it doesn’t have data to receive. This contributes to energy efficiency. The short subframe enables small latency. Since DMRS exist in the beginning, channel estimation can be done in the beginning of the slot. This means that sample level buffering of the entire subframe is not needed. The PTRS enables phase noise and Doppler tracking.


Frame structure is closely tied to numerology and waveforms, as a result these system components were jointly designed to combat challenges and requirements for the higher carrier frequencies, especially phase noise and Doppler effects.


The subframe structure was originally designed in METIS project and refined in mmMAGIC. The placement of control symbols was adjusted for smaller latency, and the fifth subframe optimized for backhauling was introduced; furthermore, the mini-slot was introduced. The baseline subframe structure and front loaded DMRS have been agreed in 3GPP. For PTRS, discussion in 3GPP is on-going.

6.4 Channel coding

mmMAGIC has investigated three channel codes, Turbo, LDPC, and Polar codes, where several important KPIs are defined, including decoding throughput, throughput per chip area, error correction capability, processing complexity, memory consumption, suitability to re-transmission schemes, latency, and code flexibility [MMMAGIC-D4.1]. Later, 3GPP agreed on combination of LDPC and Polar codes, which is aligned with the mmMAGIC final solution.


The solution comprises of LDPC and Polar codes. LDPC is used for data transmission and Polar codes for transmission of control information. Quasi-cyclic LDPC codes have been adopted. Code extension of a parity-check matrix is used for IR HARQ/rate-matching support. With Polar codes, CA-Polar is the preferred option. As decoding is one of the most computationally-intensive tasks within the baseband detection process, the solution comprises of novel decoding approaches. For LDPC codes, the solution includes a new family of Non-Surjective Finite Alphabet Iterative Decoders (NS-FAIDs). NS-FAIDs are characterized by specific Min-Sum (MS) based processing rules, which are theoretically analysed and optimized for different trade-offs between memory requirements and error correction performance. For Polar codes, the SCFlip decoder has been developed, which aims at improving the decoding performance, while keeping the complexity close to the one of the SC decoder.

The details of the proposed solution can be found in D4.2 [MMMAGIC-D4.2].

6.4.2 Benefits

This solution benefits data rate, mobility, reliability, availability, and latency. In particular, it offers the following benefits:

High throughput High error correction capability High throughput per chip area Low latency Code flexibility




58

Suitability to retransmissions


Depending on type of quality of service requirements (e.g., throughput, reliability, latency) and channel conditions, LDPC codes with different coding rates and information block sizes can enabled. LDPC codes have been evaluated with OFDM waveform for both link level and system level concept validations in D4.2 [MMMAGIC-D4.2].


The comparisons of channel codes undertaken within mmMAGIC have strong relationship with NR standardization in 3GPP, which considered Turbo, LDPC, and Polar codes as main candidates. Finally, 3GPP adopted LDPC and Polar codes for eMBB services. mmMAGIC further studied the design of QC-LDPC codes, puncturing strategies for Polar codes and comparison of Polar code candidates which are also related to NR standardization. The novel decoding algorithms provide promising results and interesting direction for future work.

6.5 Transceiver architecture and multi-antenna schemes

Multi-antenna signal processing is one of the key enablers of 5G mm-wave systems. To address the challenges of higher path loss, penetration losses and shadowing at the mm-wave bands, the large antenna array as well as the directional transmission, coupled with hardware constraints, result in significant challenges on the mm-wave transceiver architecture and design.

Detailed system level simulation based studies have been conducted on the performance and resilience of beamforming options to radio channel and hardware impairments. Hybrid beamforming has been shown to approach the performance of digital beamforming in LOS channel conditions when the users can have sufficient angular separation (otherwise, full digital BF will perform better, as it has more degrees of freedom for BF), while offering much less complexity, and has also been demonstrated to be much more resilient to channel impairments such as the main path blockage and hardware limitations [MMMAGIC-D5.2]. The hybrid beamforming option has, therefore, been recommended by mmMAGIC as the transceiver architecture based on which transceiver schemes for most of the Access, Backhaul and Relay applications have been studied in the project (see Figure 6-6). For fixed backhaul, however, where there are no user tracking requirements, the use of analogue beamforming may be more suitable. A figure illustrating the hybrid beamforming transceiver architecture is given in [AAL+14].

Further mmMAGIC studies into the hybrid beamforming architecture have resulted in the development of a simplified sub-array architecture, where the RF chains are connected to only a sub-array of antenna elements. When the losses in the combiner stages are considered, this sub-array architecture has been shown to perform as well as, or even better than the fully connected hybrid beamforming architecture. Figure 6-7 shows a schematic comparison of the two architectures (only the analogue beamforming part is shown).

Figure 6-6. Hybrid Beamforming with analogue beamformer.




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Figure 6-7: Fully connected and partially connected architecture (subarrays).

In mmMAGIC, several multi-antenna transceiver designs and schemes have been developed for access, backhaul and relaying for mm-wave communication, in particular based on hybrid beamforming architectures, including:

Access: A flexible hybrid beamforming design for the access in multiuser systems Backhauling: A massive multiple-input, massive multiple-output communication

system for short LOS links based on a simplified channel decoding scheme; analysis of beam alignment with moving hotspots;

Relaying: A dual-hop relay selection scheme to combat blockage issues.


Access: flexible hybrid beamforming design

A flexible multi-user hybrid beamforming design has been developed. The design considers the scenario where a base station (BS) serves multiple users (UEs) in the downlink. Channel knowledge focusing on the angles-of-departure (AoDs) and angles-of-arrival (AoAs) of the used channel paths is assumed to be available. Both the transmitter and receiver employ multiple antennas (and multiple RF chains) at the BS and the UE end. A hybrid transmission scheme has been proposed, similar to the one of Figure 6-6, consisting in three sequential phases based on given scenarios, available system resources and desired performance targets. In the following, a description of the phases is provided:

Phase 1: Parallel analogue beamforming at the BS and UE(s): In this phase, the BS sets the digital precoder to be and only applies analogue beamforming on the transmitted signal. At the UE side, the receiver carries out analogue beamforming , and for equalization considers only the effective channel of each transmission path, i.e.,

, is set to be a diagonal matrix. The analogue beamformers at the UE and BS are designed to use antenna array response matrices of those channel paths with the largest gains as described in [ZRG16], [MMMAGIC-D5.1]. For phase 1, the BS needs to know the angle(s) of departure of all used transmission paths, whereas each UE needs to know the angles of arrival and the complex gain(s) of the used transmission path(s) from

. . .

. . .

N R

F c

hain

s

divider 1:N sub

N s

ub a

nten

nas

Partially Connected Architecture (Subarrays)

Analogue Beamformer

N s

ub a

nten

nas

N RF subarrays

. . .

. . .

. . .

Fully Connected Architecture

adder N RF:1

N R

F c

hain

s

divider 1:N

. . .

. . .

Analogue Beamformer

N a

nten

nas

. . .

. .

.

. . .

. .

.




60

the BS to this UE. With the use of the uncoupled analogue beamforming in phase 1, the system does not cope with inter-stream interference in the single-user case and inter-stream/inter-user interference in the multi-user case. This should suffice if different paths/users are separated well in the angular domain at the BS and UE(s) sides, and/or targeted link performance is not very demanding, and/or low computational complexity and overhead is of dominant interest.

Phase 2: Digital beamforming at the UE: after conducting analogue beamforming as in phase 1, each UE estimates the effective channel from the BS to this UE including propagation channel and the used analogue beamformers. With reasonable good effective DL channel estimation, the multi-stream interferences can be removed by constructing digital combiner , using any MIMO equalization algorithm available in the literatures at the UE. Compared to digital combiner used in phase 1, , is most likely a non-diagonal matrix. After phase 1 and phase 2 processing and without precoding at the BS side, the link performance for single-user case is already close to the achievable performance of reference hybrid beamforming scheme in [ARA+14]. In multi-user scenario, if multiple users that have good angular separations at the BS are scheduled to be served at the same time, it is still possible to achieve reasonably good system performance.

Phase 3: Precoding/digital beamforming at the BS: In the last phase, if the system requires multi-user interference mitigation to achieve the targeted performance metrics, the BS can carry out multi-user interference nulling or reduction by applying precoding, e.g., using zero-forcing and minimum mean square error (MMSE) criteria. To this end, however, the BS needs to acquire knowledge of the DL effective channel (transmit CSI), which leads to extra system overhead. Meanwhile, the use of zero-forcing or MMSE precoders could potentially increase the PAPR of the transmitted signal dramatically. Thus, more intelligent and power efficient precoding schemes need to be developed. For the numerical analysis, we focus only on the first two phases which are the most relevant for mm-wave transmission.

Backhaul: A Massive Multiple Input Massive Multiple Output (MMIMMO) for short LOS

The scheme that has been developed in the project, focused on enabling a mm-wave backhaul, by addressing the large required throughput.

The developed backhaul solution includes the proposed novel decoding scheme for MMIMMO, useful in short range, LOS communications. This theoretical decoding scheme has been experimented for realistic ray traced channels (indoor and outdoor) and with directional antenna element responses. The related analysis has shown that up to 84% of the singular value decomposition (SVD) capacity in theoretical LOS channels can be achieved with the proposed new scheme under realistic channel conditions. Also, the use of directional antennas can significantly improve performance when a dominant scatter is mitigating the LOS conditions. Hundreds of bits/s/Hz of spectral efficiency can be attained between large antenna arrays deployed along structures (such as walls, lamp posts, utility poles, urban elements of architecture and furniture) that are close and roughly parallel to each other, which make it interesting for backhaul. A detailed description of this backhaul solution is given in Section 6.7.

Relaying: A dual-hop relay selection scheme

Regarding transceiver schemes for implementing mm-wave relaying, a dual-hop relay selection algorithm has been discussed to aid a user who has suffered a signal blockage. To this end, a relay selection metric has been proposed to address the trade-off between the spectral efficiency and the co-channel interference resulting from the transmission to the relay. The results indicate a reduced performance degradation with the proposed relay selection metric.




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An example of the relay selection process in a mm-wave multi-user system is given in Figure 6-8. When blockage is detected on any of the transmission links, e.g., an active user experiences a sudden decrease in spectral efficiency, the following procedures should be performed to select the optimal relay out of the idle users which maximizes the effective spectral efficiency on the relay path, while minimizing the interference to other unblocked transmissions:

Once the blockage is detected, the blocked user , e.g., user 2 in Figure 6-8 (b), broadcasts ‘help’ messages to users in its neighbourhood on the control channel, e.g., a channel operating in lower frequency;

The users that are idle, e.g., user 3 to 7 in Figure 6-8 (c), upon receiving the ‘help’ message, measure the link strength between themselves and the blocked user, and then send feedback to the BS via the control channel to inform their availability and the quantized link strength ;

The BS collects the feedback from the feasible relay candidates, e.g., user 3, 4 and 7 in Figure 6-8 (d), which have LOS links to both the BS and the blocked user, and then evaluates the link strength from itself to these candidates, i.e., to choose the optimal relay that maximizes the selection metric, e.g., user 7 in Figure 6-8 (d).

In the first hop, the BS transmits data streams to the selected relay, i.e., user , and other spatially multiplexed users, i.e., user for all , in one time slot, e.g., user 7 and user 1 in Figure 6-8 (f);

In the second hop: when the relay finishes reception, it transmits the received data to the blocked user in another time slot, as shown in Figure 6-8 (f).

6.5.2 The Benefits

All solutions given above tackle one or multiple challenges at the mm-wave band. In addition, the simplicity in terms of implementation is also considered, which is one of the major challenges of the practicality of the mm-wave system.

The flexible hybrid beamforming solution shows much improved robustness to signal blockages (which can be a common issue in mm-wave communications), on top of the design flexibility it offers. The proposed MMIMMO backhauling scheme, can achieve extremely high spectral efficiencies (and thus extreme data rates) under short range, LOS conditions. This is complemented by the novel, relatively simple decoding scheme proposed by the project. The novel user selection scheme for ad-hoc relay implementation illustrates significant benefits in system throughput gain, over random user selection. By relying on the pre-coding codebooks to estimate the user locations and avoiding the need for precise user location knowledge, the scheme is simpler to implement, as well.


The proposed multi-antenna architectures and schemes are the key transceiver technologies to enable the system concept. Many other components e.g., initial access, are based on such multi-antenna schemes.


While all these schemes are novel and can have significant impacts on their own, they consider individual problems and specific design aspects and may be beyond the current considerations in 3GPP for Rel. 15 NR standard development. However, these works can be taken as seeds for developing specific components of access, backhaul, and relay related standardization in future study items.




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6.6 Beam management

To support random access and mobility at mm-wave frequencies, a set of procedures that can be configured depending on the deployment, the capability of the hardware, the backhaul quality, the traffic type for a given UE, have been considered, and solutions are presented in the following sections.


Beam sweeping for cell discovery

Highly-directional communications make cell discovery challenging at mm-wave frequencies. In fact, compared to sub-6 GHz systems, where omnidirectional antennas are used at both the transmitter and the receiver, a time-consuming beam sweeping procedure is now required for broadcast signalling. Furthermore, a rough beam alignment is desired to be accomplished in combination with cell discovery already, to simplify the more thorough beam training process

Figure 6-8: An example of the relay selection process: (a) initial transmission scheduling, (b) blockage detection, (c) broadcasting help message, (d) relay candidate identification

(e) relay selection metric and (f) transmission via the relay path.




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expected in the subsequent initial access phases. In the following, the design of mm-wave broadcast signalling is analysed, considering cell discovery latency, and signalling overhead. The typical sub-frame structure consists of a beacon transmission period and a data transmission period. In the former, the AP broadcasts the cell discovery-related information. As mentioned above, beamforming is needed for broadcast signalling in order to enhance the SNR at the UE. Beam sweeping across different directions needs to be performed by the AP, so that the whole spatial domain can be covered. Guard intervals (GIs) between beam sweeping slots allow for the correct re-configuration of analogue circuits. As sketched in Figure 6-9, four possible beam sweeping variants for broadcast signalling can be envisioned:

Time Division (TD): one beam per time slot. In the analysis that follows, TD is treated as the baseline design;

Frequency Division (FD): multiple beams per time slot, with the signals of different beams multiplexed in the frequency domain;

Code Division (CD): multiple beams per time slot, with the signals of different beams multiplexed in the code domain;

Spatial Division (SD): multiple beams per time slot with transmit power split (e.g., equally distributed power as in the example of Figure 6-9.

It is worth noting that the proposed beam sweeping strategies require different levels of hardware complexity. For example, the TD scheme can be implemented in analogue beamforming transceivers. On the contrary, FD, CD, and SD strategies require multi-beam transmission, which can be achieved by exploiting much more complex transceiver architectures based, for example, on fully-digital beamforming or hybrid analogue-digital beamforming. More details about the cell discovery procedure when beam sweeping is employed for broadcast signalling can be found in [MMMAGIC-D4.2].

Figure 6-9: Illustration of broadcasting beam pattern examples.

Beam refinement and tracking: P-Track mechanism

Once the user has granted access to the network and has started to exchange data with the AP, the use of highly directional antennas with very narrow beams calls for the need of fast and efficient strategies for mm-wave link maintenance. In fact, user mobility could easily lead to link degradation, or even link breakage, due to the misalignment of transmit-receive beams. The IEEE 802.11ad standard handles this problem through beam refinement procedures that search around the current sector pair to determine a new combination of beams with improved link quality. In large and crowded scenarios with mobility, such procedures may fail to cope with high channel dynamics, which would necessitate fast mechanisms to scan a large angular domain (instead of just adjacent directions) to find alternative communication links. In case simply probing adjacent beams is unsuccessful, the IEEE 802.11ad standard performs a new exhaustive beam search procedure. This leads to a high latency which deteriorates the overall system performance. Motivated by this challenging problem, it is proposed a fast and efficient beam tracking strategy for link maintenance between mm-wave devices exploiting the ability of hybrid analogue-digital beamforming transceivers to receive simultaneously from multiple directions.




64

Efficient beam tracking strategies are required in order to rapidly refine the beam directions without resorting to full beam training. The probabilistic beam tracking (P-Track) mechanism is proposed, which is able to track the mm-wave channel dynamics under node mobility (and steer the device beams accordingly) without requiring dedicated training slots. It is assumed a fixed AP and a moving UE that are communicating using pure data frames (i.e., without dedicated training slots) and highly directional beam patterns. For the sake of brevity, only the UE beam tracking procedure is considered, i.e., the procedure by which the UE exploits downlink data slots to refine its beam directions. An identical strategy is applied for AP beam tracking using uplink data slots. The P-Track strategy is based on a probabilistic model which does not require the devices to perform any spatial scanning during the ongoing data communication. It is able to track the most dominant directions of the mm-wave channel using just known portions of the data packet, e.g., the preamble. To do that, it is required that two conditions are satisfied: (1) the preamble is correctly detected by the UE within, at least one, downlink slot within the frame; (2) in such a downlink slot, the UE can access the complex output from the RF combiner. In order to reduce the interference among the parallel streams, the AP adopts Golay sequences encoded with orthogonal Walsh spreading codewords for the preamble. The reason behind the choice of using the complex output from the RF combiner for beam tracking, instead of the signal after the baseband precoder, is that the former includes much more information about the channel than the latter which is defined in a lower dimensional space. In this way, it is possible to provide channel information for a wider angular domain compared to the very narrow angular sector covered by the actual data communication beam pattern.

Since the AP is transmitting relevant data to the UE using pure data frames, the UE cannot perform any beam scan, but it must keep its antennas steered towards the directions estimated in the most recent beam training execution. It is proposed that a probabilistic estimation is carried out, based on the analysis of the preamble signal received by the UE over the current antenna pattern in a downlink data slot. To this end, the probabilistic optimization problem is formulated, and solved by applying a gradient descent algorithm, whose objective function is designed so as to model the temporal evolution

of channel paths, due to device movements. Note that this problem is quite different from the problem of MIMO channel estimation using known pilot symbols.

More details about the problem formulation and the derivation of the objective functions can be found in [MMMAGIC-D4.2]. In [MMMAGIC-D4.2], a simulation framework is also developed to assess the performance of the proposed beam training strategy and compare it against existing approaches in the literature. Our simulator integrates a ray-tracing tool to accurately model the time-varying mm-wave channel, taking into account blockage, ray clustering, and mobility effects, and guaranteeing spatial consistency over time.

Beam coordination schemes

In ultra-dense standalone deployments, the presence of significant beamforming gains can lead to bursty and unpredictable inter-cell interference. Particularly, when users have limited beamforming capabilities at the receive side, inter-cell interference can be severe in scenarios comprising a high density of access nodes per unit area. This is aggravated by the fact that practical beamforming can exploit opportunistic reflections to reach users in NLOS conditions. The presence of obstacles together with beamforming can lead to strong multipath components. Such reflections can lead to unpredictable interference towards other users, unless basic coordination of the beamforming vectors is envisioned (e.g., with coordinated beamforming), or some basic radio resource management (RRM) technique is applied.




65

Figure 6-10: General condition for the appearance of inter-cell interference with strong beamforming, in the presence of reflectors.

A convenient tool to support a broad range of inter-node coordination strategies can be a beam labelling technique. All transmitted beams can be conveniently labelled in the form of a beam indicator, which could be signalled to the users by means of a Beam Indicator Channel:

Beam Indicators can comprise any label, such as e.g., an integer number in a predefined grid of beams, the corresponding angles in a spherical coordinate system, or the intended UE to be served;

The Beam Indicator Channel would contain this information so that any user from the surrounding cells can easily acquire it;

Beam indicators should be cell-dependent, or linked with a suitable cell identifier (cell ID) so that users can acquire both the interfering beam and the originating cell ID;

If a beam transmitted towards a given user comprises more than one sub-beam, it would be desirable to label it separately so that any interfering UE can discriminate between the constituent sub-beams.

In the presence of interference, the UE will detect at least two beam indicators in the same time-frequency resources: one for the serving beam and another for the interfering beam. Hence the UE should be able to acquire both by means of orthogonal sequences, cover codes, or any other suitable multiplexing technique, with the objective of protecting the information and conveying also the corresponding cell ID.

Beam indicators could be reported back to the serving cells by means of an Automatic Interference Relation Report (AIRR). This message could convey pairs of (interfering cell ID, interfering beam indicator) as detected by users, and can be reported periodically or upon certain conditions, such as a request from the BS or the appearance or disappearance of an interfering beam. Periodicity in the detection and reporting mechanism will follow a trade-off solution between signalling overhead and responsiveness to bursty interference.

The gNB will collect multiple interference control messages from the users to populate a table of interference relationships, denoted as Automatic Interference Relation Table (AIRT), in such a way that the gNB can acquire an accurate picture of the interference relationships with beams from other surrounding cells. As a result, cells can agree on any suitable coordination strategy to minimize interference, spanning from advanced coordinated beamforming (when detailed channel state information can be exchanged among the cells at the right timing), to simpler RRM mechanisms based on coordinated partitioning of the resources.

UE A UE B

Serving beamInterfering beam

CELL A

CELL BReflected beam

SERVING 5G NB

INTERFERING 5G NB




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Beam management procedures

At high frequencies, good coverage is a challenging task due to unfavourable propagation characteristics. One solution to the coverage issue is to employ high-gain beamforming to achieve a satisfactory link budget. With such high-gain beamforming, the beams are typically quite narrow, which makes beam tracking challenging, i.e., finding, maintaining, and switching between suitable beams as UEs move both within and between the coverage areas of multi-beam transmit-receive points (TRPs).

At least two broad categories for beam handling have been identified for multi-beam systems:

Beam handling between a beam transmitted by a source (or serving) TRP and a beam transmitted by a target TRP, where the target is a TRP with which the UE has not established or maintained synchronization. This is usually referred to as Connected mode mobility;

Beam handling due to UE movement, where the beams are typically transmitted by the same (serving) TRP with which the UE continually maintains time and frequency synchronization. This is usually referred to as Beam management and will be treated in this chapter.

Beam management needs to be a fast procedure using layer 1 / layer 2 (L1/L2) signalling, in contrast to active mode mobility, which will use the slower layer 3 (L3) signalling. To support beam management, a toolbox with a set of procedures has been defined (P1-3) which are outlined in [Eri17-1] and illustrated in Figure 6-11. All three procedures (P1-3) are performed by letting the TRP transmit reference signals in various candidate TRP beams. The UE then performs measurements on the reference signals in various candidate UE beams and reports the measurements to the TRP when needed.

P-1: is used to enable UE measurement on different TRP Tx beams to support selection of TRP Tx beams and UE Rx beam(s);

P-2: is used to enable UE measurement on different TRP Tx beams to possibly change inter/intra-TRP Tx beam(s);

P-3: is used to enable UE measurement on the same TRP Tx beam to change UE Rx beam in the case UE uses beamforming.

As these beam management procedures consume overhead in form of reference signal transmissions, hence, it is preferable that these transmissions are confined within a subframe or a slot.

Figure 6-11: (Top) P1 to enable UE measurement on different TRP Tx beams to support selection

of TRP Tx beams/UE Rx beam(s) (Middle) P2 to enable UE measurements on different TRP Tx beams to possibly change/select inter/intra-TRP Tx beam(s), (Bottom) P3, to enable UE

measurement on the same TRP Tx beam to change UE Rx beam in the case UE uses beamforming.




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Heterogeneous beam codebook design

Although narrow beams with high gains are beneficial for improving the coverage at higher frequencies, the narrower the beams, the longer the time is needed for covering the entire cell as discussed for the beam sweeping in Sec. 6.6.1.1. In addition, since the number of RF chains of an AP is limited, only few beams can be transmitted simultaneously. In case of narrow beams, this implies that only a small portion of the cell is covered at the same time. Given the constraints of limited number of parallel beams supported by the AP, the use of a heterogeneous codebook with beams characterized by variable beamwidths, is proposed. The use of beams of variable beamwidth increases the scheduling flexibility by allowing more UEs to be scheduled simultaneously. An example of a heterogeneous codebook is depicted in Figure 6-13(a), where the codebook consists of eight beams of width 15° (blue), four beams of width 30° (red), two beams of width 60° (green), and the largest beam of width 120°.

The proposed solution is described with the deployment of the three UEs shown in Figure 6-12 (b) and assuming that the AP has only two RF chains. The initial search in the cell is conducted by the narrowest beam (enabling a large coverage) and the presence of the three UEs in the cell and their direction is detected. As first step, by fixing the beamwidth to the narrowest one available, according to the pre-defined metric (for example based on the throughput), the optimal beam directions are determined and the throughput that will be achieved is computed. The information about the position of the UEs is further used in the refinement steps to decide whether a better choice of beamwidths (and beam directions) can be taken in order to increase the throughput. With the deployment shown in Figure 6-12 (b), if restriction is to only use narrow beams, then UE1 and UE2 cannot be served at the same time and the scheduling algorithm should decide to serve only one of them. On the other hand, with the proposed solution, UE1 and UE2 can be served at the same time (separated in the frequency domain) by a beam with a larger beamwidth. Hence, the three UEs are served by an AP that can generate simultaneously only 2 beams, where the two users sharing a beam are separated in the frequency domain. Thus, after the AP has obtained channel state information from the UEs, then the best combination of beams for the UEs can be found by refinement steps.

Figure 6-12: (a) Heterogeneous beam space (codebook); (b) three UEs served by two heterogeneous beams of 15° and 60° beamwidths.




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6.6.2 Benefits


Extensive simulations are performed in [MMMAGIC-D4.2] to evaluate the performance of the proposed beam sweeping strategies. The results show that the TD approach outperforms all other strategies in terms of cell discovery latency, while it provides the worst performance in terms of signalling overhead. In fact, in contrast to FD, CD, and SD, the TD strategy uses sequential beam sweeping (i.e., without power splitting among the beams), which leads to higher SNR at the UE and, consequently, to faster demodulation. As a downside, the TD overhead is much higher than that exhibited by FD, CD, and SD. It is worth highlighting that, as far as signalling overhead is concerned, SD beam sweeping outperforms all strategies. Furthermore, due to inter-beam interference, the SD beam sweeping strategy offers flexibility in configuring the beam combinations, as well as the number of simultaneous beams, to achieve a convenient trade-off between latency and overhead. In summary, TD can be used if high overhead can achieve low latency. SD can be used if a trade-off between latency and overhead is required. With different SD configurations, different trade-offs can be achieved [MMMAGIC-D4.2]. In [MMMAGIC-D4.2] it is shown that using beam sweeping is particularly effective when the number of available frequency diversity branches is limited. Finally, the proposed strategies are robust against mm-wave channel challenges such as high path loss, large bandwidth, large scale parameters, and frequency selectivity, and against antenna design challenges, especially the analogue phase-shifter latency.


Simulation results in [MMMAGIC-D4.2] show that, compared to both IEEE 802.11ad-like strategies, the proposed solutions for beam refinement and tracking provide a 40% to 150% data rate increase, while, at the same time using lower complexity hardware. Finally, the proposed schemes are robust against mm-wave channel challenges such as high path loss, blockage, and rapid channel variations. They are also robust against hardware impairments such as PA imperfections and antenna design challenges, especially the analogue phase-shifter resolution.

Beam-management procedures

Beam management allows for use of large analogue arrays. The main benefit with beam management is that the link budget can be increased, which will improve the coverage and the data rates of the system.


Beam coordination enables application of effective RRM schemes among interfering beams, especially when the gNBs involved are close to each other. Coordination may be essential in ultra-dense networks, where reflection and diffraction creates bursty and unexpected interference between adjacent gNBs. This interference cannot be adequately controlled by standard link adaptation based on SINR measurements, because of their longer time scales compared to the rate of appearance of interfering beams, hence demanding more advanced beam coordination techniques.


The increased scheduling flexibility achieved with the proposed scheme was evaluated in the simulation results shown in [MMMAGIC-D5.1]. Compared to the use of beams with fixed beam width, the use of beams with variable beamwidth result on average in around twice more UEs being scheduled simultaneously for the different cell loads considered. In fact, with the proposed scheme, the gain in the average number of UEs, which can be scheduled simultaneously, increases with the cell load. Moreover, the scheduling flexibility can be translated into reduced latency in some cases. As shown also in simulation results in [MMMAGIC-D5.1], the variable beamwidth beamformer can serve all the UEs in the cell with about 30% less time slots compared to when employing beams with a fixed beamwidth.




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The proposed solution can be easily integrated in the overall mmMAGIC system concept. The interworking is achieved, for example, when multiple APs are simultaneously detected during the cell discovery. In this case, the UE could select the AP that provides better received signal quality as its serving AP, but also keep track of other potential serving cells, including traditional 4G cells, to exploit multi-connectivity [MMMAGIC-D3.2].


The proposed solutions can be easily integrated in the overall mmMAGIC system concept. In fact, beam refinement and tracking can be considered not only a system component solution for initial access, but it can be also used to handle user mobility during the ongoing data transmission. For example, this solution can provide support for system component solutions such as active mobility and multi-antenna CSI acquisition. For active mobility, efficient beam tracking allows to maintain the link during user movements. For multi-antenna CSI acquisition, the proposed beam tracking mechanism helps to reduce training overhead.

Beam-management

Beam management is one of the enabling technologies for mm-wave systems, especially considering the scenarios where large numbers of users are present, therefore interference and signal coordination is needed to be considered carefully between the transmitter and the receiver. Initial access procedures can be useful to boot strap the beam management procedures P1-P3 in case the signals during initial access are transmitted in beams that are narrower than the sector. One example of this is when the TRP transmits SS (synchronization signal) blocks in narrow beams using a beam sweep. The UE can then find the best SS block beam (based on measured received signal strength) and transmit a PRACH preamble in a PRACH resource associated with the time stamp of that SS beam. In this way, the TRP implicitly gets information of which SS block beam that was the best for a given UE.

Beam coordination schemes Interworking with the system concept

The elements of the system concept with greater connections to the proposed concept are protocol stack, multi-node, and active mobility.


The beams with variable beam widths can be chosen from a pre-designed codebook: each element of the codebook is characterized by the beam width degree and the beam direction and hence, the proposed solution can be integrated in the mmMAGIC system concept. Furthermore, the proposed scheme can also be used to handle user mobility when employing a wide beam.



The proposed solution is aligned with 3GPP specifications supporting TD beam sweeping for cell discovery, other options represent also complementary schemes to enhance the performance based on current 3GPP specifications.


The proposed solutions for beam refinement and tracking are extremely novel and can be considered as a potential solution that works under the 3GPP specification.

Beam-management procedures

Beam management is one of the key technologies considered and discussed in 3GPP RAN1.




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This is an incremental enhancement to current resource coordination involving beams. This approach is transparent to mechanisms specified in 3GPP, like those involved in initial access, active mobility, or clustering techniques.


The proposed approach increases the scheduling flexibility by matching the current distribution of the users with a set of beams of variable beam width. Since a hierarchical beam search also employs beams with different beam width at each stage of the beam search, the proposed solution is either transparent to current 3GPP specifications about beam management or provides a method for possible extensions of such specifications.

6.7 M-MIMMO scheme for multi-antenna

Millimetre-wave frequency bands enable the deployment of a (very) large number of antennas, and favour the deployment of Massive MIMO systems. By focusing the transmission and reception power, very large gains can be achieved in throughput and energy efficiency. The project considers practical scenarios for high rate short-range LOS links, such as wireless backhaul for ultra-dense networks and communications between objects, and proposes a new, low complexity spatial multiplexing scheme reaching up Kbits/s/Hz of spectral efficiency.


MMIMMO schemes for Short Range LOS Links with Gigantic Spectral Efficiencies: It is proposed to deploy long and thin uniform linear arrays (ULA) of dozens to hundreds of antenna elements, i.e., MMIMMO systems, embedded in the urban architecture (lamp posts, walls and bus stops) or fitting the shape of connected objects (cars, laptops and screens). A spatial multiplexing scheme called Block Discrete Fourier Transform Spatial Multiplexing with Maximum Ratio Transmission (B-DFT-SM-MRT) is considered [mmMAGIC-D5.1], [METIS-D3.1], [PTR+14] that maps data streams onto directions of departure/arrival thanks to spatial DFT and compensates misalignments and the effects of scattering thanks to MRT. This system consists of two ULAs, separated by a distance D, occupying a length L, and composed of N antenna elements, each. The number of data streams being spatially multiplexed is Nu ≤ N. All these parameters are optimized, knowing that such channel has around (L2xf)/(Dxc) degrees of freedom, when D >> L, with f and c being the carrier frequency and the speed of light [MMMAGIC-D5.1].

6.7.2 Benefits

Simulations at 26 GHz [mmMAGIC-D5.2] with advanced antenna models and ray tracing based channel models illustrated in Figure 6-14 show that B-DFT-SM-MRT attains up to 1 Kbits/s/Hz of spectral efficiency, from 31% to 84% of SVD capacity; and is up to 10 000 and 100 times less complex than SVD, for the receiver and transceiver, respectively.


MMIMMO communications are complementary solutions to existing ones, as they need short range LOS propagation and a disruptive type of deployment. This technique would depend on and benefit from hybrid, digital, analogue architectures, where the DFT precoding / decoding could be implemented in the analogue part.




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Figure 6-13: B-DFT-SM-MRT system parameters.

Figure 6-14: Simulated environments and antenna model.


The proposed solution is disruptive in terms of deployment, hardware, and software requirements. Its feasibility for hundreds of spatial streams remains open. It is evolutionary in terms of standardization, as it is compatible with the hybrid analogue digital beamforming and spatial multiplexing approach being standardized for NR. For the short term, a lower order MIMO implementation of B-DFT-SM-MRT is recommended.

6.8 Reference signals


RS for CSI acquisition

In general, the proposed CSI-RS design can be applied to most beam acquisition techniques described in Section 6.6, relying on beam reference signals. The proposed CSI-RSs are designed for the heterogeneous, codebook-based beam management procedure described in Section 6.6.1.5. For the flexible scheduling with beams of variable beamwidth in Section 6.6.1.5, a UE associated with each detected beam defined in the heterogeneous codebook reports its corresponding CSI to the AP. The CSI can be wideband CQI or sub-band CQIs (or even the reference signal received power). To measure the beam specific CSI, the UE can be configured with multiple CSI reference signals, each of which is transmitted via a corresponding beam defined in the codebook. For instance, with the codebook example depicted in Figure 6-15, the UE in a particular sector can be configured with 15 CSI-RSs (8 of width 15°, 4 of width 30°, 2 of width 60° and 1 of width 120°). It is reasonable to assume that for the sector edge UEs, the beams close to sector edge can be configured for the CSI-RSs. One possible timing configuration of the 15 CSI-RS processes, with different periodicity, is shown in Figure 6-15. Due to the constraint of the shared physical analogue beamformer, CSI-RS scheduled on different beams need to be transmitted in different OFDM symbols. The AP can further configure the feedback mode of UEs for the CSI-RSs. For example, the UE can be requested to periodically report only the M best detected beams in terms of wideband CQI. It should be noted that the coherence time of the spatial channel depends on the channel model and especially on the UE mobility. The proposed RS transmission can be configured in a UE-specific manner.

D

LN=Number of antennas

transmit ULA receive ULA




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Figure 6-15: Timing configuration of CSI-RS processes for the UE.

DMRS design

DMRS is used for channel estimation and coherent demodulation. As discussed in Section 6.3, DMRS is front loaded to reduce latency in channel estimation and decoding. There can be different patterns and densities for DMRS. Examples of front loaded DMRS are shown in Figure 6-16, where one or two OFDM symbols in a seven OFDM symbol slot are used for DMRS.

Figure 6-16: Front-loaded DMRS pattern of 1 and 2 OFDM symbols for a resource block of 7

OFDM symbols.

As shown in Figure 6-16-(1), each DMRS port is composed of 4 Resource Elements (REs) of same colour, i.e., 2 pairs of two adjacent REs in frequency domain. In addition, 2 DMRS ports are code domain multiplexed (CDM) in the 4 REs. As a result, 6 DMRS ports, in total, are supported by DMRS of 1 OFDM symbol. To further increase the spatial layer of MU-MIMO operation, e.g., up to 12 layers, DMRS of 2 OFDM symbols can be used by applying time domain orthogonal cover codes (TD-OCC) for two adjacent DMRS REs in time domain. As shown in Figure 6-16-(2), 16 REs in same colour can support 4 layers by using both frequency and time domain OCC, and in total 12 layers MU-MIMO operation can be supported.

For those broadcast/multicast data channels, e.g., system information, the latency is not a critical issue; however, the reliability would be a more important performance metric. To consider different Doppler spread situations, an additional DMRS symbol may be necessary to ensure sufficient channel estimation performance. As a result, it is proposed to use an additional DMRS symbol for such broadcast data channel. It should be noted that broadcast data channel typically only supports single-layer transmission due to the throughput and coverage requirement. In Figure 6-17, only a single layer DMRS pattern is shown.

port 1 & 2 port 1, 2, 7, 8

port 3 & 4 port 3, 4, 9, 10

port 5 & 6 port 5, 6, 11, 12

(1) Front‐loaded DMRS of 1 OFDM symbol (2) Front‐loaded DMRS of 2 OFDM symbols




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Figure 6-17: Single layer DMRS pattern for broadcast channels.

PTRS design

Phase Noise (PN) is a problem for mm-wave communications, where the main issue is known as Common Phase Error (CPE). The CPE shifts the modulation and coding scheme (MCS) constellations across all the sub-carriers in a OFDM like system by a common off-set. This becomes problematic in higher order MCS, where the constellation points are packed closely together and any shift would likely cause errors in data reception. To mitigate this CPE impact, the solution discussed in 3GPP NR is the insertion of reference signals, known as Phase Tracking Reference Signals (PTRS). There is a known trade-off with different PTRS densities in the performance it provides in error recovery and the signalling overhead. 3GPP has agreed that PTRS can be in every OFDM symbol, every second OFDM symbol, and every fourth OFDM symbol. PTRS in every OFDM symbol is illustrated in Figure 6-18.

PTRS is UE-specific, confined in scheduled resource and beamformed (in order to obtain processing gain for the phase estimation). Moreover, the number of PTRS ports can be smaller than the total number of ports, and orthogonality between PTRS ports is achieved by means of FDM. The presence of PTRS in the transmission is configurable and depends of the quality of the oscillators in the RF equipment, the carrier frequency and the MCSs used for the transmission. In order to achieve forward compatibility, the presence and configuration of PTRS is explicitly indicated by means of RRC signalling. The project analysed the impact of different PTRS densities with a PN model discussed in the project and with parameters that will be likely applied in 3GPP NR systems. This work was also submitted to 3GPP RAN1 [3GPP-RAN1-17] and discussed in that meeting.

Figure 6-18 Illustration of PTRS in every OFDM symbol.

6.8.2 Benefits of the Solution


The proposed timing configuration considers the constraint of the shared physical analogue beamformer and enables the CSI acquisition required for the solution proposed in Section 6.6.1.5.




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DMRS design

The proposed DMRS design enables single-layer, multi-layer, single-UE, multi-UE MIMO data transmission by virtue of the same precoding scheme as the one for data transmission. Moreover, front-loaded DMRS is also beneficial to low latency service support of unicast data channel. In addition, for broadcast data channels, use of an additional DMRS symbol is proposed to enhance channel estimation performance in high Doppler conditions.

PTRS design

The PTRS signal enables the tracking of phase noise and doppler, this makes the physical interface robust to such challenges. Density of PTRS in time and frequency domain is configurable to address different scenarios (for example carrier frequency, modulation and coding scheme, and hardware quality).



The proposed solution can be integrated in the overall mmMAGIC system concept, which intends to realize flexibly configurable radio. Given this design principle, the proposed solution can be readily applied within the proposed radio interface design, for example already considered in the frame structure.

DMRS design

The proposed solution can be integrated in the overall mmMAGIC system concept. Specifically, front-loaded DMRS patters can be applied to low latency unicast data channels with the potential support of MIMO transmission, and single port DMRS patterns with additional symbols can be used for the broadcast channel for robust channel in different Doppler spread situations.

PTRS design

PTRS have been design depending on OFDM waveform and the density of PTRS will depends on OFDM numerology, such density will affect the resources used in the frame structure for phase noise compensation.



The proposed CSI-RS scheme enables the beam-management procedure, which utilizes beams of different beam-widths, i.e., the heterogeneous beamforming codebook described in Section 6.6.1.5. Reconfigurable CSI-RS has already been adopted by the 3GPP NR standard, hence, the proposed solution is fully aligned with current standardization developments.

DMRS design

DMRS has already been adopted by the 3GPP NR standard, thus, the proposed solution is in line with current standardization development. Compared to LTE, the novelty lies in the aspect of dynamically configurable DMRS patterns of 1 or 2 OFDM symbols for the support of (SU-/MU-) MIMO transmission of low latency service.

PTRS design

PTRS has already been adopted by the 3GPP NR standard.




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6.9 Summary

The chapter has presented the CoS on the PHY layer of the new radio interface, describing per each component the proposed solutions, and highlighted the benefits of each solution, along with its relation to standardization activities. The CoS presented in the chapter include waveform, numerology, frame structure, channel coding and multi-antenna technologies.




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7 Assessment of the system concept The mmMAGIC system respective component solutions (CoS) have been evaluated through different means, for example, simulations, analysis, and hardware experiments. This chapter will provide an overview of the benefits provided by the CoS to the system concept.

7.1 Contribution to the key performance indicators (KPIs)

The CoS developed in mmMAGIC have been evaluated and optimized individually, each contributing to some of the required KPIs. In this chapter, a mapping between each CoS and the addressed KPIs is provided.

In the following a brief description of the mmMAGIC’s KPIs [mmMAGICD1.1] is provided:

User data rate: refers to the user data rate (DR) at which the end user uploads or downloads a file at the application layer for a defined time period. It is measured in megabits per second (Mbps). In the project download of 50 Mbps is considered as the baseline, however up to 1 Gbps DRs are foreseen for other use cases, and uploads between 1 to 500 Mbps.

Connection density: refers to the average number of simultaneous active connections that can be supported by an operator in an area, measured in connections per square kilometre (connections/km²). The requirement may vary from 330 to 75000 user/Km2.

Traffic density: is equal to the product of the connection density and the experienced user data rate, measuring the amount of traffic exchanged from all the active connections in an area. This definition does not capture user behaviour, thus the requirements in traffic density are overestimated. The measurement unit is in bits per second per square kilometre (bps/km²). The downlink/uplink requirements vary from 16/0,32 Gbps/Km2 for the less stringent use case, up till 15000/2000 Gbps/km2.

Mobility: measures the supported end user mobility, usually measured in kilometres per hour (km/h). The use cases may require support for pedestrian mobility, up to very high speed like 500 km/h.

Reliability: an assessment criterion to describe the quality of a radio link connection for fulfilling a certain service level. The common requirement is 95% reliability, but 99.999 is demanded for ultra-reliable use cases.

Availability: corresponds to the satisfaction of the end user and is correlated with reliability. Thus, if reliability is maintained over a certain quality of experience (QoE) threshold, then also the availability is perceived as satisfactory, and the user experiences the service as available. The common requirement is 95% reliability, but 99.999 is demanded for ultra-reliable use cases.

Latency: the latency perceived by the end user is defined as the duration between the transmission of a small data packet from user equipment to the Layer 2 / Layer 3 interface of the 5G system destination. In some cases, latency may also include the equivalent time needed to carry any response back, according to Next Generation Mobile Network (NGMN). The requirements for latency vary from 50 to 1 ms.

The mmMAGIC system concept contains 23 components and has defined 43 component solutions (CoS). Each individual CoS will be contributing to improve one or more KPIs related to the use cases, and can also increase the robustness of the system concept against signal degradation/ resource utilization shortage phenomena. Table 7-1. Summary of the CoS and benefits to the system concept summarizes the CoS and their relevance to a standalone or non-standalone deployment, the benefit to the system concept mapped to the KPIs and other benefits indirectly related to KPIs.




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Table 7-1. Summary of the CoS and benefits to the system concept System Components

Component solution (CoS)

Deployment KPIs from mmMAGIC's use cases

Other benefits, robust towards:

Logical Network Architecture

CoS1: Logical network architecture (Section 4.1)

Standalone, non-standalone.

Data rate, mobility, reliability, latency.

N/A.

Interfaces CoS2: NG, Xn, NAS, AS (Section 4.2)


All KPIs. Flexibility to support (new) multiple services.

Spectrum sharing

CoS3: Spectrum sharing using interfaces (Section 4.3)


Data rate. Spectrum utilization, cost

Multi-connectivity

CoS4: Dual connectivity with MCG /SCG-split bearer (Section 4.4.1.1)


Data rate, mobility, reliability.

Blockage, high path loss.

CoS5: RRC diversity (Section 4.4.1.2)


Reliability, mobility, latency.

N/A.

Tight Interworking with LTE

CoS6: Tight interworking with LTE (Section 4.5)

Non-standalone.

Data rate, mobility, reliability.

Blockage, doppler.

Multi service support

CoS7: Network slicing (Section 4.6)

Standalone, non-standalone

All KPIs. Cost.

Protocol Stack

CoS8: RRC_inactive (Section 5.1.1)


Connection density, mobility, latency

Reduces the C-Plane latency and UE power consumption; localized RAN based paging.

Cos9: RLC Optimization (Section 5.1.1)

Standalone, non-standalone

Data rate, latency.

N/A.

Multi-node schemes

CoS10: Beam sweeping for multi-node networks considering interference (Section 5.2.1.1)


Data rate, latency, reliability.

Interference.

CoS11: Sequential hybrid beamforming for multi-link mm-wave communication (Section 5.2.1.2)


Data rate, reliability, mobility, latency.

Blockage.




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System Components




CoS12: mm-wave based RF-FSO multi-hop networks (Section 5.2.1.3)


Data rate, connection density, reliability, latency.

High path loss, moisture diversity, energy efficiency.

CoS13: mm-wave LOS coverage enhancements with coordinated high-rise APs (Section 5.2.1.4)


Reliability, data rate, connection density.

Blockage, high path loss.

CoS14: Relay-assisted access at mm-waves (Section 5.2.1.5)


Reliability, data rate, connection density.

High path loss, blockage, penetration loss, coverage.

CoS15: Joint hybrid precoding for energy-efficient mm-wave networks (Section 5.2.1.6)


Data rate, connection density, reliability.

Blockage, energy efficiency, high path loss.

Initial Access Schemes

CoS16: Cell discovery with wide beams and broadcasting (Section 5.3.1.1)


Reliability, availability, latency.

Robust against high path loss, blockage.

CoS17: Synchronization and CSI reference signals (Section 5.3.1.2)


Reliability, latency.

Robust against high path loss, blockage, coverage.

CoS18: Cell discovery assisted by auxiliary transceiver (Section 5.3.1.3)


Data rate, availability, latency.

High path loss, large scale parameters, large BW, antenna design challenges especially the analogue phase-shifter latency.

CoS19: Random access with coordinated access points (Section 5.3.1.4)



Robust against high path loss, blockage, coverage.




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System Components




CoS20: Random access based FDD (Section 5.3.1.5)

Standalone. Reliability, availability and latency.

High path loss, large BW, time and/or frequency selectivity, non-reciprocity of the hardware and antenna design challenges.

Multiple access and duplexing schemes

CoS21: QoS-centric resource allocation with Hybrid-BF (Section 5.4.1.1)

Standalone. Data rate, reliability, latency.

High path loss, antenna design challenges especially the analogue phase-shifter resolution.

CoS22: Joint scheduling and resource allocation for access and backhaul exploiting SDMA (Section 5.4.1.2)


Data rate, connection density, traffic density.

High path loss, large scale parameters, large BW.

Active Mobility

CoS23: mm-wave clustering (Section 5.5.1.1)

Non-standalone, standalone.

Availability, latency, mobility.

Minimizing signalling overhead.

CoS24: Configurable CSI-RS (Section 5.5.1.2)


Mobility. Minimizing signalling overhead, seamless connection.

Inactive Mobility

CoS8: RRC_inactive (Section 5.1.1)


Connection density, mobility, latency.

Reduces the C-Plane latency and UE power consumption; localized RAN based paging.

CoS25: Fast switching to RRC_connected (Section 5.6.1.2)


Mobility, latency.

Minimizes latency, improves power efficiency.

Self-Backhauling

CoS22: Joint scheduling and resource allocation for access and backhaul (Section 5.7.1)


Data rate, connection density, traffic density.

High path loss, large scale parameters, large BW.

Re-transmission protocols

CoS26: Single-hop and multi-hop retransmission protocols (Section 5.8.1.1)



Channel variations as long as there is some correlation between successive channel realizations, and to hardware impairments.




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System Components




CoS27: Early detection of packet errors without full FEC decoding (Section 5.8.1.2)


Data rate, connection density, traffic density, latency.

Large BW (error detection on a per-codeblock basis irrespective of whether the system BW is large or not), and robust to hardware impairments as long as the resulting SNR falls within the expected range for HARQ operation.

CoS28: Fast HARQ over finite block length codes (Section 5.8.1.3)


Data rate, reliability, latency.

Robust to channel variations as long as there is some correlation between successive channel realizations, impact of PA inefficiency on system performance.

Waveform CoS29: Scalable OFDM (Section 6.1.1)


Data rate, connection, and traffic densities.

Delay spread, Doppler spread, path-loss; blockage. Robustness to PA non-linearity, PN, synchronization errors.

Numerology CoS30: Scaled LTE numerology (Section 6.2.1)


Data rate, latency.

Phase noise, Doppler spread.

Frame-structure

CoS31: Scalable adaptive subframe (Section 6.3.1)


Data rate, latency.

Small overhead, flexible TDD support.

Channel Coding

CoS32: LDPC (for data) and Polar (for control) (Section 6.4.1)


Data rate, mobility, reliability, availability and latency.

Path-loss, delay spread, Doppler spread, and phase noise.

Transceiver architecture and schemes

CoS33: Access: flexible hybrid beamforming design (Section 6.5.1.1)


Data rate, reliability.

Blockage.

CoS34: Backhauling: MMIMMO schemes for short range LOS links (Section 6.5.1.2)

Standalone. Data rate. Very high spectral efficiency.




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System Components




CoS35: Relaying: A dual-hop relay selection scheme (Section 6.5.1.3)


Data rate, reliability.

Spectral efficiency.

Beam management

CoS36: Beam sweeping for cell discovery (Section 6.6.1.1)

Standalone. Data rate, connection density, availability, latency.

High path loss, large BW, large scale parameters, frequency selectivity, antenna design challenges especially the analogue phase-shifter latency.

CoS37: Beam refinement and tracking: P-Track mechanism (Section 6.6.1.2)

Standalone. Data rate, mobility, reliability, availability and latency.

High path loss, blockage, channel variations, PA imperfections, antenna design challenges especially the analogue phase-shifter resolution.

CoS38: Beam management procedures (Section 6.6.1.4)


Mobility, reliability, latency.

Blockage, Doppler, high path loss.

CoS39: Beam coordination schemes (Section 6.6.1.3)


Latency. Scheduling flexibility, spectral efficiency.

CoS40: Heterogeneous beam codebook design (Section 6.6.1.5)


Connection density, traffic density, latency.

N/A

M-MIMMO scheme

CoS34: MMIMMO schemes for short range LOS links (Section 6.7.1)


Data rate. Very high spectral efficiency.

Reference Signal

Cos41: RS for CSI acquisition (Section 6.8.1.1)

Standalone. Data rate, reliability.

Efficient beam management.

Cos42: DMRS design (Section 6.8.1.2)


Data rate, reliability, latency.

No need for subframe level buffering.

Cos43: PTRS design (Section 6.8.1.3)

Standalone (non-standalone)

Data rate, reliability, mobility.

Phase Noise.




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Figure 7-1 illustrates the mapping of CoS to different KPIs. The sizes of the bars indicate the number of solutions that are designed to improve the given KPI. It is notable that mmMAGIC has focused on the CoS that enhances data rate, latency, reliability, and mobility. The project main focus was to deliver a system concept for extreme mobile broadband (eMBB) services, meaning that very high data rate is expected, however since mobility and reliability will be one of the fundamental challenges in mm-waves, several of the CoS have been designed considering such challenges as KPIs. At the same time, one of the key principle in the design of the system concept is to be flexible enough to count with diverse use case, thus latency is another cornerstone for enabling diverse ultra-reliable use cases. Some of the component solutions listed in Table 7-1 contributed to reduced latency at the physical layer, with faster random access and beam management schemes.

Figure 7-1: Mapping of the component solutions (CoS) to KPIs.




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8 The technology roadmap

8.1 5G timeline in standards, regulation and mmMAGIC

The ambition of this project was to develop together a mm-wave system concept relevant for standards, thus 3GPP and ITU-R were targeted as the main industry forums to impact. mmMAGIC officially started in July 2015; at that time 3GPP worked on the study item (SI) to create a channel model for above 6 GHz (SI:CM) and the SI that defined the requirements for 5G (see the green boxes in Figure 8-1:).

Figure 8-1: Timeline for regulation and standards activities, running in parallel to mmMAGIC project.

Through the engagement of the industry partners in 3GPP, very early into the project, mmMAGIC started to send contributions to the 3GPP SI: CM. In February 2017, the SI: CM closed creating the TR38.900 document and since then the project findings focusing on the mm-wave channel were mainly provided to the ITU-R activity that defines the requirements and evaluation methods for IMT-2020 (or 5G) but also to 3GPP in the form of “request for change” to update the 3GPP channel model. The activities to define 5G in 3GPP and ITU-R were running in parallel to the project and there was continuing interaction among them, on top of the one reflected in Figure 8-1:. In total, 13 contributions to 3GPP and 6 contributions to ITU-R, related to channel model and measurements, have been considered acknowledging mmMAGIC through the life time of the project (see the list in Table 8-1). This is a list of direct contributions; however, through partners’ engagement both within 3GPP and mmMAGIC, the project has further contributed to standardization fora in an indirect manner.

Table 8-1. The project contributions on mm-wave channel to standards and regulation.

No Description of Work Doc Number Company Meeting

1 Street microcell channel measurements at 2.44, 14.8, and 58.68 GHz

R1-160846 Ericsson 3GPP RAN1 #84

2 Measurements of path and penetration losses at multiple carrier frequencies

R1-161688 Ericsson 3GPP RAN1 #AH




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No Description of Work Doc Number Company Meeting

3 Indoor and outdoor to indoor channel measurements

R1-161691 Ericsson 3GPP RAN1 #AH

4 mmMAGIC white paper R1-161700 HHI 3GPP RAN1 #AH

5 Discussion on blockage modelling R1-161707 Samsung 3GPP RAN1 #AH

6 Discussion on spatial consistency R1-161708 Samsung 3GPP RAN1 #AH

7 Discussion on blockage modelling for above 6 GHz channel model

R1-162720 Samsung 3GPP RAN1 #84-BIS

8 Discussion on spatial consistency for above 6 GHz channel model


9 Initial 60 GHz channel sounding results from the mmMAGIC project for corner diffraction and surface scattering

R1-162872 Keysight 3GPP RAN1 #84-BIS

10 On the frequency dependence of LSPs R1-163254 Ericsson 3GPP RAN1 #84-BIS

11 Remaining details on blockage modelling for above 6 GHz channel model

R1-164808 Samsung 3GPP RAN1 #85

12 Remaining details on spatial consistency for above 6 GHz channels


13 Change request TR38.900_CR_Ground_ Reflection_Model

R1-1701412 HHI, Ericsson

3GPP RAN1 NR #AH

1 Ground reflection modelling in IMT 2020 channel model

5D/336-E Samsung ITU-R WP5D

2 Remaining issues on ground reflection modelling for IMT-2020 channel model

5D/438-E Samsung ITU-R WP5D

3 Building entry loss measurements at 3, 10, 17 and 60 GHz for development of a BEL model

3M/104-E Orange ITU-R WP3J 3K 3M

4 Measurement results of building entry loss in the 2 GHz to 60 GHz frequency range

3M/73-E Ericsson ITU-R WP3J 3K 3M

5 Building entry loss measurements at 3, 10 and 17 GHz frequencies

3M/150-E Orange ITU-R WP3J 3K 3M

6 On Recommendation ITU-R P.[BEL]: Building entry loss measurement and modelling results on elevation angle dependence

3M/199-E Ericsson ITU-R WP3J 3K 3M




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At the time when this report is public, five of the contributions listed in Table 8-1 have been adopted in 3GPP and ITU-R; these contributions are listed in Table 8-2.

Table 8-2. List of mmMAGIC contributions adopted by standards or regulators

The partners in the project are interested to bring the mmMAGIC channel model into standards even after the project closure.

In April 2017, 3GPP started the work item to define the 5G architecture and radio interface, (the purple box 5G SI in Figure 8-1:. At this point in time, mmMAGIC had already identified the project’s use cases and requirements (D1.1, IR1.1), the principles for the new RAN (IR3.1), entities and enablers for the logical architecture (D3.1), and the components for the 5G radio interface (IR4.1, IR4.2, IR5.1). Given the way 3GPP standardization works and the nature of the topics that will set the standards for the new 5G RAT, a few contributions related to such topics are acknowledging mmMAGIC (see Table 8-3). However, some of the components for the 5G RAT identified early in the different deliverables (IR3.1, D3.1, IR4.1, IR4.2, D4.1, IR5.1) can be mapped to the decisions in standards, (see Table 8-4), where the mmMAGIC partners are engaged in such issues.

Table 8-3. The project’s contributions on 5G new RAT.

Description of Work Doc Number Company Standard Meeting

The impact of a new flexible mobile RAT operating in frequency bands 6-100 GHz on 5G use cases: the vision of the mmMAGIC project

S1-62253 Intel 3GPP SA1 #75

No Description of the idea or solution Standard group

Document or reference in the standard

1 The ground reflection modelling in IMT 2020 channel model. Adopted in 5D/336, 5D/438, ITU-R document.

ITU-R ITU-R M. [IMT-2020.EVAL]

2 The building entry loss (BEL) measurements at 3, 10, 17 and 60 GHz for development of a BEL model.

ITU-R,

3GPP RAN 1

Adopted in 3J/82-E, 3K/97-E, 3M/150-E

3 The explicit ground reflection model.

Adopted in R1-1701412 and TR 38.901, Section 7.6.8

4 The proposed elevation angle model was approved in ITU-R SG3 with about 20 dB/90-degree parameter value, ITU-R SG3.

ITU-R ITU-R M.[IMT-2020.EVAL]

5 The different measurements data provided as contributions in Table 8-1 have been adopted to develop 3GPP modelling.

3GPP RAN1 3GPP TR38.901




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Description of Work Doc Number Company Standard Meeting

Discussion on large antenna array modelling


Joint contribution, WF on phase noise modelling


Table 8-4. Mapping investigated technologies in mmMAGIC to standards.

No Description of the idea or solution Standard group

Document or reference in the standard

1 Waveform: CP-OFDM for below 40 GHz. The project compared ten different waveforms in a common simulator and recommended on CP-OFDM for below 40 GHz, documented in D4.1, June 2016.

3GPP RAN1 TR38.802 version 1.0.0, Section 6.1.3 on Waveform

2 Modulation Schemes: D4.1 considered QPSK, 16QAM, 64QAM and 256QAM as modulation schemes for waveform evaluations, implicitly assuming that these will be supported.

3GPP RAN1 TR38.912

3 HARQ: mmMAGIC proposed asynchronous and adaptive HARQ in detail in D4.1. However, the following paper was submitted first in Dec. 2015 to EUSIPCO i.e., before NR study item started “Retransmission Schemes for 5G Radio Interface”

3GPP RAN1 TR38.912

4 Numerology: the proposed numerology in D4.1 is fully aligned with TR38.912 on all concepts of numerology: a) scaling according to 2^n. b) considering at-least 15 kHz to 480 kHz subcarrier spacing and c) mixed numerology.

The following paper on numerology was submitted in March 2016 “Waveform and Numerology to Support 5G Services and Requirements”.

3GPP RAN1 TR38.802

5 Reference signals: PT-RS and DM-RS: front loaded DMRS and PT-RS. D4.1 proposed the same.

Also, captured in the papers: “Numerology and Frame Structure for 5G Radio Access,” and “Frame Structure Design for Future Millimetre Wave Mobile Radio Access”

3GPP RAN1 TR38.802

6 New RRC state: prose RRC_INACTIVE as a new state to improve energy efficiency and latency, identified before D3.1, March 2016.

3GPP RAN2 and RAN3

TR38.804 Section 5.5.2 UE states and state transitions.




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The timeline for 5G or NR standardization in 3GPP has been once more accelerated during the RAN plenary meeting in March 2017. 3GPP started the road to 5G standardization and is in the middle of its first release (Rel-15). Standards specifications for the 5G non-standalone RAT should be ready by December 2017, while the 5G standalone specification will be ready in the middle of 2018, shown as 5G-NR WI (working item) in Figure 8-1:. In parallel to the 5G-NR WI, several other SI are starting up to prepare the specification work for the coming releases.

Despite the accelerated timeline in 3GPP, mmMAGIC directly and indirectly impacted the standardization and regulation through noticeable contributions towards 3GPP and ITU-R. The impact to standards/regulation is important to guarantee early industry alignment, since the project has already achieved consensus in some technology solutions. At the time this deliverable was published, the partners from the project have made 16 contributions to 3GPP, 6 to ITU-R, and a number of technologies have been already adopted by these groups. However, the standardization and regulations towards 5G is being shaped and discussed in other groups where mmMAGIC have also engaged with: presentations in several ETSI workshops, active participation in different 5G PPP working groups, contributions, and presentations at COST IRACON, engagement with regulators, participation at 5G global events, monitoring and presenting at NGMN, ECC and other parts of the ecosystem. More details on the mmMAGIC engagement and dissemination/exploitation activities can be found in [mmMAGIC-D6.5].

Finally, eleven patent applications have been reported to European Commission on issues related to the system component solutions such as: initial access, frame-structure, RRC, channel coding, waveform, transceivers.

8.2 mmMAGIC component solutions and relation to standards.

This section provides an overview of the project’s interaction with the 5G regulation and standardization activities. The given interactions bring as a consequence that some of the solutions from the project have been adopted in standards, or that mmMAGIC worked on enhancing solutions to be ahead of standards, or that the solutions that mmMAGIC proposed are not yet considered in the timeline of standards and will be considered for further studies. The novelty of the CoS and its relations to standards is summarized in Table 8-5.

7 Tight integration with LTE-A with PDCP layer multi connectivity, identified in D3.1, March 2016.

3GPP RAN2 and RAN3

TR38.801 in Section 6.2 RAN functions description and TR38.804.

8 Network Slicing at RAN level, identified in D3.1, March 2016.

3GPP RAN2 and RAN3

TR38.804

9 Cell clustering / mobility without RRC, identified in D3.1 3GPP RAN2 and RAN 3

TR38.804

10 Cell clustering with centralized PDCP, identified in D3.1 3GPP RAN2 TR38.801 in

Section 11.2.3.




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Table 8-5. The novelty of the system component solutions and relevance to standards.

Component solution (CoS) Novelty and relation to standards

CoS1: Logical network architecture

The proposal is an evolution of LTE-A, and it is aligned with 3GPP in TR 38.804.

CoS2: NG, Xn, NAS, AS interfaces

The proposal is an evolution of LTE-A, and it is aligned with 3GPP in TR 38.801.

CoS3: Spectrum sharing using interfaces

The proposal can be considered for future study in standards.

CoS3CoS4: Dual connectivity with SCG-split bearer

The solution is an evolution of dual connectivity in LTE Release-12. It is aligned with 3GPP in TR 38.801/804. The novelty is that 5G RAT will allow information flow with legacy LTE RAT.

CoS4CoS5: RRC diversity The novelty is to realize redundancy on the control plane messaging. In LTE Release-12/13 the RRC signalling can be realized only via the master node.

Cos6: Tight interworking with LTE

New solution which is 5G specific, but dual connectivity is an evolution of LTE Release-12. The solution is aligned with 3GPP in TR 38.801/804.

Cos7: Network slicing New solution when applied to the 5G RAN. It is aligned with 3GPP in TR38.801.

Cos8: RRC_inactive This is entirely a new state introduced into the existing two state mobility system. The proposal has been accepted in 3GPP and is in TR 38.804.

Cos9: RLC Optimization The proposal is an evolution of an existing technique.

Cos10: Beam sweeping for multi-node networks considering interference

Aligned with 3GPP in TR 38.804, since proposed approach is transparent to the network.

Cos11: Sequential hybrid beamforming for multi-link mm-wave communication


Cos12: mm-wave based RF-FSO Multi-hop networks

Hybrid RF-FSO communications is not part of current 3GPP standards. This can be considered for future study in standards.

Cos13: mm-wave LOS coverage enhancements with coordinated high-rise APs

Relevant to beam management in current 3GPP NR discussion.

Cos14: Relay-assisted Access at mm-waves

This is an evolution of LTE-A, as relays were standardized already since 3GPP LTE Release 10.

Cos15: Joint Hybrid Precoding for Energy-efficient mm-wave Networks

Joint load balancing and precoding for cooperative hybrid beamforming systems at mm-waves is novel, and not part of current 3GPP standards.




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Cos16: Cell discovery with wide beams and broadcasting

Aligned with the 3GPP underlying assumption that initial cell discovery can be conducted entirely with omnidirectional transmissions or transmissions in fixed antenna patterns.

Cos17: Synchronization and CSI reference signals

Aligned with the 3GPP underlying assumption that initial cell discovery can be conducted entirely with omnidirectional transmissions or transmissions in fixed antenna patterns.

Cos18: Cell discovery assisted by auxiliary transceiver

This can be considered for future study in standards. This has impact on frame structure design and beam management.

Cos19: Random access with coordinated access points

Relevant to both initial access and beam management in current 3GPP NR development.

Cos20: Random access based FDD

This can be considered for future study in standards.

Cos21: QoS-centric resource allocation with hybrid-BF

This serves as an enhancement based on current 3GPP specification. It can also be for future study in 3GPP.

Cos22: Joint scheduling and resource allocation for access and backhaul


Cos23: mm-wave clustering The clustering concept is currently discussed in standards [TR 38.300], with the terminology multiple TRPs scenario. It is not only for mm-waves but also can be considered for low frequency NR deployments.

Cos24: Configurable CSI-RS The usage of CSI signalling for handover procedures is a new concept for NR. This was agreed in 3GPPP (May 2017).

Cos25: Fast switching to RRC_connected

The proposal is an evolution of LTE-A, and it is aligned with 3GPP TR 38.804.

Cos26: Single-hop and multi-hop retransmission protocols

The proposal is an evolution of LTE-A, and it is aligned with 3GPP TR 38.802, only for single hop since multi-hop protocols is not included in 3GPP yet.

Cos27: Early detection of packet errors without full FEC decoding

The proposal can be used as an extra implementation to enhancement standardized retransition protocols to further reduce latency.

Cos28: Fast HARQ over finite block length codes

This can be considered for future study in standards.

Cos29: Scalable OFDM Aligned and enhanced with 3GPP TR 38.802.

Cos30: Scaled LTE numerology

Aligned with 3GPP TR 38.802.

Cos31: Scalable adaptive subframe

Baseline frame structure and front loaded DMRS aligned with 3GPP. PTRS discussion ongoing in 3GPP.




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Cos32: LDPC (for data) and Polar (for control)

Aligned with 3GPP. Decoders developed for further enhancement.

Cos33: Access: flexible hybrid beamforming design


Cos34: Backhauling: MMIMMO schemes for Short Range LOS Links


Cos35: Relaying: A dual-hop relay selection scheme


Cos36: Beam sweeping for cell discovery

The proposal is aligned with 3GPP and enhanced. The proposed beam sweeping strategies have been specified in 3GPP documents related to beam management. This solution provides a method to select the best strategy among the specified ones.

Cos37: Beam refinement and tracking: P-Track mechanism


Cos38: Beam management procedures

Aligned to 3GPP TR 38.802.

Cos39: Beam coordination schemes


Cos40: Heterogeneous beam codebook design

The proposed solution employs beams of different beam width at each stage of the beam search, hence it is aligned with the current 3GPP specifications (TR 38.802) on beam management and consists a method for possible extensions of such specifications.

Cos41: RS for CSI acquisition

Aligned with 3GPP TR 38.802.

Cos42: DMRS design Aligned with 3GPP TR 38.802.

Cos43: PTRS design Aligned with 3GPP TR 38.802.




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9 Final summary and Recommendations This deliverable presents the mmMAGIC system concept. The system concept consists of 23 system components, that bring together 43 technical solutions into a mobile radio access technology (RAT) targeting operation in the 6 - 100 GHz frequency range. The system concept has been integrated into an overall 5G multi-RAT concept developed in parallel within mmMAGIC, 3GPP, ITU-R, and 5G PPP, among others.

The recommendations and views from this project are the following:

The system concept was designed based on the recommendation to use the logical architecture and protocol stack of LTE-A RAN with the purpose of ensuring system integration between LTE-A and 5G RAT. The logical architecture includes the next generation user equipment, the 5G node (gNB) and the interfaces: Xn which provides the interconnection between two nodes, NG which connects the nodes to the next generation core, Uu to enable the user equipment to connect to a node, and the NAS interface allowing a UE to connect to the CN. Other system components that are proposed for the integration of the radio access are: 1) Multi-connectivity: the recommended solution is based on the LTE-A dual connectivity with MCG/SCG-split bearer at the PDCP layer which allows data flows between two nodes. Given that multi-connectivity is implemented, RRC diversity can be exploited as a complementary solution to allow the control plane RRC messages to be routed using different nodes. 2) Tight interworking with LTE-A: enables the interworking between LTE and 5G RAT to provide seamless connectivity. The solution is based on but not limited to dual connectivity and handover. 3) Multi-service support: through network slicing enabling a service with a separate logical network on a shared infrastructure, the network can be optimized and configured to suit the requirements of a specific use case. 4) The protocol stack: will need modification at different layers, especially in the physical layer with the new air interface solutions listed further down, the aggregation at the PDCP layer from and to 5G nodes to and from LTE-A nodes; the RRC layer will include a new state called RRC INACTIVE to enhance latency and energy efficiency which may require modification at the MAC layer, buffer optimization at the RLC layer. 5) The spectrum sharing component proposes five architectural variants to implement spectrum pooling through either: the interface at the RAN, the interface at the core network, the RAN sharing, the core network sharing, or the introduction of a spectrum broker entity.

The deployment is classified as standalone or non-standalone, based on the combinations of low and high frequencies. The non-standalone deployment will give as a result redundant coverage and will be the first deployment for 5G. Due to the difficult propagation conditions and hardware constraints at mm-wave carrier frequencies, the system will require dense node deployment. Thus, multi-node coordination solutions are of importance, especially for standalone deployments. The project investigated a spatial multi-flow joint transmission using hybrid beamforming, sequential hybrid beamforming as a flexible way to support adaptive dual/multi-connectivity, the potential use of hybrid RF-FSO links for backhauling and the decode-and-forward relaying scheme to improve cell border throughput.

The initial access procedure is periodically repeated and mmMAGIC presents solutions for three phases: the cell discovery phase, the random-access phase, and the beam refinement and/or tracking phase. The synchronization signals and important system information is broadcasted by access points at the cell discovery phase using wide beams. In standalone deployments, random access is suggested to be performed by access points within one cluster to reduce the random access time, given that the reliability of the transmissions improved using this setup. The solutions for the beam refinement and/or tracking phase are part of the beam management component proposing: 1) Four schemes for beam sweeping per slot have been examined, namely: time division, frequency division, code division, and spatial division depending on the transceiver design. For analogue beamforming capability only time division is




92

recommended. 2) The probabilistic beam tracking (P-Track) mechanism to track the mm-wave channel dynamics under node mobility without requiring dedicated training slots. 3) The beam indicator label that will serve as input to an Automatic Interference Relation Table (AIRT), to coordinate the beams from different nodes and avoid interference. 4) Three measurement procedures depending on the beam capability of the UE. 5) A heterogeneous codebook to produce beams with variable beam width and increase the scheduling flexibility.

Due to extensive use of beamforming in mm-waves, multiple access in the spatial domain (SDMA) is a recommended solution, which can also be applied to optimized scheduling between backhaul (self-backhaul) and access traffics, and can be used for an Integrated Access-Backhaul (IAB). However, for initial phase deployment where analogue beamforming transceivers are likely to be used, TDMA will most likely be the first choice.

The system concept defines solutions for two modes of mobility. The ctive mode mobility enables detection of adjacent beams and switching of the data flow between these beams, and allows the tracking of UEs through the cells where mm-wave clustering is a solution, which can be complemented with configurable CSI-RS short transmissions when needed that are turned off when not required. The inactive mode mobility provides means for accessing the network quickly, avoiding initial access,such as RRC Inactive state.

The project proposed novel adaptive and asynchronous retransmission protocols for single hop and multi-hop scenarios. This protocol is an enhancement of LTE-A – it contains superfast (not fully reliable) HARQ, scheduled (highly reliable) HARQ, and the option of RLC ARQ for reliability. The protocol is flexible, fast, reliable, has low-overhead, and does not require fixed timing. For multi-hop scenarios, the relay RLC protocol is shown to be a promising candidate due to low latency. It is observed that early detection of packet errors without full FEC decoding can be used to speed up retransmission. HARQ latency can also be reduced if the receiver estimates the number of further retransmissions that will be required to guarantee successful decoding and inform the transmitter. Afterwards, the receiver can remain silent until the required number of retransmissions have been received. This method can be useful for uplink communication.

The recommendations for the physical layer components include: a scalable OFDM waveform for all transmissions in uplink and downlink. The numerology is designed based on the LTE numerology and should scale according to 15 2 kHz with cyclic-

prefix according to .μ , where is an integer. The frame structure proposes:

downlink only subframe, uplink only subframe, mixed subframe structures and mini-slot, aiming for low latency and fast turn time. Because of the limitation of hybrid and analogue beamforming the frame structure should enable short transmission duration. Notice that the waveform, numerology, and frame-structure were designed to be robust against phase noise and Peak-to-Average-Power-Ratio.

The beamformed reference signals investigated in mmMAGIC are: CSI acquisition, DMRS and PTRS to mitigate phase noise. Whenever possible, it is desirable to avoid having always on reference signals. The recommendation for channel code is to use LDPC for data transmission and Polar codes for transmission of control information, with low complexity decoders.

Hybrid beamforming is the selected transceiver architecture option, where mmMAGIC proposes a simplified sub-array architecture, where the RF chains are connected only to a sub-array of antenna elements.

The impact on industry standardization and regulation of the proposed CoS have been evidenced by 16 contributions to 3GPP and 6 to ITU-R which acknowledge the mmMAGIC project. At least 15 solutions are already contained in 3GPP or ITU-R technical reports. There are proposed solutions that are of interest in further standardization such as: the spectrum




93

sharing pooling, initial access and beam tracking schemes, the mm-wave cell clustering, the integrated backhaul and access scheme as well as the frame structure.

The solutions developed within the project helped us to:

Strongly contribute to the research community with a new channel model (and software) extending the state of the art, solid research and modelling to create the mmMAGIC air interface (e.g., waveform, numerology, antenna design, advanced hardware imperfection).

Impact standards and industry alignment through direct/indirect partner contributions to standards and regulations.

Create value for society by developing a 5G technology for a range of applications, for future citizens to benefit from faster data rates and reliable communication. The knowledge from the project has been used by universities to teach recent advancement on 5G mm-wave mobile networks.

Finally, the project supported the European industry in driving the development of 5G with pre-alignment to the 5G system design, engaging with standards, regulation and 5G PPP initiatives. The partners showed leadership by exploiting/disseminating the results in: twenty-four deliverables, seventy scientific publications, several keynotes and workshops, development of the software platform QuaDRiGa incorporating the mmMAGIC channel model, development of a visualization tool, contributions to standards and regulations, show-casing hardware experiments, and eleven patents reported to the European Commission.




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10 References

[AHS+16] J. M. P. Arana, J. P. Han, K. M. Saquib, and Y. S. Cho, “Random access technique for millimeter-wave cellular communications,” in 2016 International Conference on Information and Communication Technology Convergence (ICTC), pp. 997–999, Oct 2016.

[ARA+14] O. El Ayach, S. Rajagopal, S. Abu-Surra, Z. Pi, and R. Heath, “Spatially sparse precoding in millimeter wave MIMO systems,” IEEE Transactions on Wireless Communications, vol. 13, no. 3, pp. 1499–1513, Mar. 2014.

[HL98] Hajimiri, Ali, and Thomas H. Lee. "A general theory of phase noise in electrical oscillators." IEEE journal of solid-state circuits 33.2 (1998): 179-194.

[HNK16] K. Haneda, S. Nguyen and A. Khatun, “Attainable capacity of spatial radio channels: a multiple-frequency analysis,” IEEE Global Communications Conf. (GLOBECOM 2016), Washington DC, December 2016.

[Int16] R1-1611929, Codebook with beam broadening, Intel, Reno, USA, Nov. 18, 2016

[ITU-R-P.2040-1] International Telecommunications Union, Recommendation ITU-R P.2040-1, "Effects of building materials and structures on radiowave propagation above about 100 MHz", Jul. 2015.

[J58] E. Johnson, "Physical limitations on frequency and power parameters of transistors," 1958 IRE International Convention Record, pp. 27-34, New York, NY, USA, 1965.

[JPY15] C. Jeong, J. Park, and H. Yu, “Random access in millimeter-wave beamforming cellular networks: issues and approaches,” IEEE Communications Magazine, vol. 53, no. 1, pp. 180–185, Jan. 2015.

[KOK+10] G. I. Kiani, L. G. Olsson, A. Karlsson and K. P. Esselle, "Transmission of infrared and visible wavelengths through energy-saving glass due to etching of frequency-selective surfaces", in IET Microwaves, Antennas & Propagation, vol. 4, no. 7, pp. 955-961, Jul. 2010.

[Lie89] H. J. Liebe, "MPM – An atmospheric millimeter wave propagation model", International Journal of Infrared and Millimeter Waves, vol. 10, pp. 631–650, 1989.

[METIS-D3.1] METIS deliverable D3.1 “Positioning of multi-node/multi-antenna transmission technologies”, July 2013, https://www.metis2020.com/wp-content/uploads/deliverables/METIS_D3.1_v1.pdf

[MMMAGIC-D1.1] mmMAGIC Deliverable D1.1, “Use case characterization, KPIs and preferred suitable frequency ranges for future 5G systems between 6 GHz and 100 GHz”, Nov. 2015. https://bscw.5g-mmmagic.eu/pub/bscw.cgi/d54427/mmMAGIC_D1.1.pdf.

[MMMAGIC-D2.2] mmMAGIC Deliverable D2.2, “Measurement Results and Final Channel Models”, May, 2017. https://bscw.5g-mmmagic.eu/pub/bscw.cgi/d202656/mmMAGIC_D2-2.pdf.




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[MMMAGIC-D3.1] mmMAGIC Deliverable D3.1, “Initial concepts on 5G architecture and Integration”, March, 2016. https://bscw.5g-mmmagic.eu/pub/bscw.cgi/d94809/D3.1_Initial_concepts_on_5G_architecture_and_integration.pdf.

[MMMAGIC-D3.2] mmMAGIC Deliverable D3.2, “Evaluations of the concepts for the 5G architecture and integration”, June 2017.

[MMMAGIC-D4.1] mmMAGIC Deliverable D4.1, “Preliminary radio interface concepts for mm-wave mobile communications”, June 2016. https://bscw.5g-mmmagic.eu/pub/bscw.cgi/d127361/mmMAGIC_D4.1.pdf.

[MMMAGIC-D4.2] mmMAGIC Deliverable D4.2, “Final radio interface concepts and evaluations for mm-wave mobile communications”, June 2017.

[MMMAGIC-D5.1] mmMAGIC Deliverable D5.1, "Initial multi-node and antenna transmitter and receiver architectures and schemes", June 2016. https://bscw.5g-mmmagic.eu/pub/bscw.cgi/d95056/mmMAGIC_D5_1.pdf.

[MMMAGIC-D5.2] mmMAGIC Deliverable D5.2, "Final multi-node and multi-antenna transmitter and receiver architectures and schemes", June 2017.

[MMMAGIC-D6.5] mmMAGIC Deliverable D6.5, "Final dissemination and exploitation report", June 2017.

[NGMN15] NGMN Alliance, ‘NGMN White Paper,’ Feb. 2015. https://www.ngmn.org/uploads/media/NGMN_5G_White_Paper_V1_0.pdf.

[NHJ+15] S. L. H. Nguyen, K. Haneda, J. Jarvelainen, A. Karttunen and J. Putkonen, “On the mutual orthogonality of millimeter-wave massive MIMO channels," 81st Vehicular Technology Conference (VTC2015-Spring), Glasgow, Scotland, May 2015.

[PRF07] D. Petrovic, W. Rave, and G. Fettweis: "Effects of phase noise on OFDM systems with and without PLL: characterization and compensation," IEEE Transactions on Communications, Vol. 55, Issue 8, pp. 1607 - 1616, Oct. 2007.

[RMG+16] C. Rusu, R. Mendez-Rial, N. Gonzalez-Prelcic, and R. W. Heath, “Low complexity hybrid precoding strategies for millimeter wave communication systems,” IEEE Trans. Wireless Commun., vol. 15, no. 12, pp. 8380–8393, Dec 2016.

[QQL16] D. Qiao, H. Qian, G. Y. Li, “Broadbeam for massive MIMO systems”, IEEE Trans. Signal Process., vol. 64, no. 9, pp. 2365 – 2374, May 2016

[VRM01] J. Vuolevi, T. Rahkonen, and J. Manninen: "Measurement technique for characterizing memory effects in RF power amplifiers", IEEE Transactions on Microwave Theory and Techniques, Vol. 49, Issue 8, pp. 1383 - 1389, 2001.

[ZC02] Zhou, Xuefu, and J. Jr Caffery. "A new distribution bound and reduction scheme for OFDM PAPR." Wireless Personal Multimedia Communications, 2002. The 5th International Symposium on. Vol. 1. IEEE, 2002.

[ZCS+17] Y. Zou, M. Castañeda, T. Svensson and G. Fettweis, “Sequential Hybrid Beamforming Design for Multi-Link mm-wave Communication”, IEEE Globecom 2017, Singapore, submitted.




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[ZRG16] Y. Zou, W. Rave and G. Fettweis “Analog Beamsteering for Flexible Hybrid Beamforming Design in Mm-wave Communications,” in Proceedings of European Conference on Network and Communications (EUCNC) 2016.

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