Wireless Terahertz System Applications for Networks Beyond 5G · TERRANOVA Consortium Wireless...

TERRANOVA Consortium Wireless Terahertz System Applications for Networks Beyond 5G

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TERRANOVA CONSORTIUM

Wireless Terahertz System Applications for Networks Beyond 5G

Version 1.0, March 2019


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Contributors and Editors

Name Company/Institute/University Doc Version

Editorial Team

José Machado Altice Labs, Portugal Document Editor

Contributors and reviewers

José Machado Altice Labs, Portugal V0.1 First ToC version

José Machado Altice Labs, Portugal

V0.2 ToC Revision and content insertion on Section 2 - TERRANOVA system Applications, Use Cases & Scenarios. Request for partner contributions.

José Machado Altice Labs, Portugal V0.3 Remove of the Business model chapter. Edition and revision of chapters 4 and 5

Alexandros-Apostolos A. Boulogeorgos

University of Piraeus, Greece V0.4 Edition of the sub-chapter 3.1 Latency related

Robert Escher Fraunhofer HHI, Germany V0.5 Removal of the chapter relating candidate architectures

José Machado Altice Labs, Portugal V0.6 Consolidation and Minor changes. Review on the Acronym list


V0.7 Remove of the “chapter 5-Challenges and physical limitations” and integrate some of the content on the “chapter 3- system requirements”. Edition on the “3.1 - Link latency sub-chapter”.

Ricardo Ferreira PICadvanced, Portugal V0.8 Revision and edition of the “chapter 3.3 -

Joonas Kokkoniemi Oulu University, Finland

V0.8 Revision and edition of the “chapter 3.2. - THz link range considerations” and “chapter 3.5. - THz link throughput considerations”

Nikolaos Kokkalis, Dimitrios Kritharidis, Georgia Ntouni

Intracom Telecom, Greece V0.8 “chapter 3.4. Number of connections per THz node considerations”

Sajid Mushtaq JCP Connect, France V0.9 “chapter 3.1.1.Caching related aspects” contribution


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Joonas Kokkoniemi Oulu University, Finland

V0.9 Revision on “chapter 3.2. - THz link range considerations” and “chapter 3.5. - THz link throughput considerations”


V0.9 Minor context revision on chapters 3.1.1, 3.2, 3.4 and 3.5. Edition of the Executive Summary, Structure and Conclusions chapter. Also review on the references list.

Georgia Ntouni Intracom Telecom, Greece V0.10 General review on chapters 1 and 2.

Joonas Kokkoniemi Oulu University, Finland V0.10 General document review.

Alexandros-Apostolos A. Boulogeorgos

University of Piraeus, Greece V0.10 General Chapter 3 review and other minor changes.

Sajid Mushtaq, Jean-Charles JCP Connect, France V0.10 review on section 2 and chapter 3.1.1. Caching related aspects

José Machado Altice Labs, Portugal V0.10 General edition documentation review.

José Machado Altice Labs, Portugal V1.0 Official final version release


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Executive Summary The present white paper focuses on the wireless terahertz system applications for communication networks beyond 5G in the context of the TERRANOVA H2020 project. This document translates the state of the art knowledge as reflected among TERRANOVA consortium partners in what relates to the specific applications for the Optical–Terahertz system that will be expected as the outcome from the project itself. The white paper initially details the potential applications that are reflected on key use cases for the TERRANOVA technology classified into two main categories namely, Backhaul & Fronthaul and Mobile & Fixed Wireless Access. The first category refers to Fibre Extender, Point-to-point, Redundancy, Backhaul and Fronthaul applications, whereas the latter is designed to support Corporate Backup, Ad-hoc Access, Mobile & Last Mile Access, Indoor Short Range, Dense IoT and Data Centres communication. All referred applications are then classified among three different technical scenarios namely, outdoor fixed point-to-point (P2P), outdoor/indoor point-to-multipoint (P2MP), and outdoor/indoor “quasi” omnidirectional. For each technical scenario, a group of corresponding key performance system requirements was defined together with a critical analysis concerning its practical implementations. Finally, generic network and system candidate architectures are proposed for the TERRANOVA Optical – Terahertz system in the context of Fixed, Mobile and Indoor wireless communications.


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Table of Contents

Contributors and Editors ......................................................................................................................... 2

Executive Summary ................................................................................................................................ 4

Table of Contents ................................................................................................................................... 5

List of Figures ......................................................................................................................................... 6

List of Tables .......................................................................................................................................... 7

List of Acronyms and Abbreviations ........................................................................................................ 8

Polarization Multiplexed Quadrature Amplitude Modulation .................................................................. 9

1. Introduction .................................................................................................................................. 11 1.1. Scope & Contribution ......................................................................................................................... 11 1.2. Structure ........................................................................................................................................... 11

2. TERRANOVA System Applications, Use Cases & Scenarios .............................................................. 11 2.1. Backhaul & Fronthaul ......................................................................................................................... 12 2.2. Mobile & Fixed Wireless Access .......................................................................................................... 15

3. TERRANOVA System Requirements ............................................................................................... 17 3.1. THz link latency aspect considerations ................................................................................................ 18 3.2. THz link range considerations ............................................................................................................. 20 3.3. Optical link range considerations ........................................................................................................ 20 3.4. Number of connections per THz node considerations .......................................................................... 20 3.5. THz link throughput considerations .................................................................................................... 21 3.6. Other considerations ......................................................................................................................... 22

TERRANOVA System Architecture ......................................................................................................... 23

4. Conclusions ................................................................................................................................... 26

References ........................................................................................................................................... 27


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List of Figures FIGURE 1: TERRANOVA APPLICATIONS, USE CASES AND SCENARIOS ............................................................................................................. 12 FIGURE 2: THE CONCEPT OF WIRELESS FIBRE EXTENDER. ................................................................................................................................ 13 FIGURE 3: GENERAL POINT-TO-POINT APPLICATION ...................................................................................................................................... 13 FIGURE 4: REDUNDANCY APPLICATION....................................................................................................................................................... 14 FIGURE 5: BACKHAUL APPLICATION ........................................................................................................................................................... 14 FIGURE 6: 5G FRONTHAUL APPLICATION .................................................................................................................................................... 14 FIGURE 7: CORPORATE BACKUP APPLICATION .............................................................................................................................................. 15 FIGURE 8: PHYSICAL NETWORK ARCHITECTURE............................................................................................................................................. 23 FIGURE 9: SCHEMATIC DEPICTION OF OPTICAL-WIRELESS SYSTEMS FOR REPLACEMENT OF FIBRE LINK BY A WIRELESS THZ LINK (ABOVE) AND AN INDICATIVE

EXAMPLE FOR AN OPTICAL-WIRELESS SYSTEM WITH OPTICAL RF-FRONTEND BASED ON 100GBASE-LR4 QSFP 288 TRANSPONDER MODULES

(BELOW). .................................................................................................................................................................................... 25 FIGURE 10: PTMP FIXED WIRELESS ACCESS (BACKHAUL) .............................................................................................................................. 25 FIGURE 11: PTMP MOBILE WIRELESS ACCESS ............................................................................................................................................ 26 FIGURE 12: PTMP INDOOR WIRELESS ACCESS ............................................................................................................................................ 26


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List of Tables TABLE 1: MAPPING BETWEEN THE USE CASES AND THE TECHNICAL SCENARIOS. .................................................................................................. 17 TABLE 2: BASIC KPIS FOR EACH TECHNICAL SCENARIO ................................................................................................................................... 18 TABLE 3: INDICATIVE IA LATENCY IN MAC .................................................................................................................................................. 19 TABLE 4: IA SIMULATION PARAMETERS ...................................................................................................................................................... 19


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List of Acronyms and Abbreviations

Acronym/Abbreviation Description

4G Forth Generation

5G Fifth Generation

ACO Analog Coherent Optics

ADC Analog-to-Digital Converter

AM Amplitude Modulation

AWG Arrayed Waveguide Gratings

BB BaseBand

BER Bit Error Rate

BF BeamForming

BH BackHaul

BS Base Station

CC Central Cloud

CFP C-Form Factor Pluggable

CoMP Coordination Multi-Point

DAC Digital to Analog Converter

DL DownLink

DSP Digital Signal Processing

DWDM Dense Wavelength Division Multiplexing

EC European Commission

eMBB Enhanced Mobile Broadband

E/O Electrical-Optical

ETSI European Telecommunications Standards Institute

FH FrontHaul

FSO Free-Space Optics

FWA Fixed Wireless Access

G.fast Transmission Technology for Telephone Lines up to 1 Gbit/s

I/Q In-phase and Quadrature

IEEE Institute of Electrical and Electronics Engineers

IF Intermediate Frequency

IoT Internet of Things


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IM/DD Intensity Modulation/Direct Detection

IP Internet protocol layer

ITU International Telecommunication Union

ITU-R Radiocommunication sector of the International Telecommunication Union

KPI Key Performance Indicator

LO Local Oscillator

LoS Line of Sight

MAC Medium Access Control

MAL Molecular Absorption Loss

MIMO Multiple Input Multiple Output

MMIC Monolithic Microwave Integrated Circuit

mmWave Millimeter Wave

MUX MUltipleXer

NGPON2 Next-Generation Passive Optical Network 2

nLoS Non-Line Of Sight

NRZ Non-Return to Zero

OFDM Orthogonal Frequency Division Modulation

OLT Optical Line Terminal

ONUs Optical Network Units

OpEx Operating Expenses

P2MP Point-to-Multi-Point

P2P Point-to-Point

PAM Pulse Amplitude Modulation

PCB Printed Circuit Board

PDM Polarization-Division Multiplexing

PDM-QAM Polarization Multiplexed Quadrature Amplitude Modulation

PER Packet Error Rate

PHY PHYsical

PL PathLoss

PONs Passive Optical Networks

QAM Quadrature Amplitude Modulation

QoE Quality of Experience


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QoS Quality-of-Service

QSFP Quad Small Form-Factor Pluggable

RA Random Access

RAN Radio Access Network

RAT Radio Access Technology

RAU Remote Antenna Unit

RF Radio Frequency

RH Radio Head

RoF Radio over Fiber

RRM Radio Resource Management

RX Receiver

SC Small Cell

SDN Software Define Network

SFF Small Form Factor

SFP Small Form-Factor Pluggable

SMF Single Mode Fiber

SNR Signal to Noise Ratio

SOTA State Of The Art

TDM Time Division Multiplexing

TDMA Time Division Multiple Access

TERRANOVA Terabit/s Wireless Connectivity by Terahertz innovative technologies to deliver

Optical Network Quality of Experience in Systems beyond 5G

THz Terahertz

TWDM Time and Wavelength Division Multiplexed

Tx Transmitter

UE User Equipment

UL Uplink

URLLC Ultra Reliable Low Latency Communications

VLC Visible Light Communication

WLAN Wireless Local Area Network

WDM Wavelength Division Multiplexing

WiFi Wireless Fidelity


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1. Introduction Wireless data traffic has drastically increased accompanied by a growing demand for higher data rate transmissions. In particular and according to the Edholm’s law of bandwidth [1], wireless data rates have been doubled every eighteen months over the last three decades and are quickly approaching the capacity of wired communication systems [2]. In order to address this tremendous capacity demand, the mobile world has moved towards the fifth generation (5G) era, by introducing several novel wireless approaches, such as massive multiple-input multiple-output (MIMO) systems, full-duplexing, and millimetre wave (mmWave) communications. However, there is a lack of efficiency and flexibility in handling the huge amount of quality of service (QoS) and experience (QoE) oriented data services [3]. In view of the fact that the currently used frequency spectrum for 5G has limited capacity, THz wireless became an attractive complementing technology to the less flexible and more expensive optical-fibre connections as well as to the lower data-rate systems, such as visible light communications (VLC), microwave links, and wireless fidelity (WiFi) [4], [5], [6], [7], [8], [9], [10]. As a consequence, by enabling wireless THz communications, we expect not only to address the spectrum scarcity and capacity limitations [11] of the current cellular systems, but also influence the main technology trends in wireless networks within the following ten years and beyond. At this specific white paper, we will be focusing on the expected fields of application for the “fibre optic - THz wireless” novel technology organized by the specific applications, use cases and corresponding technical scenarios.

1.1. Scope & Contribution Motivated by the potentials of the optical-wireless THz system technology, this white paper is devoted to the presentation of beyond 5G envisioned TERRANOVA applications, which are expected to be enabled by THz technologies. Besides, after the definition of the potential applications, use cases and corresponding scenarios, we will also focus on the corresponding system requirements and related considerations. Basic network and system architectures will be presented, while the corresponding challenges and physical limitations will be taken into consideration.

1.2. Structure The white paper is organized as follows. In Section 2, the TERRANOVA system candidate applications are presented leading to corresponding use cases and technical scenarios. Likewise, Section 3 focuses on pointing and describing the system requirements associated with the three technical scenarios that were envisioned by the TERRANOVA consortium for the beyond 5G networks and system applications. On Section 4, generic candidate network and system architectures are presented for the TERRANOVA Optical-Terahertz technology. Finally, Section 5 summarizes our observations and concludes the white paper.

2. TERRANOVA System Applications, Use Cases & Scenarios This section summarizes the identified applications, use cases and scenarios that are considered relevant applications domain of the TERRANOVA technology. In more detail, the use cases are classified into two categories, namely Backhaul & Fronthaul and Mobile & Fixed Wireless Access. First category refers to fibre extender, Point-to-Point (P2P), redundancy and backhaul/fronthaul applications. Second category is designed to support corporate backup connections for large companies and SMEs, Internet of Things (IoT)


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dense environments, data centres, indoor wireless access, ad-hoc networks, sport and music events, and last mile general access. Each one of those applications may also be framed on the several defined scenarios namely: Outdoor Fixed P2P, Outdoor / Indoor Point-to-Multi-Point (P2MP) and Indoor / Outdoor “quasi” Omni Directional leading to corresponding technical specifications.

Figure 1: TERRANOVA Applications, Use Cases and Scenarios

2.1. Backhaul & Fronthaul The backhaul and fronthaul use case is mostly related with the classical network operator’s topologies, where the TERRANOVA outcome applications are to be fitted. Specifically, it relates with the communication to or between cell towers (backhaul) or between remote radio heads located at the cell towers and centralized baseband units (fronthaul). Backhaul & Fronthaul applications can be classified into wireless fibre extender, P2P applications, redundancy links as well as the classical backhaul/fronthaul Telco applications. Next, we focus on presenting the particularities of these applications.

2.1.1. Wireless fibre extender Data rates in both fibre-optic and wireless communications have been increasing exponentially over recent decades. For the upcoming decade, this trend seems to be unbroken, at least as far as fibre-optic communications is concerned. On the other hand, in wireless communications, the spectral resources are extremely limited, because of the heavy use of today’s conventional frequency range up to 60 GHz. Even with highly spectral efficient Quadrature Amplitude Modulation (QAM) and the spatial diversity achieved with MIMO technology, a significant capacity enhancement to multi-gigabit or even terabit wireless transmission rates requires larger bandwidths, which are only available in the high mmWave and THz region [12]. Between 200 and 300 GHz, there is a transmission window with low atmospheric losses. In contrast to Free-Space Optical (FSO) links, mmWave or THz transmission is much less affected by adverse weather conditions like rain and fog. Based on these facts, wireless fibre extender in THz band is considered as a key application for TERRANOVA, as illustrated in Figure 2. This application scenario is especially interesting, when it is intended to provide reliable data communication with very high data throughputs of up to 1 Tbps for distances up to 1 km on adverse geographies, such as lakes, rivers and dams, or even on environments,

Use

Ca

se

sA

pp

lica

tion

sS

ce

na

rio

s

Corporate Backup Ad-hoc Access Mobile & Last MileFibre Extender Point-to-Point Redundancy

Indoor Short Range Dense IoT Data CentresFronthaul

Outdoor Fixed P2P

(Scenario 1)

Outdoor / Indoor P2MP

(Scenario 2)

Indoor / Outdoor “quasi” Omnidirectional

(Scenario 3)

Backhaul & Fronthaul Mobile & Fixed Wireless Access

OLT

AW

G

ONU

ONU

PON

ONU

OLT

CU

AW

G

RU

RU RUDU

Backhaul


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where it is not allowed to perform civil work construction for a certain period of time due to regulatory constraints. Currently, rural users suffer low-connectivity problem, but in the future they will have an opportunity to enjoy high data rates, such as 10 Gbps. TERRANOVA wireless fibre extender has low-complexity setup, high flexibility and low total cost of ownership when compared to the fibre deployment, which makes it appropriate for this type of applications.

OLT

AW

G

ONU

ONU

PON

ONU Fiber

TERRANOVA Link Figure 2: The concept of wireless fibre extender.

Currently, there are some commercial alternatives, which allow data communication with high throughput on adverse geographies, such as FSO. However, FSO has significant performance limitations related to the weather conditions [13], [14], which typically limit its reliable use for distances up to 1 km.

2.1.2. Point-to-Point applications P2P application scenario refers to general P2P data communication (shown in Error! Reference source not found.), with low latency and very high data rate (up to 1 Tbps) for distances up to 1 km. P2P is an alternative solution when there is no access to fibre infrastructure at all (in opposition to the previous application), either due to technical or regulatory constrains that limit the fibre deployment, or even due to investment cost reasons. A more specific application is related to the stock exchanges applications, where very high throughput and extremely low latencies are critical for High Frequency Trading (HFT) [15]. HFT players trade on milliseconds of differences between bid and ask quotes on the same or different stock exchanges. To achieve this HFT, players install equipment in or close to the stock exchange data centre, paying a fee for the data transfer in advance of other market players.

Figure 3: General point-to-point application

2.1.3. Redundancy The redundancy application scenario is also a relevant use case, in which critical services can benefit from the use of the wireless THz technology as backup for existing fixed line technologies (namely fibre). Error! Reference source not found. represents the scenario, when there is the possibility of enabling throughputs up to 0.1 Tbps with QoS requirements for sensitive applications (availability ~99.999%) that must operate, in redundancy to the failure paths, due to natural phenomena, power outage or simply service loss.


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Figure 4: Redundancy Application

2.1.4. Backhaul / Fronthaul The classical Backhaul / Fronthaul application fits the operator’s at mobile network domain. At the backhaul TERRRANOVA link may be used to feed the cell sites from a central point to distributed geographical locations in P2P or P2MP topologies as illustrated by Error! Reference source not found.. With a very high throughput, around 0.1 Tbps, and distance range of up to 1 km, several cell sites may be feed by TERRANOVA links, especially dense urban areas assisting the increase and reinforcement of 4G and 5G mobile coverage. The central location will afterwards route the aggregated traffic to the fibre backbone.

Figure 5: Backhaul Application

TERRANOVA links may also have impact on future 5G fronthaul topologies by reducing the need for fibre extended capillarity as well as expensive radio coaxial cabling, from the Distributed Units (DU) up to the Radio Unit (RU), where the need of a very high throughput is a must to carry traffic over 10 Gbps per RU. This application will also optimize the fibre deployment resources as well as the number of DUs at the field.

OLT

CU

AW

G

RU

RU RUDU

Fiber

TERRANOVA Link5G Link

Figure 6: 5G Fronthaul Application

Fiber




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2.2. Mobile & Fixed Wireless Access This subsection details the mobile and fixed wireless access use cases for TERRANOVA’s technology applications that we consider to be in the context of the optical/THz system communications. More specifically, these applications are corporate backup connections, IoT dense environments, data centres, indoor wireless access and ad hoc networks, where sport and music events are the major part of nomadic applications. Finally, the last mile access is also one of the targeted applications within the classical telecom operator’s network.

2.2.1. Corporate backup connection The corporate backup connection is an application scenario, where it is intended to have a backup data communication link for large/medium/small enterprises with very high data throughputs needs on small distances (up to 1 km), shown by Error! Reference source not found.. In these cases, there is no access to a backup fibre access infrastructure, either due to regulatory constrains or even due to the cost of using/renting existing telecom links. Owning to the very high data throughputs (up to 0.1 Tbps), a single backup link using TERRANOVA technology could be used to serve several enterprise networks.

Fiber

TERRANOVA Link

Figure 7: Corporate Backup application

2.2.2. IoT dense environment Full adoption of digital networking in industry, commerce and public services, including traffic control and autonomous driving, remote health monitoring services, supply chain, security and safety procedures, automation of large production sites, places stringent requirements for Tbps class access subject to fast response constraints [16]. These application scenarios describe the true colours of the well-known IoT. The IoT dense environments are an interesting use case, in which TERRANOVA can be used to leverage industrial networking with very high data rates (up to 0.1 Tbps), high reliability (application dependent) and low latency (less than 1 ms). Thanks to THz band frequency range and high antenna directivity, TERRANOVA can provide a high level of immunity to environments that have high interference at typical lower frequencies (usually up to the GHz frequency range). This scenario is particularly interesting in Industry 4.0 for short- to medium-range industrial networking (up to 500 m) where, due to technical, regulatory or economic constraints, it is not viable to use optical fibre links.

2.2.3. Data centres The use of TERRANOVA technology in data centres has the potential of allowing ultra-high speed wireless data distribution (up to 0.2 Tbps) per link for short range (up to 100 m) in noisy environments. In this sense, TERRANOVA could be used for establishing multiple links between processing and/or data storage racks with the potential advantages of simplifying the installation, reducing the amount of wired circuits


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that interconnect racks and possibly reducing the rack space utilization. This application scenario is also appealing considering that data centres have high electromagnetic noise in the GHz frequency range. TERRANOVA architecture can provide a high level of immunity and reliability of connectivity in a low mobility environment.

2.2.4. Indoor short-range THz access The increasing demand for higher indoor data rates makes an attractive application scenario for TERRANOVA. In this scenario, the indoors access network would experience a significant boost in terms of aggregated data rates (up to 0.3 Tbps), enabling new dimensions of interconnectivity and QoE for very short range communications (up to 20 m), allowing wireless connectivity for specific applications, such as high-definition holographic video conferencing (i.e., virtual reality office).

2.2.5. Ad-hoc access

Ad-hoc business access is an interesting scenario for emergency situations, where it is important to have low latency, very high data throughputs (up to 0.1 Tbps) and reliable communication for small distances (up to 500 m). Music festivals in remote places, concerts or sports events with demanding special effects (like holographic images) could benefit from the temporary installation of TERRANOVA outcomes in the form of equipment. Here, a key factor is the reduced setup time, because the TERRANOVA technology is expected to require limited installation time in the order of a few hours per link.

2.2.6. Nomadic Access (sport, music events, etc.)

Nomadic outdoors events, including sports and music events, are very interesting use cases, due to the predictable short time duration and the requirement for massive data throughputs [17]. Unlike the applications in Section 2.2.5, where the event is organised in a venue without suitable infrastructure and TERRANOVA based solutions must be installed and be removed afterwards, in this case we focus on permanent installation in stadiums, entertainment centres, etc. When the number of communication devices is significantly increased in a specific area, the communication traffic will also dramatically increase. As a consequence, the users’ QoS and QoE will decrease. On the other hand, it is expected from the communication provider to guarantee the quality of the service. In such scenarios, due to technical or economic constraints, it may not be viable to use optical fibre links. To this end, TERRANOVA technology is a better alternative, due to the reduced setup time, the very high aggregated data throughputs (up to 0.2 Tbps) and the reliability of communication for the expected small average distances (up to 500 m).

2.2.7. Last mile access The last mile access is a key application scenario for TERRANOVA. This application scenario is especially interesting, when it is intended to have data communication with very high aggregated data throughputs (up to 0.1 Tbps) on the last mile access (up to 1 km) and where, due to technical, regulatory or economic constraints, it is not viable to use optical fibre links. The easy installation is also a relevant point here, since it allows a faster deployment of the last mile access network, thus increasing the revenue margin from the service subscribers. Nowadays, technologies such as G.fast (ITU-T G.9701) that use low RF frequencies over twisted pair or coax lines are commonly utilized when deploying fibre in the last mile. However, this technology is limited in terms of bandwidth and will not map with applications requiring massive data throughputs. In this


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sense, TERRANOVA may become a natural evolution for extending higher bandwidth fibre access including NG-PON2 and future PON standards. In all the above scenarios, TERRANOVA is an attractive complementary technology to more expensive and less flexible optical fibre connections and to the lower data rate wireless technologies including VLC, FSO, mmWave and WiFi. It is widely known that FSO have significant performance limitations related to the weather conditions including rain and fog, which typically limit its reliable use for long links [13]. On the other hand, microwave and mmWave links are more robust to adverse weather conditions, but the state of the art (SOTA) throughputs are far below the required in the above application scenarios [18], [19].

3. TERRANOVA System Requirements The implementation of wireless THz systems will have to leverage breakthrough novel technological concepts. Indicative examples can be considered the joint-design of baseband digital signal processing (DSP) for the complete optical and wireless link, the development of broadband and highly spectral efficient radio frequency (RF) frontends operating at frequencies higher than 275 GHz, and new standardized electrical-optical (E/O) interfaces. Additionally, in order to address the extremely large bandwidth and the propagation properties of the THz band, improved channel modelling and the design of appropriate waveforms, multiple access control (MAC) schemes and antenna array configurations are also required. This section is devoted to derive the requirements on the link performance for the relevant use case scenarios for the co-designed THz and fibre-optical network. Following link and system key performance indicators (KPIs) should then be evaluated and taken into account:

o Aggregate throughput of wireless access for any traffic load/pattern; o Throughput of the point-to-point ‘fibre optic - THz wireless’ link; o Link latency of the ‘fibre optic - THz wireless’; o Range of the ‘fibre optic - THz wireless’ link; o Reliable communications (probability of achieving a target bit error rate - BER and packet error rate

PER); o Availability (‘Always’ available connectivity of ‘infinite’ number of devices).

Table 1 illustrates the relationship between application use cases, the technical scenarios as defined at Figure 1 and the corresponding fundamental requirements.

Table 1: Mapping between the use cases and the technical scenarios.

Application Use Case Scenario Use case fundamental requirements

Wireless fibre extender 1 Data Rate: 1 Tbps, Range: 1 km

P2P 1 Data Rate: 1 Tbps, Range < 1 km

Redundancy 1 Data Rate: 0.1 Tbps, Availability: 99.999 %

Corporate backup connection

1 Data Rate: 0.1 Tbps, Range: 1 km

IoT dense environment 2 and 3 Data Rate: 0.1 Tbps, Latency: < 1 ms,

Reliability (target BER): Application dependent

Data centres 2 Data Rate: 0.2 Tbps, Range < 100 m

Indoor Short range THz access

2 and 3 Data Rate: up to 0.3 Tbps, Range < 20 m


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Ad-hoc access 2 and 3 Data Rate: 0.1 Tbps, Range 500 m, Target

setup time < 1 hour

Backhaul / Fronthaul 1 and 2 Data Rate: 0.1 Tbps, Range: 1 km / Data

Rate: 1 Tbps, Range: 1 km

Nomadic Access (sport, music events, etc.)

2 Data Rate: 0.2 Tbps, Range 500 m

Last mile access 2 Data Rate: 0.1 Tbps, Range: 1 km

following Table 2 details the KPI values that should be taken in consideration according with the considered technical scenarios.

Table 2: Basic KPIs for each technical scenario

KPI Scenario 1 Scenario 2 Scenario 3

Max. THz link latency (ms) Application dependent < 1ms

Max. THz link range (m) 1000 500 10

Max. optical (wired) link range (km) 50 10 1

Number of connections per THz node 1 10 100

Max. THz link throughput x range (Gbps x m) 1000 x 1000 100 x 1000

10 x 10

Max. THz link throughput (Gbps) x connections (= aggregate throughput)

1000 x 1 100 x 10 10 x 100

Target BER 10-12 Application dependent

Application dependent

Availability Critical Critical Application dependent

As previously considered the several KPI parameter values are defined by the numerical boundaries according with the considered application scenarios in the context of TERRANOVA scope of work. Subsequently, we will further comment on the several considerations currently being taken into account, when individually assessing each one of the KPIs considering the defined scope of applications.

3.1. THz link latency aspect considerations Latency is often referred as a relevant KPI when assessing communications performance parameters. On our study, it was defined as application dependent having in any cases reference values below 1ms considering the several application scenarios (please see Table 2). In the context of the TERRANOVA project, the consortium has identified the initial access (IA) latency as the source of latency that may be under our scope of study that is directly verified at the MAC layer level. For the IA, the system time is divided into different IA cycles, with a predefined period, T. In each cycle, the base-station (BS) or the access point (AP) initiates a cell search (CS) procedure, during which it sweeps through M non-overlapping sectors and broadcasts synchronization signals with period Ts. Meanwhile, the user equipment (UE) sweeps through N receive beamforming directions to detect the received signal. This procedure is followed by a random access (RA) phase, in which the UE transmits a connection request of period, Ts, to the BS. Note that this IA procedure is in line with several directional MAC protocols as well as many standards, such as the IEEE 802.11ay. Next, we present in Table 3 indicative simulation results in order to quantify the IA latency. The simulation parameters that were used are summarized in Table 4.


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Table 3: Indicative IA latency in MAC

Ts (μs) L = M N Latency IA latency

(ms) CS RA CS (ms) RA (ms)

10

16 16

3.2 3.2 6.4

50 16 16 32

100 32 32 64

10

64 64

12.8 12.8 25.6

50 64 64 128

100 128 128 256

10

1024 64

204.8 12.8 216.6

50 1024 64 1088

100 2048 128 2166

Table 4: IA simulation parameters

Parameter Value

Synchronization signal frequency 300 GHz

Synchronization/RA signal bandwidth 50 kHz

Relative humidity 50 %

Pressure 101235 Pa

Temperature 296 oK

Communication distance 40 cm

Beamforming type Analogue

Transmission to noise power ratio 40 dB

Number of IA cycles 1

Note that the IA Latency simulations as evaluated by the TERRANOVA consortium, are not in any case comparable with the application dependent latency (which refers the general round trip Latency term) that should be considered on a communication oriented perspective (application dependent) that is currently out of the study scope of TERRANOVA.

3.1.1. Caching related aspects The high-frequency communication networks provide high bandwidth, but they suffer communication blockage as transmission signals unable to penetrate walls or even people, and it is a main hurdle for a practical communication network. Similarly, THz system faces the problem of blockage and deafness, which result in communication interruption. The communication interruption caused by blockage could be temporary and frequent. In this case, the caching system caches the data at the edge network which saves backhaul bandwidth, reduces latency, and also UE or AP local caching directly improve the user QoE in two aspects. First, the availability of high bandwidth allows the end-user to download required content into local storage, so that the impact of blockage time will be seamless for the end-use. Second, the prefetching method, which makes it possible that required contents will be available in advance at the edge network. The cached content at the edge network device reduces the overall latency, as it reduces the frequent communication with the core network. In other words, if the needed contents are already prefetched in the edge, the end-user has an opportunity to download the contents during the connection time. During blockage time, the end-user does not face blockage, because the contents are partially retrieved from the edge network in UE local storage or local Access Point, thanks to high-bandwidth availability, and data rate characteristics of THz networks.


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The prefetching in caching system also plays a vital role during the handover, as the end-user required content already prefetched at the edge network that avoids content retrieval interruption, and improves the user QoE. Thanks to caching system, which decreases the latency and improves both backhaul and fronthaul resource utilization by removing redundant retrievals of the same contents. The placement of contents at the edge network not only decreases the latency, but it also compensates the deafness delay.

3.2. THz link range considerations Channel losses aggregate quickly when moving to higher frequencies. This in general limits the THz regime to short range communications. Large signal path loss at these frequencies is compensated by very large antenna gains. Arbitrarily high antenna gains theoretically increases the link distance to infinity. In the context of TERRANOVA, the project consortium have shown (deliverable D2.1 Table 3) the calculations on theoretical minimum transmit powers for 1000 meter link with aggregated throughputs about 1 Tbps. Depending on the band, the minimum transmit power ranges from 130 – 200 dBm. This means that if the link is enabled with a single antenna, a 100 + 100 dBi (Tx + Rx) gain is required in worst case for 0 dBm source. Adding hardware imperfections as well as other real (non-ideal) system conditions make such a link very hard to achieve in a practical system. Theoretically it is possible, but many challenges lie ahead from the hardware point of view.

3.3. Optical link range considerations At this point in time the optical technology is mature enough in order not to be a bottleneck for the distance range communication throughputs in the context of the TERRANOVA foreseen applications. Optical distance ranges up to 80 km @ 100Gbps optical bitrate are currently market available having multiple wavelengths travelling over a single SMF. For the considered high bitrates Chromatic Dispersion and Polarization Mode Dispersion are the main optical impairments to take into account. The Chromatic Dispersion is a phenomena that depends on the fiber reach and also on the symbol rate operation. Since most of the high bandwidth available systems are based on coherent detection using digital signal processing (DSP) units at the receiver side, the dispersion is easily compensated for distances beyond hundreds of kilometers using for instance finite impulse response (FIR) filters. However, a solution without DSP may be limited in terms of fiber reach. As an example, if 32 Gbaud signals are employed, the Chromatic Dispersion may be critical for distances beyond 10 km. Therefore, the optical link range will depend on the employed transceiver and also on the target bitrate. If a coherent transceiver combined with advanced DSP is used, the fiber link can range up to tens or hundreds of kilometers. On the other hand, if a coherent solution without DSP or a solution based on direct detection is employed, the fiber's range may be limited to a few tens of kilometers.

3.4. Number of connections per THz node considerations In any Point-to-Multipoint system, one of the key system parameters that needs to be defined from the outset, based on the specific requirements of the targeted deployment, is the number of users to be served by the system. The number of users comes usually as a requirement, according to which, important design decisions will be taken for key system parameters. Such a parameter in the original specification, originating from customer requirements, is the user profile. For example, in order to serve users requiring an average of 100 Mbps during peak time, with an available


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throughput of 10 Gbps, it makes no sense to design for fewer than 100 users. Then, depending on the quality of service required, an appropriate multiplier will be applied. Another important parameter is the airframe, which must be appropriately designed in order to support the selected number of users. In such a design decision, the main trade-offs are usually the number of users vs. desirable latency and vs. required resources. Additional elements taken into account are the number of users that could be served in a frame - as it is not necessary that all users are served in each frame - as well as the frame duration. At L1 and L2/L3 stages, adequate processing power must be designed in, to meet both the throughput and number of users’ requirements. Other features that play a role in the design, are the type of multiplexing used in the downlink direction (e.g., TDM, FDM, SDM, OFDM, etc.), the multiple access scheme used in the uplink (TDMA, FDMA, SDMA, OFDMA, etc.), as well as the type of transmission in the downlink (e.g., burst or broadcast). Depending on the use case, downlink and uplink directions will have in general different characteristics; for example, in a broadcast downlink it is easier to support a larger number of users with small latency, whereas the uplink, due to its multiple access nature, presents increased latency and will effectively determine the maximum number of users. In a PtMP system with beam switching though, the downlink latency will increase, since each terminal will have to wait for its beam’s turn. In a MU-MIMO case, users could be served in parallel, but then, limiting factor becomes the available processing power. Furthermore, a beamforming system will introduce increased overhead due to necessary gaps in time, frequency or space, in order to adequately separate the beams. Network slicing types, as in general all QoS considerations, will have to be taken into account, as all slices should be simultaneously served (e.g., eMBB and URLLC), while guaranteeing each slice’s required characteristics. Concluding, as there is a multitude of parameters that need to be taken into account in such a network dimensioning, a combination of trade-offs on key system parameters, is required.

3.5. THz link throughput considerations The THz frequency domain offers vast allocation resources resulting on very high transmission throughputs. This is especially the case for short distance ranges (in the range of few meters), where atmospheric losses have a small impact. Spatial multiplexing technologies with large antenna gains theoretically allow large data rates for larger number of users in a small area. As the link distance increase, the atmospheric losses start to cut the frequencies out into smaller low loss regions. Going to extreme use cases and taken into account the work that has been developed within TERRANOVA consortium, 1000 meters range with 1000 Gbps throughput connection scenarios will unavoidably require frequency aggregation. But, as mentioned in Section 3.2, the extreme distances require extreme antenna gains. From the throughput point of view, the problem here becomes with the frequency aggregation. At 1000 meters of link distance, 1 Tbps bandwidth connection most likely requires aggregation of all atmospheric windows between 275 – 1000 GHz. This causes very tight linearity requirements for the hardware. This is possible from the theoretical point of view, but it also faces some severe practical limitations either the distance or the bandwidth. However, the aggregation could be done by separate receivers, each focusing on certain transmission window. This way the hardware requirements are easier to realize, but a lot of development is required until this is possible. From a practical implementation perspective, 100 users at 10 Gbps throughput each is far easier to achieve than a single Tbps link with 1000 meter distance. At shorter distances antenna gain requirements are not as high, and by combining smaller channel loss into spatial multiplexing gives much more room to move what comes to number of users and maximum throughput. A 1 Tbps aggregated throughput is a great challenge in any case due to large bandwidth required for it. A


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significant part of the TERRANOVA project is to be among the first entities to show hardware solutions that can demonstrate that this goal is possible.

3.6. Other considerations We should also refer some of the fundamental considerations that will affect the design of the wireless THz system. These characteristics can be summarized as follows:

In the THz region, because of the small wavelength, we are able to design high directional transmit

antennas and receive antennas with low acceptance angle. These antennas are employed to

countermeasure the high channel attenuation. However, at the same time, they require an

extremely accurate alignment between the communication nodes.

The high material absorption in the THz band makes doubtable the use of the NLOS

communications. As a consequence, beam tracking schemes as well as coordination multi-point

needs to be used in order to guarantee uninterrupted communication.

Molecular absorption in the THz frequencies causes frequency and distance dependent pathloss,

which makes specific frequency windows unsuitable for establishing a communication link.

Therefore, although the high bandwidth availability in the THz region, windowed transmission with

time varying loss and per-window adaptive bandwidth usage is expected to be employed.

In order to increase the links capacity, suitable MIMO techniques in combination with beamforming

need to be employed.

In order to countermeasure the impact of deafness and support tracking of mobile or moving UE,

adaptive beamsteering is expected to be utilized. Adaptive beamsteering enables the low-

complexity link installation and guarantees that the TX and RX antennas are aligned.

In all cases, due to small wavelengths, there are high requirement on intra and inter beam

coherence.

Furthermore, due to small wavelengths, multi-path fading in case of NLOS link will be quickly

changing already at small spatial movements leading to highly time-variable non-flat channel

characteristics in nomadic applications.

Finally, adaptive modulation coding (AMC) schemes will be employed in order to increase both the

range and the throughput of the THz system, while, at the same time, guarantee a pre-defined

degree of reliability.

Due to the fundamental characteristics of the THz systems, it is also evident that the propagation environment suffers from sparse-scattering. This causes to the majority of the channel direction of arrivals to be below the noise floor. As a consequence, a channel in a wireless THz system can be established in a specific direction with a range that varies according to the directionality level. However, the directionality of wireless THz channels result in two consequences, namely:

Blockage, which refers to the high penetration loss, due to obstacles and cannot be solved by just

increasing the transmission power; and

Deafness, which refers to the situation, in which the main beams of the transmitter and the

receiver are not aligned to each other. This prevents the establishment of the communication link.

In order to overcome blockage, the wireless THz system is required to search for and identify alternatives directed spatial channels, which are not blocked. However, this search entails a new BF overhead of significant amount and hence it introduces a new type of latency, in which we will refer to as BF latency. As a consequence, the MAC design for cellular networks is more complicated than the one of the conventional wireless local area networks, in which short range communications can be also established


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through nLOS components. Additionally, the conventional notion of cell boundary becomes questionable in these systems, due to the randomly located obstacles. As a result, a redefinition of the notion of the “traditional cell” into “dynamic cell” is required. On the other hand, deafness has a detrimental effect on the complexity of establishing the link and causes a synchronization overhead increase. This indicates the importance of redesigning the initial access (IA) procedures.

TERRANOVA System Architecture

WDM Metro

OLT

AW

G

ONU

ONU

PON

Central Cloud

ComputeCompute

PNF/VNFPNF/VNF

StorageStorage

Backhaul

Fiber

TERRANOVA Link5G NR

ONU

Indoor Wireless Access

OLT

CU

AW

G

RU

RU RUDU

Fronthaul

Redundancy

Fiber Extender

Point-to-PointCorporate

Backup

Nomadic

Mobile Access

Fixed Wireless Access

Figure 8: Physical network architecture

By observing Figure 8, we realize that TERRANOVA optical/THz links will have relevant impact on future network topologies especially in the interconnection between the WDM metro and access networks as well as at the mobile access itself. Given that in the near future, high-speed access should be made ubiquitously available to guarantee equal opportunities in the global competition, rural or remote regions that are difficult to access (e.g., mountains, cliffs and islands), all are expected to be connected with high data rates up to 10 Gbps per user. This is either infeasible or prohibitively costly, when using solely optical fibre solutions. As a result, the use of wireless terahertz (THz) transmission as wireless backhaul extension of the optical fibre is an important building block to bridge the ‘divide’ between rural and dense urban areas and guarantee high-speed internet access everywhere in a cost-efficient manner, in the beyond 5G era. Once THz wireless link bandwidth can reach up to 1 THz, it could also be considered as promising candidate to be integrated into beyond 5G networks as another wireless access branch for several bandwidth-hungry use cases (both indoor and outdoor, fixed and mobile users). Wireless THz access may also be used as a complement to the 5G new radio (NR) to offer connectivity between ultra-high-speed wired networks, achieving full transparency and rate convergence between the two links. In addition, THz links may also be used on 5G fronthaul topologies looking forward to optimize and speed up the 5G field rollouts also in urban network domains by means of implementing a radio link between the distributed units (DU) and the radio units (RU) avoiding the need for heavy and expensive coaxial cabling and also optimizing the fibre capillarity needs on 5G deployments.


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Other indicative examples of bandwidth-hungry applications are sport stadiums and concert halls during events. The unique characteristic of extremely high concentration of mobile equipment for a short period of time asks for ultra-high throughput that can be covered by terahertz wireless access links. Furthermore, all this traffic can be aggregated and sent back to the network by wireless means. Since this extremely high traffic is not continuous in these cases, it can be forwarded to the network in an ad-hoc fashion, by employing dedicated equipment to other links. The metro network domain is responsible for providing reliable connectivity between the access branches and the regional/core data centres, as well as to the global internet. Metro solutions employ dense WDM (DWDM) in metropolitan areas [20]. As a consequence, up to 100 wavelength channels can be transported in parallel over the same fibre to satisfy the large aggregated traffic from the access branches. Besides advantages for longer distances typical in the metro domain (using coherent optical transport technologies), DWDM leads to aggregate capacity in the range of some Tbps over a single optical fibre [21]. For the optical access network, TERRANOVA employs a passive optical networks (PONs) component. PONs have been considered as an effective solution for the fibre access networks, because of the fact that they are able to provide huge bandwidth in a cost efficient manner [22]. A PON consists of an optical line terminal (OLT) system, which serves as the central office endpoint to an optical link that will be launched into an optical arrayed waveguide grating (AWG) that splits the primary fibre into up to 128 or 256 fibres, reaching each one of the subscriber’s premises optical network units (ONUs). Optical power is then spread out along the several fibres and the optical bandwidth uses an uplink and downlink wavelength by also enabling a time division multiplexing (TDM) scheme where each one of the ONUs has its own time slot reaching up to 2.5 Gbps (downstream) and 1.25 Gbps (uplink) for GPON (ITU-T G.984) technology. Current PON technologies need further evolution in order to achieve the 1 Tbps goal. Therefore, NG-PON2 (ITU-T G.989), which uses wavelength division multiplexing (WDM) in addition by joining multiple 10Gbps optical channels, promise to multiply capacity to Nx10 Gbps, thus being an attractive solution in order to deal with the intense telecommunication traffic, which is expected to be caused in the 5G and beyond 5G era [23]. The central cloud (CC) is where the traffic from multiple cell sites is aggregated. The CC is placed in a centrally located data centre hosting a large collection of processing, storage, networking, and other fundamental computing resources. In this node, tenants are allowed to deploy and run arbitrary software, such as operating systems and service applications. Figure 9 illustrates a schematic depiction of the TERRANOVA optical-wireless system. Note that, in order to ‘transparently’ extend the capacity, range and reliability of the fibre optic link to wireless, novel THz transceiver designs should be presented. The baseband unit incorporating the MAC and PHY layers will generate the electrical signal that is after converted and coursed through different I/Q optical wavelengths along the fibre. Convenient optical to electrical conversion and corresponding I/Q processing will after be enabled at the high frequency frontend and finally delivered to the H-Band Antenna.


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Figure 9: Schematic depiction of optical-wireless systems for replacement of fibre link by a wireless THz link (above) and an indicative example for an optical-wireless system with optical RF-frontend based on 100GBase-LR4 QSFP 288

transponder modules (below).

Wireless THz links can be used for P2MP Fixed Wireless access (see Figure 10), thus, constituting a backup for the backhaul of cell sites in several application scenarios, such as redundant infrastructure for disaster recovery or for wireless coverage in emerging markets. As an example, that might be the case corresponding to “Corporate Backup” application where TERRANOVA technology is used to backup existing fixed lines in the case of critical services, which require a failsafe alternative.

Figure 10: PtMP Fixed Wireless Access (Backhaul)

Wireless access scenarios are defined for shorter distance links and, thus, relax the directivity demands, while, on the other hand, intensify the problem of discovering and tracking the users and controlling the interference in multi-user access. As pencil beamforming is needed to realize both high available link SNR and narrow beamsteering angle, UE detection and tracking will be major challenges for point-to-multipoint scenarios, both at the physical and MAC layers. For this scenarios, where independent THz pencil beams are potentially directed to individual users (see Figure 11), user equipment (UE) detection and tracking could be realized using (legacy) lower frequency regimes with omni-directional antennas, while only the high capacity connection is realized via the THz wireless connection. Interference issues between the beams will likely result in limitations with respect to how close the individual users can be located.


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Figure 11: PtMP Mobile Wireless Access

In the indoor scenario depicted in Figure 12, uninterrupted communication can be guaranteed by using beam tracking as well as coordination multi-point (CoMP). One option to solve the issue of UE tracking and interference is to realize a system with many fixed beams pointing in slightly different directions, achieving full coverage of the indoor environment. In all the P2MP scenarios, the key technology challenge is the realization of antenna arrays with sufficient number of antenna elements to implement the beam steering. The required array factors will depend on the use case, as well as the question for a one- or two-dimensional array. The same holds for the different beamforming options (local oscillator, RF or digital beam steering), which have different advantages and disadvantages.

Figure 12: PtMP Indoor Wireless Access

4. Conclusions The conclusions of the presented white paper reflect the work that has been done by the TERRANOVA consortium in the preliminary phase of the project and in what specifically relates to the network and system candidate architectures that were derived from the set of defined applications foreseen by the consortium for the Optical-Terahertz technology as the outcome of the project. Please refer bellow for the main conclusions of this white paper:

o Identification of the TERRANOVA network system applications leading to the corresponding network use cases.

o Determination of the technical scenarios corresponding to the defined application use cases.


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o Definition of the key performance requirements and corresponding critical analysis for relevant parameter range.

o Presentation of the technological enablers that will allow us to achieve these requirements and parameters.

o Description of the preliminary network and system architecture of the envisioned TERRANOVA Optical-Terahertz system.

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