Nokia Solutions and Networks Smart Scheduler

NSN White paper February 2014

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CONTENTS

1. Introduction 32. SmartSchedulerFeaturesandBenefits 43. Smart Scheduler wit Explicit Multi-Cell Coordination

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3.1 Distributed RAN with X2+ and non-ideal backhaul

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3.2 Distributed RAN with slow centralized scheduling and non-ideal backhaul

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3.3 Centralized RAN (C-RAN) 11 3.4 Enhanced Inter-Cell Interference Control (eICIC) with co-channel small cells

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4. Further Evolution of LTE Scheduling 145. Summary 156. Abbreviations 15

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1. IntroductionAs of January 2014, Long Term Evolution (LTE) has been successfully deployed by more than 250 operators, with more than 200 million customers enjoying high mobile broadband data rates. LTE in FDD and TDD mode (TD-LTE) is designed for a so-called frequency reuse of one where all the cells use the same frequency. Reuse of one provides the highestnetworkefficiencyandenableshighdataratesclosetothebase station.

The challenge with reuse of one is the high inter-cell interference when the terminal (User Equipment UE) is located between two cells. The data rate over the cell area is illustrated in Figure 1. Boosting the cell edge performance is the main motivation of Smart Scheduler.

Smart Scheduler can also enhance the average data rates and system capacity by considering signal fading and interference in packet schedulingdecisions.SmartScheduleralgorithms,benefits,impactonthe network architecture and further evolution are discussed in this white paper. If not otherwise explicitly stated, all statements are valid for both LTE (in FDD mode) as well as for TD-LTE.

Frequency f1

Cell A Cell BUE

High data rateclose to BTS

Data rate

Frequency f1

Low data rateat cell edge

Fig. 1. Frequency reuse of one creates high inter-cell interference

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2. Smart Scheduler features andbenefitsLTE radio technology is highly standardized by 3GPP but only with regard to the interfaces the network algorithms including link adaptation, power control and packet scheduling are not standardized. Therefore,therecanbedifferencesinnetworkperformanceduetothedifferentalgorithmsbeingusedbydifferentvendors.Themostrelevantfeaturesandbenefitsaredescribedinthissection.Packetschedulingcanusedifferentinputinformationforresourceallocationand for interference coordination:

Channel Quality Information (CQI) from UE to BTS for downlink scheduling.

Sounding Reference Signal (SRS) measurements and interference measurements in the frequency domain for uplink scheduling.

Load and other information exchange over the X2 interface between base stations. X2 interface in Release 8 allows some exchange of information between the base stations, but further extensions will be discussed in 3GPP and can also be added proprietarily.

Quality of Service (QoS) parameters from the packet core network

ThesedifferentinputinformationoptionsareillustratedinFigure2.

Cell A Cell BUE

GatewayQoS QoS

Channel qualityinformation (CQI)

Coordination over X2+

Fig. 2. Input information for coordinating the resource usage

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SmartSchedulercanutilizethedifferentinputvaluestooptimizepacket scheduling and link adaptation. LTE allows considerable freedomtodefineallocationsinthetime,frequencyandpowerdomains.Anumberofdifferentfeaturesarerequiredforthedifferentuse cases. The same features are utilized both in Frequency Division Duplex (FDD) and Time Division Duplex (TDD) based LTE. The Smart Scheduler utilizes the following main features:

Frequency Selective Scheduling (FSS) improves performance in the case of frequency selective fading and fractional inter-cell interference. FSS consists of Channel Aware Scheduling (CAS) and InterferenceAwareScheduling(IAS).Thefieldmeasurementsshow+30% gains for the cell edge data rates.

Interferenceshapingfurtherimprovestheefficiencyoftheinter-cell interference avoidance by FSS. When the cell loading is low, the number and set of physical resource blocks is adapted only slowlyaccordingtotrafficfluctuations.Thisapproachmakesitmoreefficientfortheadjacenthighlyloadedcellstoavoidinter-cell interference based on UE CQI reporting. Studies show gains exceeding 100%.

QoSdifferentiationimprovescelledgeperformancebyallocatingmore resources for users in weak channel conditions. QoS can be utilized to maintain the data rate, for example for video streaming services.FurtherflexibilityisobtainedbyusingoperatorspecificQoSClassIdentifier(QCI)values.Theminimumguaranteedcelledge data rate can be obtained also by Nominal Bit Rate (NBR) which works even without guaranteed bit rate QoS classes. Cell edge prioritization has only a minor impact on the cell aggregate throughput capacity, in typical case 30% cell edge throughput improvement can be obtained at the cost of 5% cell throughput capacity.Thecapacitymeasuredinnumberofsatisfiedsubscribersis still higher.

Interference aware uplink power control considers the adjacent cells when allocating the uplink transmission power. The feature minimizes inter-cell interference and helps to boost uplink data rates.

Intra-frequency load balancing helps when the load in the adjacent cells is not balanced. The idea is to modify handover parameters based on the information exchange of the X2 interface. If there are double the users in the adjacent cell, the intra-frequency load balancing can improve the cell edge data rate by 30%.

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Multi-cell scheduling can reduce the power levels (muting or related variants) in adjacent cells to minimize the interference. The multi-cell scheduling coordinates resource allocation between multiple cells in time and in frequency, using a selection of users and power levelsinmultiplecellstocombinethebenefitsoffrequency-selectiveschedulingandspectralefficiencygainduetoreducedinterference. The coordination happens between the sectors of one base station, or over the X2 interface between the base stations. Multi-cell scheduling can improve cell edge performance by 20%. Multi-cell scheduling requires inter base station time synchronization. TD-LTE base stations need to be synchronized while the synchronization of LTE FDD base stations is not mandatory and is typically not used by operators for FDD deployments. Note also that the reference signals are overlapping in adjacent cells in a synchronized network. Therefore, UEs should preferably support cancellation of common reference signals for better performance.

The Smart Scheduler use cases, features and gains are shown in Figure 3 and Figure 4. Figure 4 shows the gains of the individual scheduling functionalities when used jointly. More gain can be obtained in HetNet scenarios with eICIC.

100%

Cell edge Average

Intra-frequency load balancing

Multi-cell scheduling

Nominal bit rate and QoS

Frequency selective scheduling

20%

40%

60%

80%

120%

0%

Fig. 4. Smart Scheduler downlink data rate gains with non-ideal backhaul

Fig. 3. Smart Scheduler use cases and solutions

Use case Feature

Fractional inter-cell interferenceLower gain

Highestgain

Unbalanced loading between cells

HetNet

Minimum cell edge rate required

Fractional inter-cell interference

Frequency selective fading

Multi-cell scheduling

Intra- and inter-frequency load balancing

eICIC

QoS differentation and nominal bit rate

Baseline scheduler

FSS including Interference Aware Scheduling (IAS) and Channel Aware Scheduling (CAS)

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Fig. 5. Frequency Selective Scheduling to minimize fading impact

Lets now analyze Frequency Selective Scheduling (FSS) the most important part of the Smart Scheduler. The multipath propagation in the mobile environment makes the fading frequency selective. The typical coherence bandwidth of the macro cell channel is 1-2 MHz, therefore, there are faded and non-faded frequencies within one LTE carrier. LTE radio uses Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink. Therefore, FSS allows use of those parts of the carrier (called Physical Resource Blocks) not faded for the transmission. The concept is illustrated in Figure 5. Information about channel fading can be obtained from UE CQI reports in downlink and from Sounding Reference Symbols (SRS) in uplink.

Transmit on those resource blocks that are not faded

Carrier bandwith

Resource block

Frequency

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Interfering cell Target cell

Freq

uenc

y

CQI 1 (low)CQI 2 (high)CQI 3 (high)CQI 4 (low)CQI 5 (low)CQI 6 (high)CQI 7 (high)CQI 8 (high)

Fractional load in adjacent cell

UE reports subband CQI

Frequency selective scheduling

UE A

No transmissionTransmission in adjacent cell

Transmission to UE A Transmission to other UEs

Fig. 6. Frequency Selective Scheduling (FSS) to minimize inter-cell interference

FSS can also be applied to avoid inter-cell interference. An example is shown in Figure 6 where the interfering cell is partially loaded. The UE is connected to the target cell but receives strong interference from the adjacent interfering cell. The UE reports sub-banded CQI values in the frequency domain to the target cell. Low CQI values are reported on those sub-bands where the interfering cell has on-going transmission while high CQI values are reported in other sub-bands. The target cell with FSS tends to allocate those downlink physical resources blocks to the UE where the interference is lowest. The other resource blocks in the target cell can be allocated to other UEs thatdonotreceiveinterferencefromtheadjacentcell.Benefitsof FSS include:

Effectiveinter-cellinterferencecoordinationwithouttheneedforexplicit inter-BTS coordination

Utilization of UE CQI reports for interference mitigation and without the need for coordination signaling between the base stations

Improved cell edge data rates as well as total cell capacity.

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Fig. 7. Field measurements with FSS in downlink

Cell edge throughput

Mbp

s

Mbp

s

Cell capacity

FSS off FSS on

2.5

3.0

0.5

1.0

1.5

2.0

3.5

0.0

25

30

FSS off FSS on

5

10

15

20

35

0

As part of the Smart Scheduler concept, the underlying link adaptation function is critical for the success of features such as FSS. The quality of reporting from each active terminal is always monitored and compensation is constantly conducted in order to improve the value of the scheduler decisions. With such methods, NSN has in numerous commercial LTE networks shown the practical value of FSS. An example fieldmeasurementresultwith10MHzbandwidthisshowninFigure7.

Fig. 8. Interference shapingformoreefficientinterference avoidance

Interferenceshapingfurtherimprovestheefficiencyoftheinter-cellinterference avoidance by FSS. Interference shaping is illustrated in Figure 8. When the cell loading is low, the number and set of physical resourceblocksisadaptedonlyslowlyaccordingtotrafficfluctuations.Thisapproachmakesitmoreefficientfortheadjacenthighloadedcells to avoid inter-cell interference based on UE CQI reporting. The studies show major gains for the cell edge data rates in those cases where the loading is unbalanced between the cells: the gains can exceed 100%. High gains can be achieved in the distributed solution with cleverer scheduling without any fast signaling over the X2 connection and without any centralized network element.

Low loaded cell: Only slow changes infrequency domains

High loaded cell: Robust inter-cellinterference avoidance based on CQI reports

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3. Smart Scheduler with explicit multi-cell coordinationFurther performance improvements can be obtained by coordinating resource allocation in adjacent base stations. The network architecture options for supporting multi-cell scheduling are shown in Figure 9.

a) Distributed RAN with X2+ and non-ideal backhaul

X2+

eNB#1Coordinated scheduling (inter-eNB)Fast local scheduling

eNB#1Coordinated scheduling (inter-eNB)Fast local scheduling

eNB#1Fast local scheduling

X3

eNB#NFast local scheduling

Coordinated scheduling

...

b) Distributed RAN with slow centralized scheduling and non-ideal backhaul

Super-eNB (baseband pool)Common packet scheduling

Direct fiber with multi-Gbps

...

c) Centralized RAN with fast centralized scheduling and dark fiber connection

Fig. 9. Network architecture options for explicit multi-cell scheduling

3.1 Distributed RAN with X2 and non-ideal backhaulTodays LTE architecture (99% of deployments) is shown in Figure 9a usingnon-idealbackhaulwithmicrowaveradio,IPconnectedfiberorcopper based transport. The multi-cell scheduling needs to coordinate the resource usage in adjacent base stations over non-ideal backhaul while still fully utilizing FSS gains in fast scheduling. The coordination betweencellsofdifferentbasestationswillutilizetheX2interface.

Each scheduler that requests coordination from its neighboring base stations to aid a user at the cell edge can still take into account FSS gains for that user, thus FSS gains can be fully preserved while adding the gains from multi-cell coordination. The evolution from fully distributed architecture to multi-cell coordination over X2 is a straightforward software upgrade no new network elements or interfaces are needed. Note that fast local coordination can be implemented between the cells in one base station without any inter-base station coordination.

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3.2 Distributed RAN with slow centralized scheduling and non-ideal backhaul

Another architecture alternative is shown in Figure 9b with a new centralized network element for coordinating the distributed schedulers. A new interface between base stations and the centralized scheduler is required. Involving an additional interface and information exchange to an additional entity has a negative impact on the responsiveness of this architecture. The distributed base stations still run the fast scheduling while the centralized element can only set scheduling limitations to minimize the interference. The performance gain of the centralized element is similar to the coordination over the X2 interface.

Coordinated multi-cell scheduling and muting over non-ideal backhaul was studied in 3GPP under the title Enhanced Coordinated Multipoint (eCoMP) during 2013. The conclusion taken in December 2013 was that the gains for inter-site macro-macro scenario are below 5% in the best case over intra-site and less if the backhaul latency increases (several tens of ms). The further focus of eCoMP will be in the HetNet scenarios between macro cells and small cells.

3.3 Centralized RAN (C-RAN)Thefinalmulti-cellarchitectureshowninFigure9ciscentralizedscheduling in the baseband pool. This is the architecture for a network with ideal transport. The baseband pool requires a low latency direct darkfiberconnectionbetweentheRFheadsandthebasebandpool. The baseband pool is also referred to as Centralized Radio Access Network (C-RAN).

C-RAN is like a super-sized base station. C-RAN enables the most advanced multi-cell coordination because all the functionalities are in the same location: link adaptation, power control, fast FSS and multi-cell coordination. C-RAN architecture also enables Joint Transmission and JointReceptionCoordinatedMultipoint(CoMP)betweendifferentsiteswhile intra-site CoMP can be implemented also in the distributed RAN architecture.

CoMPfunctionalityisdefinedin3GPPRelease11butuplinkCoMPcanbe implemented also with legacy Release 8 UEs while the downlink CoMP requires Release 11 UEs. Uplink CoMP gives more gain, while downlink CoMP gains are limited. An excellent use case for C-RAN is to boost capacity in stadiums and other mass event locations. These events tend to be uplink limited because many people want to send pictures from the event. The UE transmission is received by a single cell in the traditional solution while the same UE transmission can be received by multiple cells and combined in the baseband module. The inter-cell interference turns into a constructive signal. The solution is illustrated in Figure10.TheinstallationoffiberbetweenbasebandmodulesandRFisrelatively simple in these event areas.

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NSN Flexi Multiradio base station enables CoMP by providing fast interconnections between the baseband modules. NSN C-RAN has been validated in commercial networks in large stadiums and the practical gains exceed 100%.

3.4 Enhanced Inter-Cell Interference Control (eICIC) with co-channel small cells

Small cells are an attractive solution for boosting hot spot capacity and coverage. The interference management needs to be considered when the small cells are deployed on the same frequency as the macro cells.

3GPP Release 10 brings a solution for managing the interference in the time domain. The solution is called enhanced Inter-Cell Interference Coordination (eICIC) and is shown in Figure 11. The macro cell leaves some empty sub-frames called Almost Blank Subframes (ABS). During these sub-frames, the small cell can serve UEs that would otherwise receive too much co-channel interference from the macro cell.

Fig. 10. Centralized RAN for boosting mass event capacity

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ThebenefitofeICICcomeswhenseveralsmallcellscanbenefitfrommacro cell empty subframes. eICIC performance is further boosted in Release 11 by using UE interference cancellation for the minimization of inter-cell interference, which is known as further enhanced ICIC (feICIC). Optimized eICIC requires that the number of ABS frames and the handover parameters are adjusted dynamically according to the instantaneoustrafficconditionsandUElocations.Thesemi-staticsolution is a slow approach for modifying the feICIC parameters over several seconds. The fast feICIC adaptation uses quick adaptation for the number of ABS sub-frames to reallocate resources between macro cells and small cells depending on the instantaneous requirements. NSNs unique algorithm is based on the fast adaptation of ABS-blankingandcellrangeextensionformaximumbenefitfromsmall cell deployments. The throughput gains are shown in Figure 12: dynamic eICIC can nearly double the user throughputs in heterogeneous networks.

Fig. 11. Time domain interference management with enhanced ICIC

Fig. 12. Throughput gain from enhanced ICIC

= Sub-frame with normal transmission

= Almost blank sub-frame (ABS)

Pico cell can reuse same frequencyas macro when UE is closer to pico

Pico cell can serve also such UEs that receive stronger macro cell signal

Sub-frame (1 ms)

Macro

Pico

Baselinew/o felCIC:0%

Semi-staticfelCIC70%

FastfelCIC90%

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4. Further evolution of LTE scheduling 3GPP is working with Inter-site carrier aggregation in Release 12. The feature allows the UE to receive data simultaneously from the macrocellandfromthesmallcell.Thetwocellsdonotneedanyfiberbackhaul,althoughwirelessbackhaulwithsomedelayisfine.TheX2interface is used between the macro cell and small cell for scheduler coordination. The macro cell and the small cell can share the same frequency or the small cell can use a dedicated frequency. The feature is illustrated in Figure 11.

Inter-site carrier aggregation uses Dual Connectivity where the UE has simultaneous radio connection to both macro and to small cell. That bringsbenefitsintermsofreliablemobility.

3GPP is also working on a solution where UEs can cancel the inter-cell interference by obtaining assistance information from the network. This feature is called Network Assisted Interference Cancellation and Suppression (NAICS) and it is part of Release 12. If UEs can cancel interference,itmaybemoreefficienttouseallresourcesinco-channel cells instead of muting resources. The multi-cell scheduling andmutingalgorithmsneedtobedesignedinsuchaflexiblewaythattheycanbenefitfromthefutureadvancedUEcapabilities.

Fig. 13. Inter-site carrier aggregation in Release 12

Inter-site Carrier Aggregationand Dual Connectivity

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5. SummaryWhile LTE has been highly standardized by 3GPP, the network algorithms including packet scheduling are not standardized. The packet scheduling in LTE has the freedom to control the resource allocation in the time and in the frequency domain.

Smart Scheduler can push cell edge data rates by more than 100% in the presence of inter-cell interference compared to baseline wideband scheduling, and improve the cell capacity by more than +20%. The main component of Smart Scheduler is frequency selective scheduling that avoids the fading and interference in the frequency domain combinedwithQualityofServicedifferentiationandintra-frequencyload balancing. NSNs innovation Interference Shaping increases the cell edge throughput further by up to 100% when the cell loading is unbalanced.

Additional cell edge gains can be obtained by multi-cell scheduling. Multi-cell scheduling is a simple software upgrade to distributed base stations. Scheduling information is shared between base stations over the X2 interface. The detailed standardization of multi-cell coordination is considered in 3GPP Release 12.

The most advanced multi-cell coordination can be obtained with baseband pooling in Centralized RAN. The baseband pool deployment assumesdirectfiberconnectionbetweenbasebandandRFsites.CentralizedRANprovidesthebiggestbenefitsinuplinkcapacity,whichismostusefulinhighcapacityevents.TheefficiencyofsmallcelldeploymentcanbeboostedbyusingdynamiceICICconfigurationtomanage the interference between macro cells and small cells.

6. Abbreviations3GPP Third Generation Partnership ProjectBTS Base StationCoMP Coordinated MultipointCQI Channel Quality InformationC-RAN Centralized Radio Access NetworkeCoMP Enhanced CoMPeICIC Enhanced Inter-Cell Interference CoordinationFDD Frequency Division DuplexFSS Frequency Selective SchedulingLTE Long Term EvolutionOFDMA Orthogonal Frequency Division MQoS Quality of ServiceRRH Remote Radio HeadSC-FDMA Single Carrier Frequency Division Multiple AccessSRS Sounding Reference SymbolsUE User Equipment

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Public NSN is a trademark of Nokia Solutions and Networks. Nokia is a registered trademark of Nokia Corporation. Other product names mentioned in this document may be trademarks of their respective owners, and they are mentioned for identification purposes only.

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