nsn lte smart scheduler white paper

7/27/2019 Nsn Lte Smart Scheduler White Paper

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Nokia Solutions and NetworksSmart Scheduler

NSN White paperOctober 2013


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CONTENTS

1. Introduction 3

2. Smart Scheduler Features and Benets 4

2.1 Frequency Selective Scheduling 7

3. Multi-cell Smart Scheduler 8

3.1 Distributed RAN with X2+ and non-

ideal backhaul

9

3.2 Distributed RAN with slow centralizedscheduling and non-ideal backhaul 9

3.3 Centralized RAN with fast centralizedscheduling and dark ber connection

10

4. Further Evolution of LTE Scheduling 10

5. Summary 11

6. Abbreviations 11


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1. IntroductionLong Term Evolution (LTE) has been successfully deployed by morethan 200 operators and the subscribers can enjoy high LTE data rates.LTE radio in FDD and TDD mode (TD-LTE) is designed for a so-calledfrequency reuse of one where all the cells use the same frequency.Reuse one provides the highest network eciency and enables highdata rates close to the base station. The challenge with reuse one isthe high inter-cell interference when the terminal (User EquipmentUE) is located between two cells. The data rate over the cell area isillustrated in Figure 1. The boosting of the cell edge performance isthe main motivation for the Smart Scheduler. The Smart Scheduler

can also enhance the average data rates and system capacity byconsidering signal fading and interference in the packet schedulingdecisions. The Smart Scheduler algorithms, benets, implications tothe network architecture and further evolution are discussed in thiswhite paper. If not explicitly stated otherwise, all statements are validfor both LTE (in FDD mode) as well as for TD-LTE.

Frequency f1

Cell A Cell BUE

High data rateclose to BTS

Datarate

Frequency f1

Low data rateat cell edge

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


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2. Smart Scheduler features andbenefts

LTE radio is highly standardized by 3GPP when it concerns interfaces,but the network algorithms including link adaptation, power controland packet scheduling are not standardized. Network performancecan therefore dier according to which algorithms are used. The mostrelevant scheduling features and benets are described in this section.Packet scheduling can use various input data for resource allocationand for interference coordination:

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

scheduling.

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

Load and other information exchange over X2+ interface betweenbase stations. X2+ is an enhanced version of X2 interface andallows fast exchange of further information between base stations,for example, handover measurements, load information, or CQImeasurements. X2 interface in Release 8 allows only very limitedexchange of information between the base stations, but furtherextensions will be discussed in 3GPP and can also be added

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

These input options are illustrated in Figure 2.

Cell A Cell BUE

Gateway

QoS QoS

Channel qualityinformation (CQI)

Coordination over X2+

Fig. 2. Input information for coordinating the resource usage


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Fig. 3. Smart Scheduler use cases and solutions

The Smart Scheduler can utilize the dierent input values to optimize

the packet scheduling and the link adaptation. LTE radio allowssignicant freedom in dening the allocations in the time, frequencyand power domains. A number of dierent features are requiredfor the dierent use cases. The Smart Scheduler utilizes the samefeatures both in Frequency Division Duplex (FDD) and Time DivisionDuplex (TDD) based LTE systems. These include:

Frequency Selective Scheduling(FSS), which improves performancein the case of frequency selective fading and fractional inter-cellinterference. FSS consists of Channel Aware Scheduling (CAS)and Interference Aware Scheduling (IAS). The eld measurementsdemonstrate 30% gains for the cell edge data rates.

QoS diferentiation, which improves cell edge performance byallocating more resources for the users in weak channel conditions.QoS can be utilized to maintain the data rate for example for videostreaming services. Further exibility is obtained by using operatorspecic QoS Class Identier (QCI) values. The minimum guaranteedcell edge data rate can also be obtained using a Nominal Bit Rate(NBR) solution, which works even without any guaranteed bit rateQoS classes. The cell edge prioritization has only a minor impactonthe cell aggregate throughput capacity; in a typical case, celledge throughput can be improved by 30% and the number ofsatised subscribers increased, at a cost of 5% sacrice in cell

throughput capacity.

Interference aware uplink power control, which considers theadjacent cells when allocating the uplink transmission power. Thefeature minimizes inter-cell interference and helps to boost theuplink data rates.

Intra-frequency load balancinghelps when the load in the adjacentcells is not balanced. The idea is to modify handover parametersbased on the information exchange of X2 interface. If there aredouble the users in the adjacent cell, the intra-frequency loadbalancing can improve the cell edge data rate by 30%.

Use case Feature

Fractional inter-cell interferenceLowergain

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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Multi-cell schedulingcan reduce the power levels (muting or related

variants) in adjacent cells to minimize the interference. The multi-cell scheduling coordinates resource allocation between multiplecells in the time and frequency domains, using a judicious selectionof users and power levels in multiple cells to maximally combine thebenets of frequency-selective scheduling and spectral eciencygain due to reduced interference. The coordination occurs betweenthe sectors of one base station, or over X2+ interface betweenthe base stations. The multi-cell scheduling requires base stationtime synchronization and can improve cell edge performanceby 20%. TD-LTE base stations must be synchronized, while thesynchronization of LTE FDD base stations is not mandatory and nottypically used by FDD operators. As in a synchronized network, thereference signals overlap in adjacent cells. Therefore, UEs shouldpreferably support cancellation of common reference signals forbetter performance.

Enhanced Inter-Cell Interference Coordination (eICIC) minimizesinter-cell interference in heterogeneous networks (HetNet) betweenmacro and small cells in the time domain when deployed in thesame frequency carrier. eICIC provides data rate benets especiallywhen there are multiple small cells under one macro cell. TheeICIC technique can improve the data rates by over 50% in a highdensity small cell deployment scenario. These techniques benet

from Release 10 and 11 terminals due to their special eICIC andinterference cancellation capabilities. Some gains can be achievedwith Release 8 legacy terminals as well. eICIC can be furtherenhanced with fast coordination control running at the macro site,either via X2+ or when the small cells are deployed as low-powerRemote Radio Heads (RRH).

Smart Scheduler use cases, features and gains are presented inFigures 3 and 4. Figure 4 illustrates macro cell gains achieved bycombining individual scheduling functionalities, with the exception ofeICIC. Additional gains can be obtained with eICIC in HetNet scenarios.

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. FSS and QoS gains are obtained from eldmeasurements and the other gains from demo/trial setups


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Fig. 5. Frequency Selective Scheduling to minimize fading impact

2.1 Frequency Selective Scheduling

Multipath propagation in the mobile environment results in frequencyselective fading. For this reason, Frequency Selective Scheduling(FSS) is the most important part of the Smart Scheduler. The typicalcoherence bandwidth of the macro cell channel is 1-2 MHz, andtherefore, faded and non-faded frequencies exist within one LTEcarrier. LTE radio uses Orthogonal Frequency Division Multiple Access(OFDMA) in downlink and Single Carrier Frequency Division MultipleAccess (SC-FDMA) in uplink. FSS thus allows using those parts of thecarrier (called Physical Resource Blocks), which are not faded for thetransmission. The concept is illustrated in Figure 5. The information

about channel fading can be obtained from UE CQI reports in downlinkand from Sounding Reference Symbols (SRS) in uplink.

Transmit on those resource blocks that are not faded

Carrier bandwith

Resource block

Frequency

FSS can also be applied to avoid inter-cell interference. Figure 6provides an example where the interfering cell is partially loaded. TheUE is connected to the target cell but receives strong interferencefrom the adjacent interfering cell. The UE reports subband CQIvalues in the frequency domain to the target cell. Low CQI values arereported on those subbands where the interfering cell has on-goingtransmission, while high CQI values are reported in other subbands.FSS in the target cell prioritizes an allocation of those downlinkphysical resource blocks to the UE where the interference to this UE islowest. Other resource blocks in the target cell can then be allocatedto other UEs that do not receive interference from the adjacent cell.Benets of FSS include:

Eective inter-cell interference coordination without the need forexplicit inter-BTS coordination.

Utilization of UE CQI reports for interference mitigation and without

the need for coordination signalling between the base stations.

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


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Interfering cell Target cell

Frequency

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 transmission

Transmission in adjacent cell

Transmission to UE A Transmission to other UEs Fig. 6. FSS to minimize inter-cellinterference

Fig. 7. Field measurements withFSS in downlink

As part of the Smart Scheduler concept, the underlying link adaptationfunction is very critical for the success of features such as FSS.The quality of the reporting from each active terminal is constantlymonitored and evaluated in order to improve the quality ofthe scheduler decisions, overruling the UE recommendations whereneeded. With such methods, Nokia Solutions and Networks (NSN) hasin numerous eld and lab trials demonstrated the practical value of

FSS. Figure 7 documents the benet of FSS for cell edge performanceand total cell capacity based on eld measurement results from asystem with 10 MHz bandwidth.

Cell edge throughput

Mb

ps

Mb

ps

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

3. Multi-cell Smart Scheduler

Further performance improvements can be obtained by explicitlycoordinating the resource allocation in the adjacent base stations. Thenetwork architecture options for supporting multi-cell scheduling areshown in Figure 8.


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3.1 Distributed RAN with X2+ and non-idealbackhaul

Todays LTE architecture is depicted in Figure 8a using non-idealbackhaul with microwave radio, IP connected ber or copper basedtransport. The multi-cell scheduling needs to coordinate the resourceusage in adjacent base stations over non-ideal backhaul while still fullyutilizing FSS gains in fast scheduling. The coordination between cellsof dierent base stations will utilize X2+ interface. Each schedulerrequesting coordination from its neighboring base stations to aid acertain user at the cell edge can still take into account FSS gains forthat user. FSS gains can thus be fully preserved, while also addingthose achieved from multi-cell coordination. The evolution fromfully distributed architecture to multi-cell coordination over X2+ isa straight-forward software upgrade no need for any new networkelements or new interface. It is important to note that fast localcoordination can be implemented between the cells in one basestation without any inter-base station coordination.

This approach is also the basis for HetNets, where small cells arecoordinated by the nearest macro cell using Enhanced Inter-CellInterference Coordination (eICIC).

3.2 Distributed RAN with slow centralizedscheduling and non-ideal backhaul

Another architecture alternative is shown in Figure 8b adding anew centralized network element for coordinating the distributedschedulers. A new interface between base stations and the centralizedscheduler is denoted as X3. Involving an additional interface andinformation exchange to an additional entity has a negative impacton the responsiveness of this architecture. The distributed basestations still run fast local scheduling, while the centralized element

can only set scheduling limitations to minimize the interference.The performance gain of the centralized element is similar to thecoordination over X2+ interface. 3GPP standardization is required forX3 to become multivendor capable.

a) Distributed RAN with X2+ andnon-ideal backhaul

X2+

eNB#1Coordinatedscheduling

(inter-eNB)Fast localscheduling

eNB#1Coordinatedscheduling

(inter-eNB)Fast localscheduling

eNB#1Fast localscheduling

X3

eNB#NFast localscheduling

Coordinated scheduling

...

b) Distributed RAN with slowcentralized scheduling andnon-ideal backhaul

Super-eNB (baseband pool)

Common packet scheduling

Direct ber with multi-Gbps

...

c) Centralized RAN with fastcentralized schedulingand dark ber connection

Fig. 8. Network architectureoptions for explicit multi-cellscheduling


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3.3 Centralized RAN with fast centralizedscheduling and dark fber connection

The nal multi-cell architecture shown in Figure 8c is centralizedscheduling using a baseband pool and assuming that ideal backhaulis available. The baseband pool architecture requires low latency anddirect dark ber connection between the RF heads and the basebandpool. The baseband pool is also referred to as Centralized RadioAccess Network (C-RAN) and is comparable to a super-sized basestation. C-RAN enables the most advanced multi-cell coordinationbecause all the functionalities are in the same location: linkadaptation, power control, fast FSS and multi-cell coordination. C-RANarchitecture also enables Coordinated Multipoint (CoMP) functionalityas dened in 3GPP Release 11.

C-RAN provides clear benets but the practical implementation ischallenging given the limited availability of wide dark ber connectionto RF sites. The centralized architecture also implies that there is asingle point of failure and requirements for availability are higher thanin a distributed architecture.

The most typical architecture scenario globally is multi-cellcoordination over X2+ interface. Studies show that other schedulingfeatures, like FSS, are more important than multi-cell scheduling and

FSS should be implemented rst. Multi-cell scheduling also increasesdeployment complexity given the synchronization requirement andimplementation should take into account the 3GPP Release 12 workto be completed during 2H/2014. Preferably, multi-cell schedulingshould be implemented through a software upgrade without any newnetwork element.

4. Further Evolution of LTE Scheduling3GPP is working with enhanced LTE performance in Release 12,and3GPP initiated a new study item Enhanced CoMP for LTE in June

2013. The target is to evaluate the benets of multi-cell schedulingwith non-ideal backhaul, and to dene the required interfaces andsignalling messages to support multi-cell scheduling. The optionsof Figure 8a and Figure 8b will likely be considered as referencearchitectures. The earlier CoMP work in 3GPP Releases 10 and11 considered only ideal backhaul, which is not feasible for mostLTE deployments.

Further related work items in 3GPP are UE interference cancellationand network assisted UE interference cancellation. The target isto cancel the inter-cell interference by UE baseband algorithms.These advanced UE receivers can improve cell edge performance

considerably. Both performance enhancements in 3GPP should beconsidered jointly in optimizing the cell edge performance: if UE isable to cancel inter-cell interference, there is less gain in explicitcoordination and no need to mute the strongest interferer in theadjacent cells.

Enhanced CoMP(Multi-cell

scheduling)

Mute the

strongest

interferer

Cancel the

impact of

the strongest

interferer

UE interferencecancellation

Fig. 9. Work items in 3GPPRelease 12 for improving celledge data rates


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5. SummaryWhile LTE radio has been highly standardized by 3GPP, the networkalgorithms including packet scheduling are not standardized. Packetscheduling in LTE has the freedom to control the resource allocationin both the time and frequency domains. The Smart Scheduler canpush the cell edge data rates by more than100% in the presence ofinter-cell interference compared to baseline wideband scheduling,and improve the cell capacity by over 20%. The essential componentof the Smart Scheduler is Frequency Selective Scheduling that avoidsfading and interference in the frequency domain combined withQuality of Service dierentiation and intra-frequency load balancing.

Further cell edge gains can be obtained through multi-cell scheduling.Multi-cell scheduling is a simple software upgrade to the distributedbase stations. Scheduling information is shared between basestations over X2+ interface. The detailed standardization of multi-cellcoordination is expected in 3GPP Release 12.

The most advanced multi-cell coordination can be obtained withbaseband pooling in Centralized RAN. The baseband pool deploymentassumes direct ber connection between baseband and RF sites. A3GPP study item started in June 2013 aims to evaluate multi-cellscheduling options with non-ideal backhaul.

6. Abbreviations3GPP Third Generation Partnership ProjectBTS Base StationCoMP Coordinated MultipointCQI Channel Quality InformationC-RAN Centralized Radio Access NetworkeICIC 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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Nokia Solutions and Networks

P.O. Box 1FI-02022Finland

Visiting address:Karaportti 3, ESPOO, FinlandSwitchboard +358 71 400 4000

Product code C401-00822-WP-201310-1-EN

2013 Nokia Solutions and Networks. All rights reserved.

PublicNSN is a trademark of Nokia Solutions and Networks. Nokia is a registeredtrademark of Nokia Corporation. Other product names mentioned in thisdocument may be trademarks of their respective owners, and they arementioned for identication purposes only.

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