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Adaptive Frequency Reuse in IEEE 802.16mDocument Number: S80216m-08/702r3Date Submitted: 2008-07-17Source: Clark Chen, Hongmei Sun, Hua Yang Email: {clark.chen, hongmei.sun, hua.yang, shilpa.talwar, Shilpa Talwar, Vladimir Krasvtov, vladimir.kravstov, yuval.lomnitz, himayat.nageen, Yuval Lomnitz, Hujun Yin hujun.yin} @intel.com Intel Corporation Kwanhee Roh, Jaehee Cho, Jeongho Park, E-mail: kwanhee.roh@samsung.com Sooryong Jung Samsung ElectronicsVenue: IEEE 802.16m-08/024, Call for Comments and Contributions on Project 802.16m System Description Document (SDD), Interference Mitigation Base Contribution: C80216m-08/702r3Purpose: For discussion in TGm and adoption of proposed SDD text into the IEEE 802.16m System Description DocumentNotice:

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S80216m-08_702r3

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Outline

• Motivation• AFR Requirements• AFR Architecture• AFR Algorithm• Standard Support• Conclusions and recommendations

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IEEE 16m SRD Requirements

Meet 802.16m downlink Data requirement

4

Interference is serious problem with Reuse 1

-10 0 10 20 30 40 50 600

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Geometry SINR (dB)

F(x

)

Empirical CDF

Reuse1Reuse3/2Reuse3

Geometric SINR distribution in hexagon cell structureGeometric SINR under different reuse factors

• More than 30% of subscribers have average SINR below 0dB• Worst SINR conditions seen at cell-edges• Higher reuse helps improve SINR, but lowers cell capacity• Solution: mixed reuse to tradeoff cell-edge performance with cell-capacity

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Adaptive Frequency Reuse Concepts

PHYM

ACRRM

Tx Beamforming

Interference Randomization

UplinkPower Control

Rx Interference Mitigation

Interference-aware Scheduling

FFR (semi-static)

FFR (dynamic)

Interference-aware BS Coordination

AFR

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AFR Components

• Fractional frequency reuse (reuse partitioning): Restrict usage of certain frequencies or power level of these frequencies in a sector to minimize interference. Ex. allocate fraction of frequency sub-channels to reuse 3 for cell-edge users

• Interference aware scheduling: Scheduler allocates resources to users based on FFR partition and interference aware CQI metrics

• Interference-aware BS co-ordination: Base stations dynamically adjust FFR partition by exchanging information across the network backbone, to adapt to time-varying user traffic loads & distributionsa) Static - no information exchange

b) Semi-static - on order of 100msec to seconds c) Dynamic - on order of frame duration

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AFR Design Requirements

• Support multiple reuse settings: 1, 3, 3/2• Support diversity & contiguous permutation modes • Support hard reuse (AFR-H) and soft reuse (AFR-S)• Flexibility with non-uniform user distributions• Adaptation to time-varying traffic conditions • Exploit channel aware scheduling gains• Robustness to mobile environments• Low system complexity

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AFR Architecture

• Reuse partition: N = [K]N = [K] partitions (to support reuse 1, N, M)• Attributes of partition: A triplet of N-dim vectors: [[WW, , PP, , CC]]

– Bandwidth partition W: – Power level P: – System cost C:

• Soft reuse achieved by setting power level of each partition group• Optimal resource allocation achieved by setting system-widesystem-wide CostCost for each partition

],,,,,,[ 123132312321 WWWWWWWW

],,,,,[ 123,132312321 PPPPPPPP

],,,,,[ 123,132312321 CCCCCCCC

P1-3

P2-2

P3-1

P1-5

P2-4

P3-6

P1-6

P2-5

P3-4 P3-7

P1-7

P2-7P2-1 P2-3

P3-2 P3-3

P1-1 P1-2 P1-4

P2-6

P3-5

Reuse M Reuse N Reuse 1

Power

Power

Power

Frequency

Sector 3

Sector 2

Sector 1

w1 w2 w3 w23 w12 w13 w123

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Various deployments with AFR Architecture

Supported reuse factor Number of partition groups /(corresponding size vector)

Single reuse factor system

Reuse 1 1 /Reuse 3/2 3 /Reuse 3 3 /

AFR system

2 mixed reuse partitions

Reuse 1 and reuse 3/2 4 /Reuse 1 and reuse 3 4 /

3 mixed reuse partitions

Reuse 1, reuse 3/2, reuse 3

7 /

],0,0,0,0,0,0[ 123W

]0,,,,0,0,0[ 132312 WWW]0,0,0,0,,,[ 321 WWW

],,,,0,0,0[ 123132312 WWWW

],0,0,0,,,[ 123321 WWWW

],,,,,,[ 123132312321 WWWWWWW

],0,0,0,,,[ 1ReuseHighLowLow PPPPP

],0,0,0,,,[ 1ReuseLowLowHigh PPPPP

],0,0,0,,,[ 1ReuseLowHighLow PPPPP

Example:Reuse 1 and soft reuse 3 AFR system

Power level of Sector 1Power level of Sector 2Power level of Sector 3

Example:Reuse 1 and soft reuse 3/2 AFR system

Power level of Sector 1Power level of Sector 2Power level of Sector 3

Hard Reuse

Soft Reuse],,,,0,0,0[ 1ReuseHighLowLow PPPPP

],,,,0,0,0[ 1ReuseLowLowHigh PPPPP

],,,,0,0,0[ 1ReuseLowHighLow PPPPP

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AFR Algorithm Details

• Problem definition and theory introduction• Cost and BW partition adaptation • Performance• Summary

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Problem and Definition

• Optimal resource allocation problem:– How power loading level and AFR partition size is adapted?– What measurements are needed? – What information is fed back from SS?– How SS’s are scheduled on different resource types?

• Definitions– Radio Resource Type: FFR partition with different average S/I– Cost: a real value that is a measurement of system resources used by

a particular resource type – Normalized SE: a value represents the normalized efficiency

achievable on particular resource type in terms of system resource. – User Distribution: position and corresponding S/I distribution of users

(SS) in the system – Fairness Constrain: a pre-defined percentage curve that specifies the

throughput CDF of all SS* Complete mathematic proof in a separated document

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Optimal Resource Allocation Theory Solution

• Assumptions– The user distribution and corresponding average signal/interference

level doesn’t change during the optimization time.– A user’s average spectrum efficiency is a rising function of their average

SINR at different radio resource types.• Theory Solution

– Given a user distribution and a fairness constraint , for every power loading factor , there is a optimal resource allocation solution achievable, that yields the highest average SE, with a unique ( , ), where the system partition is proportional to SS’s relative bandwidth request on different resource types, and all SS are allocated resources that yield maximum normalized SE amongst all resource types.

• Reference: – Problem of optimal radio resource allocation, by Vladimir Kravstov, Intel (unpublished doc)

optW optC

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AFR Scheme Details – Theory Solution Exemplary

• Intuitive example: AFR system with Hard Reuse 3– Cost of reuse-3 is 3 while cost of reuse-1 is 1

• Each resource block in Reuse 3 occupies 3 times system resource compared with resource block in Reuse 1

– SS should be allocated with FFR partition that yields highest nSE• Only SS that can get more than 3 times SE gain should use Reuse 3

– The optimal Reuse partition would be decide by all SS’ choice of reuse partition and their corresponding bandwidth required (fairness constrain)

• Key learning– Cost of different reuse partition is different

• Must find optimal cost vector online– Reuse Partition should be proportional to SS’ relative bandwidth request

• Must adapt online

* Complete mathematic proof in a separated document

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AFR Implementation and Procedure

• Given SS distribution and S/I, find the optimal {P, W, C}• Power loading

– predefined according to engineering experience or network planning• Initial Partition

– Enable SS measurement on different AFR partition• Initial Cost

– Can start from any real value or choose on engineering experience• Two steps adaptation

– cost adaptation and channel partition adaptation. – the cost adaptation is a necessary step for partition adaptation.

],,,,,[ 123,132312321 CCCCCCCC

* Complete mathematic proof in a separated document

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AFR Scheme Details – Cost adaptation

• ‘Market Price Iteration’ algorithm to find the ‘optimal’ C incrementally

• Decreases the cost values if there are too many/few bandwidth requests from SS for the corresponding channel partitions

• Target is to achieve balanced bandwidth request on three reuse-3 partitions

• Theory proves the convergence and unique of cost vector

• BS autonomous decision process in cost adaptation, and deals good with un-balanced SS distribution among BS

Updated cost vector

BS SS

Broadcast

SINR measurementfor different reuse

partition

Calculate the nSE by:SE/cost

Estimate the SE of different subchannel

Collect bandwidth request of different

reuse

Preferred sub-channel and reuse

choicePreferred feedback

by CQI

Initial Price

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AFR Scheme Details – BW Partition adaptation

• Require the cost converged in each BS individually

• BS report all SS preference of different reuse partition to RRM

• RRM take system wide fairness rule in addition to the relative bandwidth request

• RRM (function) unit takes charge of BW partition changes for all neighboring BS in a system

• Partition adaptation is relatively slow, from tens of minutes or hours, on carrier’s choice, depending on system dynamic

• Cost/Partition adaptation approaches optimal system operation point

BS

All BS change AFR configuration in sync

Report signalling

Signalling

RRM Unit

Collect all bandwidth request on different

reuse partition

Cost converged of diff reuse partition

Statistic bandwidth request of different

reuse partition

Notice the same trend of change in

system

Adjust system wide AFR configuration

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AFR Scheme Details – Soft Reuse with AFR Architecture

w1

w2

w3

1

2

w123

3

4

1,2,3

4

1,3,4

21,2,43

1,2,3

4

1,3,4

21,2,43

1,2,3

4

1,3,4

21,2,

43

1,2,3

4

1,3,4

21,2,

43

1,2,3

4

1,3,4

21,2,43

1,2,3

4

1,3,4

21,2,43

1,2,3

4

1,3,4

21,2,

43

High Power

AFR-S: AFR with reuse 1 and soft reuse 3

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AFR Scheme Details – AFR Performance

• With standard proportional fair scheduling, AFR soft reuse provides– limited gain on both cell edge and cell capacity in localized (AMC-like)

resources– 89% gain on cell edge with +23% gain on cell capacity in distributed

resources

* Initial data and due to change according to updates

Scheme Channel Model

Gross SE(bps/Hz/

cell)

Gain on SE

Cell edge user throughput

Gain on cell

edge user throughput

Note

AMC baseline PedB-3kmph 5.84 667k standard PF

AMC AFR-S 5.93 1.6% 689k 3.3% standard PF

Dist-baseline PedB-3kmph 3.68 315k standard PF

Dist AFR-S 4.52 22.8% 595k 88.9% standard PF

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AFR Scheme Details – AFR Performance (continued)

Scheme Channel Model

SE(bps/Hz/

Cell)

gain on SE

cell edge user

Throughput(kbps)

gain on cell edge user throughput

Note

baseline PedB-3kmph 5.84 667 standard PF

AFR-S 5.93 1.6% 689 3.3% standard PF

AFR-S 5.71 -2.2% 772 15.7% Weighted PF

AFR-S 5.29 -9.4% 819 22.8% Weighted PF

AFR-S 5.18 -11.2% 843 26.4% Weighted PF

• AFR can further improve cell edge user throughput at the expense of cell capacity loss by changing scheduling policy

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AFR Summary

• Theory indicates the optimal (power loading, partition, cost) combination that AFR system achievable

• AFR architecture approaches the optimal solution point with self-learning process

• AFR architecture well deal with the dynamic facts in real system with un-predictable SS distribution and changing propagation environment

• AFR architecture is simple, scalable, and has light overhead

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Standard Support

• Symbol Structure• Signaling Support• Measurement and feedback• Inter-BS Co-ordination

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AFR Implementation Details

1) FFR partition needs to be inherent in Symbol StructureP

hysi

cal f

requ

ency

Reo

rder

ed P

RU

Freq

. Par

t1Fr

eq. P

art2

Freq

. P

art3

Localized

Distributed

(1)Distribute PRU to Freq

Partitions

(3) Distribute PRUs to localized (LLRU) and

distributed (DRU) resources

Localized

Distributed

Localized

Subcarrier permutation

(4) Distribute subcarriers to

subchannels (LRU)

Subcarrier permutation

010203040506070809

00

...(Outer) Permutation of PRU to

Freq. Partitions

Inter-cell (semi static) Intra-cell (potentially dynamic)

Resource groups Single resource

(2) Renumber PRUs for each Freq

partition

Symbol structure with localized and distributed resources per FFR partition

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AFR Implementation Details

2) Downlink Signaling– AFR system configuration needs be broadcast in BCH/SFH or DL MAP– AFR information includes Bandwidth Partition, Power Level, and System

Cost of each partition– Needed for each SS in initial entry process and measurement

3) CQI Indication: – CQI feedback is needed to support frequency selective scheduling

• For example, best-M CQI feedback scheme– CQI is interference-aware, ex. post-processing SINR after LMMSE

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AFR Implementation Details

4) Measurements: – Long-term measurements: Geometric SINR on different reuse

partitions is needed to select best partition for each SS • Can be computed from AFR-friendly preamble

– Short-term measurements: Instantaneous SINR (localized or distributed) on different partitions is needed to support frequency selective scheduling • Can be measured from dedicated pilots in symbol structure design

• Symbol structure should support boosted pilots proportional to data

for soft reuse

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AFR Implementation Details

5) Inter-BS Co-ordination– BS ex-change information for system BW adaptation by:

• Bandwidth partition: W, • Power level: P• Cost vector: C• Load on different FFR partition, etc

– A RRM (function) unit should decide whether the BW partition should be adapted according to fairness constrain and performance metric (system SE and/or edge throughput)

– The adaptation of system wide AFR configuration can be dynamic, semi-static or static

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Conclusions and Recommendations

Conclusions– Adaptive Frequency Reuse (AFR) can effectively improve cell edge

performance while keep or gain in SE– Adaptive Frequency Reuse (AFR) provides very flexible solution to

dynamic deployment environments which approaches the optimal resource allocation in system

– Symbol structure, Signaling design, and Measurement and feedback support are needed to implement AFR

We would like to recommend the following to be incorporated in SDD to support AFR

– Support Adaptive Frequency Reuse partition framework– Support Symbol structure design that accommodates AFR framework– Support Downlink Signaling necessary for AFR– Support Measurements and feedback necessary for AFR, including

preamble and dedicated pilots

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Proposed Text• 11.x Inter-cell interference mitigation• 11.x.1 Flexible Frequency Reuse• Flexible Frequency Reuse can be used to help users suffering from severe inter-cell interference.

With FFR enabled, subcarriers across the whole frequency band are grouped into [K] frequency partitions. Associated with partitions are the parameters of power and cost. The usage of frequency partitions is limited in each BS such that parts of frequency partitions can be unused or have lower/higher transmission power. There is a system cost associated with partitions which is a measurement of the system resource used by the partitions due to its different transmission power, in terms of interference caused to other cells. For example, the cost of a partition is high if this partition is restricted in other sectors to create higher reuse pattern (3, 3/2), or if the partition uses higher transmission power and causes interference to neighbor cells. To control system-wide interference, the BS can adjust partition and power in coordination with other BSs.

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Proposed Text (Cont’d)The following diagram shows an example of frequency partition and power control to mitigate the interference in DL. The system may have [K] frequency partitions in mixture of reuse-1, reuse-M, reuse-N according to deployment needs.

P1-3

P2-2

P3-1

P1-5

P2-4

P3-6

P1-6

P2-5

P3-4 P3-7

P1-7

P2-7P2-1 P2-3

P3-2 P3-3

P1-1 P1-2 P1-4

P2-6

P3-5

Reuse M Reuse N Reuse 1

Power

Power

Power

Frequency

Sector 3

Sector 2

Sector 1

w1 w2 w3 w23 w12 w13 w123

Figure x: Example of Flexible Frequency Reuse

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Proposed Text (Cont’d)11.x.2 Control signaling for inter-cell interference mitigationBS can transmit necessary information through signaling channel or message to MS to support Flexible Frequency Reuse. The necessary information includes frequency partition and additional partition information such as power and cost.

MS can transmit measurements through signaling channel or message to BS to support Flexible Frequency Reuse. The necessary information includes CINR measurements (long-term and instantaneous) or their representatives. The feedback from MS across the frequency partitions should be minimized.

BS can coordinate and exchange information with neighbor BSs to support Flexible Frequency Reuse through the backbone network. The necessary information includes frequency partition, cost and interference level.

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Q & A

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Backup - System Simulation Assumptions

Number of cells 19 cells, with 3 sector per cell(with wrapping around)

Number of sectors per cell 3Site-to-site distance 1.5kmCarrier Frequency 2.5GHzUser Number 10 users/sectorPermutation mode AMC, PUSCMIMO 2x2 with rank adaptation (STBC/SM)

with MMSE aware receiverRepetition ONTarget PER 0.1Strong interference number 14Channel model PedB 3kmphCQI feedback Full feedback with 5ms, 10ms, 15ms

delayHARQ Chase-combing with 4 retransmission

with 4 frames of retransmission delayRB size 48 sub-carriers x 6 symbolsFrame length 5 msScheduler PF