1

Differential Feedback Scheme for Closed-Loop Beamforming

IEEE 802.16 Presentation Submission Template (Rev. 9) Document Number:

IEEE C80216m-09_0927Date Submitted:

2009-03-07Source:

Qinghua Li, Yuan Zhu, Eddie Lin, Shanshan Zheng, E-mail: guangjie.li@intel.com Jiacheng Wang, Xiaofeng Liu, Feng Zhou, Guangjie Li, qinghua.li@intel.comAlexei Davydov, Huaning Niu, and Yang-seok ChoiIntel Corporation

Venue: Session #61, Cairo, EgyptRe: TGm AWDBase Contribution:

NonePurpose:

Discussion and adoption by TGm AWDNotice:

This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein.

Release:The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that

this contribution may be made public by IEEE 802.16.

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Outline• Introduction• Differential feedback

– Scheme I: C80216m-09_0528r4, Qinghua Li, et al.– Scheme II: C80216m-08_1187, Bruno Clerckx, et

al.

• Comparison of throughput, reliability, overhead, and complexity

• Conclusions• Proposed text

3

Signal model — a sense about matrix dimensions

• is channel matrix of dimension .

• is beamforming matrix of dimension .

• is transmitted signal vector of dimension .

H tr NN

V̂ st NN

s 1sN

= + nH V̂ sy

4

One-shot reset and differential feedback

Initial feed-backSS:

Differential feedback

Differential feedback

... ...

Reset period

Time

BS:Beamforming using

... ...Time

2V 10V 1VInitial feed-back 11V

Differential feedback 12V

1VBeamforming using 9V

5

There is always correlation between adjacent precoders.

1tV

Set of ideal beamforming matrixes

tV

• Adjacent beamforming matrixes are nearby. • Distance between adjacent matrixes is determined by channel variation and determines beamforming gain degradation within each feedback period.

Beamforming accuracy

TimeFeedback t-1 arrival

Feedback t arrival

6

Differential codebook — polar cap

1ˆ tV

1ˆ tV

dC

tV

tV̂

Set of ideal beamforming matrixes

Polar cap

7

Scheme I

• Differentiation at SS:

• Quantization at SS:

• Beamforming matrix reconstruction at BS:

• Beamforming at BS:

ttH VQD 1

FiH

Cdi

DDDD

maxargˆ

DQV ˆ1ˆ tt

nsVHy tˆ

tttttt H VVQQDQVI

11ˆ1ˆSanity check:

Beamforming matrix needs to be fed back.

8

Actual implementation

• Actual quantization at SS:

• Beamforming matrix reconstruction at BS:

• Beamforming at BS:

DQV ˆ1ˆ tt

nsVHy tˆ

i

HHHi

sCtt

Ndi

DHQHQDIDD

11detmaxargˆ

Note that quantization criterion here is better than that in C80216m-09_0058r4.

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Computation of Q(t-1)

• Low computational complexity – Householder matrix for rank 1– Gram-Schmidt for rank 2

• Add no hardware– Reuse hardware of the mandatory, transformed codebook

1sN

1ˆ tV 1tQ

1

ˆt

V

1ˆ tVΩ

4,2 ts NN

1ˆ tV M

1b 2b BB ImRe

1b 2b

1

1

M

1b 2b

1

2*

2,11*1,11 bbe bb

3

3

m

m

3b 1b 2b4

4

m

m

3b

3*

3,42*

2,41*

1,44 bbbe bbb

1tQ

4b

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Scheme II

• Actual quantization at SS:

• Beamforming matrix reconstruction at BS:

•Beamforming at BS:

1ˆˆˆ tt VDV

nsVHy tˆ

1ˆ1ˆdetmaxargˆ tt

N iHH

iH

sCdi

VHDHDVIDD

Note that quantization criterion here is better than that in C80216m-08_1187.

11

Updates of Scheme II

• Identity matrix is included into the codebook lately. • Polar cap size is reduced through increased correlation 0.95.

12

Scheme I and Scheme II track perturbations of e1 and [e1 e2 e3 e4], respectively.

• Scheme I’s codewords quantize perturbations of e1 on unit circle with 6 degrees of freedom (DOF), while Scheme II’s codewords quantize perturbations of whole identity matrix on Stiefel manifold with 12 DOF.

Scheme I

00.0

01.0

05.0

0

0

0

0.9

D

1

i

Scheme II

0.9

0.9

0.9

0.9

D

1

1

1

1

04.000.000.0

07.005.001.0

01.003.006.0

00.000.002.0

i

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Scheme I codebook matches to precoder delta distribution while Scheme II’s doesn’t. • Differential matrix is symmetric about center [e1] or identity matrix.

• Scheme I’s codebook is symmetric about [e1], while Scheme II’s codebook is asymmetric about identity matrix with uneven quantization errors.

Rotation angle

V(t-1)

V(t)

Scheme I

Rotation angle

V(t-1)

V(t)

Scheme II

Grassmainian manifold Grassmainian manifold

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Other differences

• Correlation adaptation– Scheme I has two codebooks for small and high correlation scenarios, respectively. The two codebooks are pre-defined and stored. – Scheme II varies codebook using measured correlation matrix and costly online SVD computation.

• Rank adaptation– Scheme I can change precoder rank anytime. – Scheme II can not change precoder rank during differential feedbacks and has to wait until next reset.

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The complexity of Scheme II’s 4-bit version is more than triple of Scheme I's 3-bit because Scheme II uses 4x4 matrix operation rather than 4x1 or 4x2.

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Scheme II's 4-bit complexity is more than 50% of Scheme I's 4-bit because Scheme II uses 4x4 matrix operation rather than 4x1 or 4x2.

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Summary of DifferencesScheme I Scheme II

Principle Track perturbation of e1 or [e1 e2].

Track perturbation of [e1 e2 e3 e4].

Codebook dimension 4x1 and 4x2 4x4

Computational Complexity Low High

No. of codebooks One small codebook for each rank

One large codebook for all ranks

Codeword distribution Even distribution on Grassmannian manifold

Uneven distribution on Grassmannian manifold

Rank adaptation Rank can be changed at anytime.

Rank changes only after reset.

Correlation adaptation Two predefined codebooks for small and large correlation scenarios, respectively.

Adaptive codebooks with online SVD computation for different correlation scenarios.

Support of 8 antennas Easy Difficult because of wasted codewords and high complexity.

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Comparison of throughput and reliability

• System level simulation• Single-user MIMO• Implementation losses are included

– Feedback error and error propagation– Quantized reset feedback

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General SLS parameters

Parameter Names Parameter Values

Network Topology 57 sectors wrap around, 10 MS/sector

MS channel ITU PB3km/h

Frame structure TDD, 5DL, 3 UL

Feedback delay 5ms

Inter cell interference modeling Channel is modeled as one tap wide band

Antenna configuration 4Tx, 2 Rx

Codebook configuration Baseline 6 bits, Diff 4 bits or 3 bits

Q matrix reset frequency Once every 4 frames

PMI error free

PMI calculation Maximize post SINR

System bandwidth 10MHz, 864 data subcarriers

Permutation type AMC, 48 LRU

CQI feedback 1Subband=4 LRU, ideal feedback

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Codebook related parameters

Samsung diff. codebook

Intel diff. 4-bit codebook

Intel diff. 3-bit codebook

Codebook size 4 bits i.e. 16 codewords

4 bits i.e. 16 codewords

3 bits i.e. 8 codewords

Feedback overhead

18 bits / 4 frames / Subband including 6-bit reset

18 bits / 4 frames / Subband including 6-bit reset

15 bits / 3 frames / Subband including 6-bit reset

CQI erasure rate 10%

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3-bit Scheme I vs. 4-bit Scheme II

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4 Tx, 2 Rx, 1 stream

0.5λScheme I 5o, Samsung

0.9

4λScheme I 20o, Samsung 0.9

Uncorrelated, Scheme I 20o, Samsung 0.9

SE gain over 16e

5%-ile SE gain over 16e

SE gain over 16e

5%-ile SE gain over 16e

SE gain over 16e

5%-ile SE gain over 16e

Scheme I: 3-bit

11.6% 36.3% 3.7% 21.9% 2.5% 16.2%

Scheme II: 4-bit

10.7% 35.3% 1.3% 19% -1% 7.7%

Scheme I over Scheme II

0.8% 0.7% 2.7% 2.4% 3.5% 7.9%

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Remarks

• Scheme I's 3-bit scheme has higher throughput and reliability than Scheme II's 4-bit scheme. In addition, Scheme I's feedback overhead and complexity are lower than Scheme II's.

• Scheme II's codebook is optimized for highly correlated channels and scarifies uncorrelated/lowly correlated channels.

• Scheme II's codebook can not track channel variation in uncorrelated channel and performs even poorer than 16e codebook.

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Scheme I's 4-bit vs. Scheme II's 4-bit

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4 Tx (0.5λ), 2 Rx, 1 stream

SE gain over 16e

5%-ile SE gain over 16e

Scheme I: 4-bit, 5o

11.7% 37.7%

Samsung: 4-bit, 0.95 ρ

10.6% 35.4%

Scheme I over Samsung

1.06% 1.37%

0 1 2 3 4 5 6 70

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

14Tx 0.5L, 2Rx, rank 1 user Rx SE CDF

user RX SE (b/s/Hz)

16e413

16m416 Intel Diff414,5, 4F reset

16m416 Samsung Diff414,0.95, 4F reset

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4 Tx(4λ), 2Rx, 1 stream

SE gain over 16e

5%-ile SE gain over 16e

Scheme I: 4-bit, 20o

3.9% 21.5%

Samsung: 4-bit, 0.9 ρ

1.36% 19%

Scheme I over Samsung

2.5% 2.1%

0 1 2 3 4 5 6 70

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

14Tx 4L, 2Rx, rank 1 user Rx SE CDF

user RX SE (b/s/Hz)

16e413

16m416 Intel Diff414,20, 4F reset

16m416 Samsung Diff414,0.9, 4F reset

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4 Tx (uncorrelated), 2 Rx, 1 stream

SE gain over 16e

5%-ile SE gain over 16e

Scheme I: 4-bit, 20o

0.2% 17.4%

Samsung: 4-bit, 0.9

ρ

-2.9% 13%

Scheme I over Samsung

3.1% 3.9%

0 1 2 3 4 5 6 70

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

14Tx U, 2Rx, rank 1 user Rx SE CDF

user RX SE (b/s/Hz)

16e413

16m416 Intel Diff41-24,20, 4F reset

16m416 Samsung Diff41-24,0.9, 4F reset

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Comparison on rank-2 codebooks

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4 Tx (4λ), 2 Rx, 2 streams

SE gain over 16e

Scheme I: 4-bit, 20o

10.29%

Samsung: 4-bit, 0.9

ρ

10%

Scheme I over Samsung

0.27%

1 1.5 2 2.5 3 3.5 4 4.5 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

14Tx 4L, 2Rx, rank 2 user Rx SE CDF, Round Robin

user RX SE (b/s/Hz)

16e41-23

16m41-26 Intel Diff41-24,20, 4F reset

16m41-26 Samsung Diff41-24,0.9, 4F reset

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4 Tx (uncorrelated), 2 Rx, 2 streams

SE gain over 16e

Scheme I: 4-bit, 20o

9.45%

Samsung: 4-bit, 0.9

ρ

9.28%

Scheme I over Samsung

0.15%

1 1.5 2 2.5 3 3.5 4 4.5 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

14Tx U, 2Rx, rank 2 user Rx SE CDF, Round Robin

user RX SE (b/s/Hz)

16e41-23

16m41-26 Intel Diff41-24,20, 4F reset

16m41-26 Samsung Diff41-24,0.9, 4F reset

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Remarks

• Scheme I's 4-bit scheme has higher throughput and reliability than Scheme II's 4-bit scheme.

• Scheme I's complexity is lower than Scheme II's.• Scheme I's 4-bit has 0.3% higher throughput tha

n Scheme I's 3-bit.

• Scheme II's codebook is optimized for highly correlated channels and scarifies uncorrelated/lowly correlated channels.

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Conclusions

• Scheme I's 3-bit outperforms Scheme II's 4-bit in all cases in terms of throughput and reliability.

• Scheme I's 3-bit scheme has feedback overhead and computational complexity lower than Scheme II's 4-bit by 17% and 60%, respectively.

• Scheme I's 4-bit has even higher throughput than Scheme I's 3-bit.

• Scheme II's new design solves vibration problem by adding identity matrix but it can not track channel variation in uncorrelated channels.