1. 11ac Explicit Sounding and Feedback
Month Year
Month Year
doc.: IEEE yy/xxxxr0
doc.: IEEE yy/xxxxr0
11ac Explicit Sounding and Feedback
Date:
Authors:
Page 1
John Doe, Some Company
John Doe, Some Company

3. Overview
The following BF/MU items were accepted in Sept IEEE meeting (1105r0):
Only explicit sounding and feedback are applied for SU beamforming, and MU-MIMO.
NDP as the only sounding format.
NDP PPDU frame format.
Immediate feedback only: FB SIFS after NDP or polling frame.
General feedback frame format: Action-No-Ack frame as below.
This presentation continues the discussions of the compressed feedback format for both SU and MU.
Order
Information 1
Category (=VHT) 2
Action (TBD) 3
VHT MIMO Control (TBD) 4
Compressed Beamforming Report (TBD)

4. Compressed V Matrix Feedback
Compressed V matrix FB (as in 11n spec and ) is a good candidate for both SU and MU feedbacks.
Unified format between SU and MU.
Unified format between 11ac in 5GHz and 11n in 2.4GHz and 5GHz.
Reduced overhead (important for MU) compared with raw CSI feedback, by using quantized angles in V to replace raw I/Q values in H.
Overhead reduction from compressed V FB was largely discussed back in 11n—refer to [1].
Refer to Appendix I for size comparisons.
Enables feeding back partial rank, in LOS or ill-conditioned channels.

5. V Feedback v.s. H Feedback
Feeding back V matrix in MU performs similarly as feeding back CSI matrix, for a wide range of precoder designs.
Assume H=U.S.V*, when per-tone substream SNR (equivalent to S) is fed back with V matrix, S.V* is equivalent to H (up to a receiver side rotation matrix).
Refer to the simulations in the subsequent slides.

6. Simulation Settings SU-BF: DL-MUMIMO PER: DL-MUMIMO Throughput:
Ntx=4, Nrx=2, DNLOS 80MHz, MCS15, Compressed V FB with b_phi=4, b_psi=2, Ng=1,2.
DL-MUMIMO PER:
2 Clients, with (Ntx, Nrx1, Nrx2) = (4,2,2), and (6,2,2)
40MHz, DNLOS channels, equal path loss for two clients.
CV FB with (9,7) angle quantization.
Linear interpolation at AP, if Ng=2; no smoothing if Ng=1.
PER of user 1 is measured
DL-MUMIMO Throughput:
4 Clients, with (8,3,3,3,3), and (4,2,2,2,2)
40MHz, DNLOS channels.

7. SU-BF PER: DNLOS 80MHz, MCS15, 4x2, b_phi=4, b_psi=2

8. MU PER: DNLOS 40MHz, (4,2,2) Nss=1 MCS7 per user, b_phi=9, b_psi=7
Note: We may reduce the gap by more
advanced interpolation/smoothing at AP.

9. MU: DNLOS 40MHz, (6,2,2) Nss=2 MCS7 per user, b_phi=9, b_psi=7
Note: We may reduce the gap by more
advanced interpolation/smoothing at AP.

10. MU Throughput: (8,3,3,3,3)
Tables below show throughput comparison of compressed V with tone grouping of one and four and with angle quantization relative to perfect channel feedback
Further overhead reduction can be achieved with rank one feedback
Throughput Loss in % of CV with tone grouping relative to perfect feedback
40MHz, 8x3, 4STA, 11ac Chan D
10dB
13dB
16dB
19dB
22dB
25dB
28dB
31dB
Perfect H
Avg. Tput - Mbps
325
420
537
649
821
968
1066
1166 CV
Ng=1
0.1
0.2
0.3
0.4
Ng=4
0.7

11. MU Throughput: (4,2,2,2,2)
Throughput Loss in % of CV with tone grouping relative to perfect feedback
40MHz, 4x2, 4STA, 11ac Chan D
10dB
13dB
16dB
19dB
22dB
25dB
28dB
31dB
Perfect H
Avg. Tput - Mbps
175
237
303
388
479
540
612
702 CV
Ng=1
0.1 0.
0.3
0.2
Ng=4
0.4
1.0
1.1

12. On Time Domain Compressed H FB [3][4]
Time/Transfer domain compressed H introduces a completely different feedback approach from 11n.
One additional feedback mode for 11ac devices that also support 11n BF mode(s).
Higher complexity, memory requirement and power consumption by using FFT engine to generate and decode the feedback, especially when only immediate feedback is allowed in 11ac.
Feeding back time/transfer domain H matrix doesn’t allow reduced-rank feedbacks, which is definitely sufficient in SU, and somewhat sufficient in MU (e.g. in LOS or ill-conditioned channels).
Example: In a 3x8 H channel with strong LOS, CV FB allows only feeding back 3x1 matrix, but H feedback needs to feed back the whole 3x8 matrix.
This might be a common scenario as MU-MIMO is mainly useful at low range, e.g. refer to the measurement results in [5][6].
The CV Feedback (in frequency domain) in this proposal does not prevent the BFmer/AP to apply advanced interpolation/smoothing technology (e.g. use time domain interpolation) to improve the feedback quality.

13. CV FB Format
Propose to define compressed V matrix FB, which is based on the n subclause , , and , with appropriate changes for 11ac.
Propose to define the 11ac compressed V FB frame for SU and MU be the Action-No-ACK format based on , with appropriate extensions:
Category: VHT
Action: Compressed Beamforming
Define VHT MIMO Control Field, as in the next slide
Define MU exclusive beamforming report that contains per-tone SNR
Refer to [2] on detailed descriptions of VHT MIMO Control Field, Compressed Beamforming Report field, and MU-exclusive Beamforming Report field.
Order
Information 1
Category (=VHT) 2
Action (=Compressed Beamforming) 3
VHT MIMO Control 4
Compressed Beamforming Report 5
MU-exclusive Beamforming Report

14. Conclusions
Propose Compressed V Matrix Feedback as the only feedback format for SU and MU.
Unified between 11ac SU and MU
Unified between 11ac and 11n
Reduced overhead for SU and MU
Good performance in both SU-BF and MU-MIMO.

15. Appendix I: Comparing Sizes of Raw H and CV Feedbacks

16. H vs CV Feedback Size (Bytes)
Follow 11n CSI and CV FB Definitions.
40MHz, Ng=1, H with Nb=8, CV with b_phi=8, b_psi=6 2 3 4 5 6 7 8 1
473
689
905
1121
1337
1553
1769
2201
2633
3065
3497
1985
3281
3929
4577
5225
4361
6089
6953 Nc Nr H: 2 3 4 5 6 7 8 1
189
378
567
756
945
1134
1323
1701
2079
2457
2268
2835
3402
1890
2646
4158 Nr Nc
CV:

17. Discussions The tables are mainly for MU FB (high bit-width required).
Overhead savings are big, e.g.
4x2: %47 reduction
4x4: %68 reduction
8x4: %41 reduction
Overhead saving for SU-BF is more (see [1]).
With reduced rank CV feedback, the saving could be more.

18. Pre-Motion #1
Do you agree to adopt compressed V matrix feedback (based on and in 11n spec) as the only feedback format for SU beamforming and MU-MIMO, and add the same statement into the spec framework?
Yes No
Abstain

19. References
[1] n-BEAM-LB97-CID2969, Joonsuk Kim, et al.
[2] ac-explicit-feedback-format.ppt
[3] ac-csi-report-for-explicit-feedback-beamforming-in-downlink-mu-mimo
[4] ac-time-domain-csi-report-for-explicit-feedback
[5] ac-indoor-channel-measurements-for-tgac
[6] ac_Corridor_channel_measurements_for_tgac