1. An improved non-compressed beamforming feedback format for 11ac
July 2011
doc.: IEEE /0xxxr0
July 14, 2011
An improved non-compressed beamforming feedback format for 11ac
Date:
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Fei Tong, CSR
Fei Tong, CSR

2. An improved non-compressed beamforming feedback format for 11ac
July 14, 2011
An improved non-compressed beamforming feedback format for 11ac
CSR
Fei Tong, CSR

3. Summary Propose an improved non-compressed beamforming feedback format
July 14, 2011
Summary
Propose an improved non-compressed beamforming feedback format
Demonstrate that, for equal performance, non-compressed BF offers similar or lower feedback overhead (bits per matrix) to compressed BF, for 1 or 2-antenna beamformee devices
Slide 3
Fei Tong, CSR

4. Focus on beamforming feedback with one or two column vectors
July 14, 2011
Focus on beamforming feedback with one or two column vectors
Beamforming matrix has one or two column vectors
The only option in devices with one or two Rx antennas
Feedback matrix dimension is small : (Ntx, 1) or (Ntx, 2)
Complexity of BF matrix calculation is low
Why?
A large proportion of portable devices with embedded modules will have one or two Rx antennas due to physical size and power consumption constraints
For middle to long propagation distances, 1 or 2 spatial streams is still preferable even if there are >= 2 Rx antennas for reliable communication and option to reject interference
Fei Tong, CSR

5. Improved non-compressed feedback format
July 14, 2011
Improved non-compressed feedback format
Each column vector is normalised by its element with maximum magnitude before quantisation
Beamforming vector is norm and phase invariant
Benefits
Can maximise the quantisation resolution by choosing different scaling for each beamforming vector
Reduced bit count with same quantisation accuracy: only feedback (Nt-1) complex values and one index to represent the position of the maximal magnitude in each vector
1 bits for this index for 2 Tx antennas
2 bits for this index for 3 and 4 Tx antennas
3 bits for this index for 5, 6, 7 and 8 Tx antennas
Fei Tong, CSR

6. Quantisation accuracy and overhead study
July 14, 2011
Quantisation accuracy and overhead study
Accuracy
Compare the CDF of quantisation error (in dBc) over a set of beamforming matrices (8x1 or 8x2) obtained from IEEE fading channel B random realisations
Overhead
The total number of bits needed with a given accuracy to feedback a beamforming matrix
Fei Tong, CSR

7. Nt x 1 BF matrix, lowest resolution
July 14, 2011
Nt x 1 BF matrix, lowest resolution
Compressed format (Feedback type = 0, Codebook Information = 0) (2,4)
Non-compressed format with Nb=2 uses fewer bits than the compressed format (2,4)
Slide 7
Fei Tong, CSR

8. Nt x 2 BF matrix, lowest resolution
July 14, 2011
Nt x 2 BF matrix, lowest resolution
For greater than 3 Tx antennas, non-compressed format with Nb=2 requires fewer bits than compressed format (2,4).
Slide 8
Fei Tong, CSR

9. Nt x 1 BF vector, medium resolution
July 14, 2011
Nt x 1 BF vector, medium resolution
Compressed format (Feedback type = 0, Codebook Information = 1) (4,6)
Non-compressed format with Nb=4 uses fewer bits than compressed format (4,6)
Slide 9
Fei Tong, CSR

10. Nt x 2 BF vector, medium resolution
July 14, 2011
Nt x 2 BF vector, medium resolution
For greater than 3 Tx antennas, non-compressed format with Nb=4 requires fewer bits than compressed format (4,6).
Slide 10
Fei Tong, CSR

11. Nt x 1 BF vector, highest resolution
July 14, 2011
Nt x 1 BF vector, highest resolution
Compressed format (Feedback type = 1, Codebook Information = 1) (7,9)
Non-compressed format with Nb=6 uses fewer bits than compressed format (7,9)
Slide 11
Fei Tong, CSR

12. Nt x 2 BF vector, highest resolution
July 14, 2011
Nt x 2 BF vector, highest resolution
For greater than 3 Tx antenna, non-compressed format with Nb=6 uses fewer bits than compressed format (7,9)
Slide 12
Fei Tong, CSR

13. SU-MIMO performance simulation setting
July 14, 2011
SU-MIMO performance simulation setting
SU-MIMO mode using IEEE Fading channel model B
No channel variation due to feedback delay
Equal 64 QAM, BCC 2/3 coding rate, 125 Bytes PPDU
MMSE receiver with channel estimation
SVD on estimate channel and quantised feedback (FB)
Beamforming based on FB; number of streams equals to number of column vector in FB; equal power on each stream
Non-compressed FB: 2 or 3 Bits quantisation per real value
Compressed FB: low resolution feedback format (2,4)
Fei Tong, CSR

14. Frame error rate results for 1 SS (1)
July 14, 2011
Frame error rate results for 1 SS (1)
Total = 5 bits per tone for Non-comp Nb=2
Total = 7 bits per tone for Non-comp Nb=3
Total = 6 bits per tone for Compressed format
Total = 14 bits per tone for Non-comp Nb=2
Total = 20 bits per tone for Non-comp Nb=3
Total = 18 bits per tone for Compressed format
Fei Tong, CSR

15. Frame error rate results for 1 SS (2)
July 14, 2011
Frame error rate results for 1 SS (2)
Total = 23 bits per tone for Non-comp Nb=2
Total = 33 bits per tone for Non-comp Nb=3
Total = 30 bits per tone for Compressed format
Total = 31 bits per tone for Non-comp Nb=2
Total = 45 bits per tone for Non-comp Nb=3
Total = 42 bits per tone for Compressed format
Fei Tong, CSR

16. Frame error rate results for 2 SS (1)
July 14, 2011
Frame error rate results for 2 SS (1)
Total = 10 bits per tone for Non-comp Nb=2
Total = 14 bits per tone for Non-comp Nb=3
Total = 6 bits per tone for Compressed format
Total = 28 bits per tone for Non-comp Nb=2
Total = 40 bits per tone for Non-comp Nb=3
Total = 30 bits per tone for Compressed format
Fei Tong, CSR

17. Frame error rate results for 2 SS (2)
July 14, 2011
Frame error rate results for 2 SS (2)
Total = 46 bits per tone for Non-comp Nb=2
Total = 66 bits per tone for Non-comp Nb=3
Total = 54 bits per tone for Compressed format
Total = 62 bits per tone for Non-comp Nb=2
Total = 90 bits per tone for Non-comp Nb=3
Total = 78 bits per tone for Compressed format
Fei Tong, CSR

18. Complexity of compression
July 14, 2011
Complexity of compression
Counts of number of complex multiplication and arctan/arccos for one BF matrix
The number of arctan/arccos is quite large
Fei Tong, CSR

19. MU-MIMO performance simulation setting
July 14, 2011
MU-MIMO performance simulation setting
MU-MIMO mode using IEEE Fading channel model B
No channel variation due to feedback delay
2 Rx antenna per user, total 2 users
Each user feedback the vector corresponding to the strongest spatial stream
ZF beamforming based on FB vector from 2 users
Equal 64 QAM, BCC 2/3 coding rate, 125 Bytes PPDU for all users
MMSE receiver with channel estimation per user
Fei Tong, CSR

20. FER results for 1 SS (Nt x 1 FB)
July 14, 2011
FER results for 1 SS (Nt x 1 FB)
Total = 9 bits per tone per user for Non-comp Nb=4
Total = 12 bits per tone per user for Compressed format (5,7)
Total = 26 bits per tone per user for Non-comp Nb=4
Total = 36 bits per tone for Compressed format
Fei Tong, CSR

21. July 14, 2011
Conclusions
For 1 or 2 column BF matrix, non-compressed feedback has similar or lower overhead (with all quantisation resolution level) than compressed feedback with similar BF performance
No extra complexity to beamformee introduced by compressing process
Including non-compressed feedback format does not require changes to BF frame exchange procedure
Fei Tong, CSR

22. Only minor changes needed to the draft
July 14, 2011
Only minor changes needed to the draft
Extra bit is needed to indicate the feedback format in VHT MIMO control field; reserved bits are available
Extra bit is needed to indicate the compressed feedback format is supported in VHT capabilities field; reserved bits are available
Fei Tong, CSR

23. July 2011
doc.: IEEE /0xxxr0
July 14, 2011
Strawpoll 1
Do you support including non-compressed format for low rank feedback (number of streams no greater than 2)
Yes No
Abstain
Fei Tong, CSR
Fei Tong, CSR

24. July 2011
doc.: IEEE /0xxxr0
July 14, 2011
Strawpoll 2
Specify that the non-compressed feedback format support is mandatory at the beamformer and compressed feedback is optional at the beamformee for Nss=1 and 2. For Nss>2, compressed feedback format remains the only option
Yes No
Abstain
Fei Tong, CSR
Fei Tong, CSR

25. July 2011
doc.: IEEE /0xxxr0
July 14, 2011
Strawpoll 3
Modify the 11n non-compressed format: before quantisation, the beamforming matrices are right-hand multiplied by a diagonal matrix so that the largest element of each column vector is one. For each column vector, only feedback (Nt-1) complex values and index of the element equal to one.
Yes No
Abstain
Fei Tong, CSR
Fei Tong, CSR