1. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 1 SINR and Inter-STA Interference Indication Feedback in DL MU-MIMO Date: 2012-03-06 Authors:

2. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 2 Sounding protocol for MU-MIMO in 11ac AP sounds the channels to obtain SNR and Channel feedbacks before MU-MIMO transmission –NDP is unbeamformed –The SNR in compressed beamforming feedback report does not include inter- STA interference /leakage among the MU Group –The actual SINR (including the inter-STA interference /leakage ) during MU- MIMO transmission can be much smaller than the SNR feedback in NDP.

3. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 3 Inter-STA Interference in MU-MIMO Inter-STA interferences in MU-MIMO can be introduced by: –Channel estimation errors –Channel variations or Channel aging Lack of inter STA interference and interference source at AP may lead to: –Wrong MCS selection for MU-MIMO transmission. The interference power can be much larger than the noise power especially when SNR is high. Example: If the SNR at STA1 is 20dB, and the leaking interference power is 10% of the signal power, the SINR became 10 dB. This may lead to 3 MCS level difference. –More Resounding overhead and inaccurate group selection

4. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 4 Benefits of resolvable LTFs are not fully Utilized For a STA without interference suppression capability, it is a waste of using resolvable LTF. –In current 11ac spec, inter-user interference metrics is calculated and then abandoned in this case. Feedback of inter-user interference metrics allows AP to make adjustment and reduce the inter-user interference, especially when a STA does not has interference suppression capability.

5. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Proposals and Comments In The Previous LB Lacking of inter-STA interference information/metric feedback in MU-MIMO is an recurring concern –IEEE 802-11/0131r0: Minimizes trial-and-error of MU MCS rate selection Fine tune MU pre-coding algorithm –CID 3739: In MU-MIMO, the SINR at each STA is not known at the AP –CID 2116 and CID 2122: To include an Interference metric allowing to enhance the pre-coding matrix and beamforming mechanism.

6. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Related Comments In LB187 CID 4610 –NDP sounding in 11ac allows AP to obtain channel information and SNR from STAs via feedback. The channel information and SNR are used for pre-coding and link adaptation in Downlink MU- MIMO transmission. However, such channel information and SNR do not include inter-STA interference during the pre-coded downlink MU-MIMO transmission because sounding NDP is not MIMO pre-coded. Channel estimation errors, channel aging and channel variations can introduce inter-STA interference. Dealing inter-STA interference should not be put on STAs only. AP should have capability to alleviate performance degradation caused by inter-STA interference too. Inter-STA interference can cause performance degradation and even frequent channel re-sounding. Re-sounding overhead is significant because of channel feedback. Slide 6

7. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Related Comments In LB187 (con’d) CID 5275 –In MU-MIMO, AP may not know the accurate SINR at each STA. This is because the SINR at each STA is very sensitive to the varying of channel state of the STAs participated in the MU- MIMO transmission. Channel estimation errors, channel aging and channel variations may all lead to inaccuracy of SINR. Slide 7

8. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 8 SINR and LIS Feedback in MU-MIMO Interference Indication includes two elements –Interference level: indicated by SINR (signal-to-interference-noise-ratio) The 4 bits Delta SINR feedback is defined as: where is the average SINR over all subcarriers and spatial streams. is the average SNR over all subcarriers and spatial streams. The state ‘0000’ represents no information provided and ‘0001’-’1111’ represents 0-14dB delta SINR.

9. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 9 SINR and LIS Feedback in MU-MIMO (con’d) Interference source: Indicated by LIS (Largest Interference Source within the MU group) –“00” indicates no information on LIS. –STA # corresponds to the user position in Group ID after excluding itself. Note: –The bits used here are only for MU-MIMO; –For SU-MIMO, the BA frame is the same as before.

10. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 10 Usage of Interference Indication Efficient Link adaptation –SINR indicates the true condition at the STAs –Fast convergence to best MCS Smart User Selection –E.x., AP can set Nsts=0 to un-select a STA within the group Channel Re-sounding Selection –E.x., AP can choose the right STA with an inaccurate channel to perform Resounding Fine tuning of Pre-coding –E.X., user power allocation (not TPC) can be applied based on SINR and LIS –User power allocation during MU-MIMO transmission is completely transparent to receivers

11. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Proposed Changes Change Figure 8-22 as shown (using 6 reserved bits for Delta SINR and LIS) Insert the following as the ninth paragraph of 8.3.1.9.1: MU_INT field contains six bits. The first four bits (B 3 to B 6 ) indicate the average delta Signal-to-Interference-Noise-Ratio (SINR) feedback defined as Where and are the average SINR and SNR over all subcarriers and spatial streams. The four bits, B 3 to B 6 indicate 0-14dB of delta SINR in one dB step as follows MU_INT (B 3 to B 6 ) =‘ 0000’ represents the delta SINR is not provided; Otherwise, MU_INT (B 3 to B 6 )

12. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Proposed Changes (con’d) The last two bits (B 7 to B 8 ) indicate the largest interference source (LIS) within the MU group. LIS is defined as follows. –00: indicates there is no information on LIS –01: indicates LIS is the STA in the first position within the MU group except the STA itself –10: indicates LIS is the STA in the second position within the MU group except the STA itself –11: indicates LIS is the STA in the third position within the MU group except the STA itself The bits in MU_INT field are reserved in SU case.

13. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 13 Straw Poll Do you agree the resolution for CID4610 and CID5275 provided in slides 11 and 12?

14. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 14 Back Ups Resolvable LTFs can be used to extract delta_SINR and Inter_STA interference/leakage easily

15. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 15 Resolvable LTFs For instance, consider a 4-antenna AP transmitting 1 ss each to 4 STAs each with 2 Rx antennas, the received signal at the k-th STA is At the k-th STA, due to resolvable LTFs, channel can be resolved as where is the equivalent channel (seen by the k-th STA) including pre-coding matrix for j-th STA where is the pre-coding matrix for the j-th STA

16. Submission March 2012 doc.: IEEE 802.11-11-12-0395 Slide 16 Extraction of inter-STA Interference Suppose the equalization matrix is that includes interference suppression if possible. For Example, MMSE equalizer with interference suppression is ( IEEE 802.11-09/1234r1 ) For the k-th STA, the interference brought by the j-th STA is the k-th element of. For example, the interference strength can be defined as For non-linear equalization, Serial Interference Reduction can be used to find interference from other STAs.