DL MU-MIMO MAC efficiency issue Month Year doc.: IEEE 802.11-yy/xxxxr0 DL MU-MIMO MAC efficiency issue Date: 2010-05-18 Authors: John Doe, Some Company

Abstract MU-MIMO technology essentiality for TGac is no longer to demonstrate We need to make sure that this technology is efficient and performant. MU-MIMO process can be divided into two phases The feedback phase and the transmission phase Previous works have shown that the feedback phase should be made approximately every 20 to 25ms (coherence time of 800ms) [1] In this presentation we want to point out that such a low feedback period can strongly reduce the MAC efficiency of MU-MIMO, and reduce its usage. To efficiently solve this problem, Using only explicit feedback, compression of the CSI report needs to be supported If we use explicit and implicit feedbacks, we could also update the feedbacks during transmission phase Slide 2

MU-MIMO feedback phase duration MU-MIMO process can be divided into two phases, the feedback phase and the transmission phase If we assume explicit CSI feedback, the time duration of the feedback phase can be quite important For the feedback phase of a group of 4 users with 2 antennas each and an AP with 8 antennas with 6 bits quantization MCS2 applied on UL CSI reports: Feedback phase time duration 40MHz 80MHz 160MHz Size of CSI report 4x2,7 kB 4x5,4 kB 4x10,9 kB 12Mbps / MCS2 2,8 ms 5,1 ms 9,6 ms Slide 3

MU-MIMO feedback phase duration Example for 80MHz, with a feedback phase (overhead) of 5,1 ms (MCS2 for CSI report) every 25ms Feedback TXoP Data TxOP 25 ms Overhead Data 4 users 1 MU-MIMO group 8 users 2 MU-MIMO groups 12 users 3 MU-MIMO groups … 16 users 4 MU-MIMO groups time (ms) 0 5 10 15 20 25 Slide 4

Impact on MU-MIMO MAC efficiency The MAC efficiency of MU-MIMO is strongly affected by the increase of the number of users with 25 ms feedback period Calculations with explicit feedback, 80MHz, MCS2 for CSI reports, "8 antennas AP" and 4 "2antennas STA" TxOP of 3ms for transmission phase Slide 5

MU-MIMO MAC efficiency analysis The MAC efficiency is strongly affected by a low duration between feedbacks For high number of users (MU-MIMO full load) For higher bandwidth In highly loaded environnement This will reduce its usage: in our example, after 10 users, it seems not worth performing MU-MIMO We therefore need to improve MU-MIMO MAC efficiency The MAC efficiency gains provided by an increase of the feedback period are significant if the feedback period increase is significant Interference cancellation's benefits may not be sufficient With only explicit feedback, we need to provide strong compression mechanisms With explicit and implicit feedbacks, we can extend the feedback period by refreshing the CSI during the feedback phase Slide 6

One of the potential feedback architecture solution During the feedback phase, implicit feedback could be used, but it seems that explicit feedback is favored We therefore assumed that explicit feedback would be used during the feedback phase During the transmission phase, implicit feedback could then be performed using Sounding PPDU (staggered pilots) for BlockAck UL transmissions. The implicit feedback, which follows the time variation of the channel, would be used to update the CSI and re-compute the MU-MIMO beams This would increase the period of the explicit feedback phase Slide 7

One of the potential feedback architecture solution Calibration performed during the first TxOP of the transmission phase CSI updates and beam-recomputation performed on the following TxOPs Feedback phase Transmission phase

One of the potential feedback architecture solution Calibration performed during the feedback phase CSI updates and beam-recomputation performed on TxOPs in the transmission phase Feedback phase Transmission phase

Conclusion Analysing MAC efficiency for MU-MIMO with explicit feedbacks leads to strong inefficiencies with a feedback period of 25ms We believe that this problem needs to be solved, for the quality of user experience With explicit feedback as the only feedback mechanism, strong compression of CSI report is needed With explicit and implicit feedback mechanism, CSI updates during the transmission phase could extend the feedback period Slide 10

References [1] Ermani, S and Van Zelst, A, Interference cancellation for Downlink MU-MIMO, IEEE 802.11-10/1234r1, Mar. 2010 [2] Ishihara, K. and Yasushi, T., CSI Report for Explicit Feedback Beamforming in Downlink MU-MIMO, IEEE 802.11-10/0332r0, Mar. 2010 Slide 11