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DL MU-MIMO MAC efficiency issue

Explore ways to boost the MAC efficiency in MU-MIMO by optimizing feedback mechanisms. Analyzing the impact of feedback duration on system performance and proposing solutions for better efficiency.

sharonmurray
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DL MU-MIMO MAC efficiency issue

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  1. DL MU-MIMO MAC efficiency issue Date: 2010-05-18 Authors:

  2. 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

  3. 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 Slide 3

  4. 16 users 4 MU-MIMO groups … 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 time (ms) 0 5 10 15 20 25 Slide 4

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

  6. 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

  7. 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

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

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

  10. 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

  11. 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

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