Reducing Channel Dimension in MU-MIMO CSI Feedback

1. Reducing Channel Dimension in MU-MIMO CSI Feedback Authors: Date: 2010-09-15 Nir Shapira et al, Celeno Communications

2. Abstract • CSI feedback overhead is a major issue in MU mode effectiveness • Last meeting [1] was presented. Reducing channel dimension per user was shown as a good method for grouping MU users for increased performance • Explicit Dimension Reduction (DR) was defined – AP notifies each user on its allocated dimension size • [1] showed that DR done locally by user has good performance with less management overhead • In this submission we present detailed suggestion for inclusion in Specification Framework document Nir Shapira et al, Celeno Communications

3. Explicit DR Principles • AP controls just the dimension size per user • AP sends dimension νk to the k’th user • νk ≤nk (nk number of user antennas) • User makes reduction operation on CSI feedback • User only sees channel from AP to user. DR decision is suboptimal • Simulations show ([1]) performance loss vs. a “know-all” user is small • User has several options to reduce dimension, amongst them: • Opt 1 (V/CV feedback): return first rows corresponding to best eigenmodes of V matrix • Opt 2 (CSI FB): receive chain (antenna) selection. User deletes rows of unused antennas from CSI matrix • Opt 3 (CSI FB) : Eigen-mode selection. User returns rows corresponding to largest eigenmodes of effective channel , where U is the receive matrix in the SVD of the channel matrix (H=UDV*) Nir Shapira et al, Celeno Communications

4. Group ID AID1 Detailed Suggestion • AP allocates dimension size νk per user • Allocation is part of MU group definition ([2]). • Allocation can be changed using dedicated action frame for dimension control: • Each user returns a channel of dimension νk as a response to subsequent NDPs • In case user belongs to several MU groups, it can have a different reduced dimension in each group AID2 … AIDN ν1 ν2 … νN AID stands for Associated Identifier AID may be replaced by some other field that identifies STAs Group ID ν1 ν2 … νN Nir Shapira et al, Celeno Communications

5. Override Reduction Option • Add bit in HTC field in sounding announcement to indicate DR override for the upcoming NDP • Enables AP to occasionally sound entire channel without having to redefine MU group Nir Shapira et al, Celeno Communications

6. Resolvable and non-Resolvable LTFs • With resolvable LTFs DR operation mainly effects sounding operation • AP is free to make any further DR decision on top of Users’. User can detect actual DR operation from LTFs in each DL-MU frame • In the non-resolvable case the AP must use entire reduced channel as returned by user • User must store DR information (DR receive matrix) for proper MU packet decoding Nir Shapira et al, Celeno Communications

7. Straw Poll #1 • Do you support adding dimension reduction capabilities, whereby the AP allocates dimension size per user and user performs dimension reduction operation to the specifications framework document (IEEE 802.11-09/0992)? Nir Shapira et al, Celeno Communications

8. Straw Poll #2 • Do you support controlling dimension size allocation by adding fields to MU Group definition frame, and by a specific Dimension Allocation action frame, as detailed in slide 4, to the specifications framework document (IEEE 802.11-09/0992)? Nir Shapira et al, Celeno Communications

9. Straw Poll #3 • Do you support adding a Reduction Override bit to NDP Announcement to enable AP to sound entire channel as detailed in slide 5, to the specifications framework document (IEEE 802.11-09/0992)? Nir Shapira et al, Celeno Communications

10. References [1] Nir Shapira, et. al., Channel Dimension Reduction in MU Operation, TGac, IEEE 802.11-10/0803r0 [2] Joonsuk Kim, et. al., Group ID Concept for Downlink MU_MIMO Transmission, TGac, IEEE 802.11-10/0073 Nir Shapira et al, Celeno Communications