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Determination of Ng in MU mode (CID 3432)

Determination of Ng in MU mode (CID 3432). Date: 2011-01-05. Authors:. Abstract. We suggest means to facilitate usage of Ng<4 in MU mode. Problem Description. In current draft BFee can use any Ng value in both SU and MU Ng value is not conveyed in advance to BFer

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Determination of Ng in MU mode (CID 3432)

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  1. Determination of Ng in MU mode(CID 3432) Date: 2011-01-05 Authors: Nir Shapira, Celeno Communications

  2. Abstract • We suggest means to facilitate usage of Ng<4 in MU mode Nir Shapira, Celeno Communications

  3. Problem Description • In current draft BFee can use any Ng value in both SU and MU • Ng value is not conveyed in advance to BFer • BFer is “surprised” by Ng value in each new Beamforming Matrix report • Using Ng<4 incurs high complexity in case where user is first in NDPA list and needs to reply within SIFS • It is expected many vendors will opt for Ng=4 to reduce complexity • Users that reply to a Poll should have no problem using Ng<4 • Ng value has implications on performance in MU mode • A small Ng value involves high protocol overheads • A large Ng value impacts performance ([1], and slides ahead) • Ng value should be taken into account in MU grouping decision Nir Shapira, Celeno Communications

  4. Discussion • Only the AP has the full picture of the implications of Ng value, in terms of both MU performance (since it depends on stream/power allocation for all users), and feedback overhead • Optimal Ng value should be determined dynamically according to SNR, group size, stream allocation, overhead (sounding frequency), channel flatness, etc. • From feedback overhead perspective, determination of matrix dimension size and tone grouping should be taken jointly. • While it is true that a user with Ng=4 harms its own performance, if all users tend to use Ng=4, the entire network performance will be impacted and MU mode effectiveness will be seriously compromised Nir Shapira, Celeno Communications

  5. Simulation Results • We show effect of grouping with a 4T Bfer and three users in channel D. Each user is served by one stream. • Performance of single antenna users is compared to that of dual antenna users • Degradation effect of Ng=4 is drastic with single antenna users. Interference cancelation in dual antenna users partially limits loss Nir Shapira, Celeno Communications

  6. Simulation Results, cont’ • With 4 users, effect is even more drastic • Ng=2 performance is almost identical to Ng=1. Ng=4 is distinctively inferior • Usage of Ng=1 or 2 enables single antenna clients to effectively participate in MU-MIMO operation Nir Shapira, Celeno Communications

  7. Impact on Single Antenna User, cont’ • Usage of Ng=4 degrades 2-antenna users’ performance to that of a single antenna user employing Ng=1 • Would be a pity to invest in 2 RF chains and get the effective performance of a single antenna device in MU mode Nir Shapira, Celeno Communications

  8. Suggested Solution • We suggest two alternative solutions • Solution A: • Ng Value in MU mode must be less than 4 (can be 1 or 2) • Optionally, BFer sends a bit per user in NDPA to signal “Ng=4 allowed” (to control overhead in cases where Ng=4 is tolerable). If set, BFee is encouraged to use Ng=4 • Solution B: • BFer sends bit per user in NDPA to signal “Ng=4 allowed” • In case the bit is not set, a BFee that cannot reply with Ng<4 (for whatever reason) returns a Null VHT Compressed Beamforming Report (Remaining Segments = 7) • No need for BFee to declare Ng capabilities in advance Nir Shapira, Celeno Communications

  9. Signaling “Ng=4 Allowed” in NDPA • Use one reserved bit in Sounding Sequence field to signal: • 0 – Nc index in STA Info field has 3 bits (as in spec today) • 1 – Nc index in STA Info field has 2 bits (max Nc=3). The remaining bit signals “Ng=4 allowed” • In SU mode STA Info field remains as in spec today B13 signals “Ng=4 Allowed” Nir Shapira, Celeno Communications

  10. Summary • We showed simulation results indicating usage of Ng=4 will seriously compromise MU performance • Two alternative solutions were presented to enable BFer to get matrix with Ng<4 when needed • “Ng=4 Allowed” bit per user was suggested in NDPA • The suggested schemes do not impact SU operation Nir Shapira, Celeno Communications

  11. Straw Poll 1 Do you support mandating BFee support for Ng<4 in MU mode? Nir Shapira, Celeno Communications

  12. Straw Poll 2 Do you support mandating BFee support for Ng<4 in MU mode and dedicating a bit in STA Info field in NDPA to indicate “Ng=4 allowed”? Nir Shapira, Celeno Communications

  13. Straw Poll 3 Do you support dedicating a bit in the STA Info field in the NDPA frame to indicate “Ng=4 allowed” and that a STA that is unable to comply to this dictum will feedback an empty VHT Compressed Beamforming Report frame? Nir Shapira, Celeno Communications

  14. References [1] Hongyuan Zhang et al. “11ac Explicit Sounding and Feedback”, 11-10-1105-01-00ac-explicit-sounding-and-feedback.ppt, Nov 2010 Nir Shapira, Celeno Communications

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