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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
Determination of Ng in MU mode(CID 3432)

Date: 2011-01-05

Authors:

Nir Shapira, Celeno Communications


Abstract
Abstract

  • We suggest means to facilitate usage of Ng<4 in MU mode

Nir Shapira, Celeno Communications


Problem description
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


Discussion
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


Simulation results
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


Simulation results cont
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


Impact on single antenna user cont
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


Suggested solution
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


Signaling ng 4 allowed in ndpa
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


Summary
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


Straw poll 1
Straw Poll 1

Do you support mandating BFee support for Ng<4 in MU mode?

Nir Shapira, Celeno Communications


Straw poll 2
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


Straw poll 3
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


References
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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