doc.: ieee 802.11-11/0995r0 submission july 2011 nir shapira, celeno communications determination of...
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doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
Determination of Ng in MU mode(CID 3432)
Date: 2011-01-05
Name Affiliations Address Phone email Nir Shapira Celeno
Communications 26 Zarhin st’ Raanana Israel
+972-54-4449370
Authors:
Slide 1
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
Abstract• We suggest means to facilitate usage of Ng<4 in MU
mode
Slide 2
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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
Slide 3
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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
Slide 4
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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
Slide 5
15 20 25 30 35100
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average SNR [dB]
rate
[M
BPS
]
4 2,2,2, with estimation errors, dim.=1, single stream all users
Ng=1Ng=2Ng=4
15 20 25 30 35100
150
200
250
300
350
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4504 1,1,1, with estimation errors
rate
[M
BPS
]
average SNR [dB]
Ng=1
Ng=2
Ng=4
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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
Slide 6
15 20 25 30 350
50
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average SNR [dB]
rate
[M
BPS
]
4 1,1,1,1, with estimation errors
Ng=1
Ng=2
Ng=4
15 20 25 30 3550
100
150
200
250
300
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450
500
4 2,2,2,2, no estimation errors, dim.=1, single stream all users4 2,2,2,2, no estimation errors, dim.=1, single stream all users4 2,2,2,2, no estimation errors, dim.=1, single stream all users
4 2,2,2,2, with estimation errors, dim.=1, single stream all users
average SNR [dB]
rate
[M
BPS
]
Ng=1
Ng=2
Ng=4
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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
Slide 7
15 20 25 30 35100
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450CH. D, VHT 40, including estimation errors
thro
ughp
ut [
MB
PS]
mean SNR [dB]
4 1,1,1,1, Ng=14 2,2,2,2, Ng=4
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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
Slide 8
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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
Slide 9
B13 signals “Ng=4 Allowed”
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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
Slide 10
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
Straw Poll 1
Do you support mandating BFee support for Ng<4 in MU mode?
Slide 11
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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”?
Slide 12
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
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?
Slide 13
doc.: IEEE 802.11-11/0995r0
Submission
July 2011
Nir Shapira, Celeno Communications
References
[1] Hongyuan Zhang et al. “11ac Explicit Sounding and Feedback”, 11-10-1105-01-00ac-explicit-sounding-and-feedback.ppt, Nov 2010
Slide 14