1. Another resource to exploit: multi-user diversity
May 2007
doc.: IEEE /2187r0
July 2007
Another resource to exploit: multi-user diversity
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Marc de Courville (Motorola)
Marc de Courville (Motorola)

2. Motivation for 11vht multi band evolution
July 2007
Motivation for 11vht multi band evolution
Reality of the market:
Wi-Fi penetration in the home/enterprise/outdoor is growing
Need to face diversity of device classes and features in a robust way
Need to support multiple connections: throughput shouldn’t drop when increasing the number of devices (in the home simultaneous support for IPTV/MP3 streaming & VoIP calls is a reality)
All the more true for public access networks and office/enterprise communication solutions
Core questions: How to support various BW devices without too much overhead? Is there a value to have several devices transmitting at the same time?
OFDMA exists and provides solutions to:
Unbalanced link budgets
Improved spectral efficiency/higher capacity through more optimal resource scheduling
Dynamic adaptive bandwidth enables spectrum allocation to be sized to traffic need
Marc de Courville (Motorola)

3. Padding loss: it increases with the bandwidth!
July 2007
Padding loss: it increases with the bandwidth!
Fact: when increasing the bandwidth the OFDM granularity becomes quite large and even larger with MIMO to accommodate various packet sizes resulting in increased padding inefficiencies!
Average: bit per carrier considering coverage of MCSs (SISO TGnD, 5% PER): around 1.47bit/carrier
Padding loss on the useful message versus bandwidth utilization: 1.7dB (average) and up to 2.2dB for VoIP packet (ACELP AMR codec at 12.2kbps, 20ms sampling; counting RTP+UDP+IP=40B header and 11 MAC header=34B)
Conclusion: with increased granularity we can gain some dBs…
Marc de Courville (Motorola)

4. Materializing multi-user diversity gain
July 2007
Materializing multi-user diversity gain
In order to increase granularity let assume several STAs are allowed to transmit at the same time with an orthogonal divisions of subcarriers (OFDMA).
Goal: determine upper bound provided by frequency selective scheduling a.k.a. multi-user diversity gain
Assumptions: OFDMA, frequency multiplexing of the users
SISO TGnD channel TRMS=50ns, coherent bandwidth Bc=20MHz thus as a rule of thumb channel is almost constant over 12 consecutive subcarriers (Bc/5).
Same carrier spacing as for 20MHz is assumed: 312.5kHz
100MHz bandwidth representing 320 subcarriers
8 adjacent subcarriers are grouped into one chunk
Nu users are present: 320/Nu/8 chunks are allocated to each user
Chunks are chosen so as to maximize the sum rate capacity
Acronyms:
OCA: optimized chunk allocation
RCA: random chunk allocation
Marc de Courville (Motorola)

5. PHY over the air throughput
July 2007
PHY over the air throughput
6 users: 66% rate increase
2 users: 33% rate increase
Marc de Courville (Motorola)

6. Equivalent SNR Gain of OCA vs. RCA function of SNR
July 2007
Equivalent SNR Gain of OCA vs. RCA function of SNR
>2dB gain already with 2 users!
Marc de Courville (Motorola)

7. PHY Rate CDF July 2007 ~50Mbps >75Mbps ~50Mbps
Marc de Courville (Motorola)

8. Some conclusions on the simulations and implications
July 2007
Some conclusions on the simulations and implications
Simulation results:
BTW OCA represents also a one user full bandwidth usage
Multi-User diversity gain starts with a 2dB for 2 users served and reaches 5dB for 6 users
In terms of throughput benefit: at 10dB SNR, 33% gain with 2 users and 66% gain with 6 users
Marc de Courville (Motorola)

9. Next steps and challenges
July 2007
Next steps and challenges
Thoughts on link budget: is the OFDMA power boost required?
AP 1W limitation (30dBm: 5GHz upper band), STA 200mW (SAR regulation 23dBm)
DL: 100MHz (5x20MHz) yields same power spectral density as for 200mW 20MHz transmissions
UL: 200mW limitation for STA indicates an unbalanced link budget that can be compensated with OFDMA serving 5 users increasing the energy per subcarrier by a factor of 5
Challenge: this presentation deals only with a potential PHY gain to be materialized by dedicated MAC protocols
Open questions:
What are the required ingredients to feed the PAR that will allow to capitalize on this new source of diversity?
Can OFDMA be coupled with smart interference management to handle overlapping BSS for efficient deployments?
From SISO to MIMO: does the frequency selective scheduling gain remain due to the “implicit antenna averaging”? It might depend on the MIMO mode.
Conclusion:
Multi-user diversity is a resource not exploited by IEEE802.11n
OFDMA is a proven technology already present in IEEE802.16, 3GPP LTE is now investigating Multi-User MIMO (e.g. PU2RC).
IEEE802.11vht should take these existing and new trends into consideration in the PAR and in the specification drafting process.
Marc de Courville (Motorola)

10. Backup: influence of the chunk size
July 2007
Backup: influence of the chunk size
Context: comparison with chunks of size 4, 8, 32
Slight improvement with smaller chunks but 10 chunks of 32 carriers would require less MAC overhead
Marc de Courville (Motorola)