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40 MHz Vs 20 MHz for video

40 MHz Vs 20 MHz for video. Authors:. Date: 2009-07-13. Abstract. Compare Two links using shared 40 MHz channel Two links using two independent 20 MHz channels Show that shared 40 MHz is at least as efficient as 2 independent 20 MHz channels

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40 MHz Vs 20 MHz for video

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  1. 40 MHz Vs 20 MHz for video Authors: Date: 2009-07-13 Robert Stacey, Intel

  2. Abstract • Compare • Two links using shared 40 MHz channel • Two links using two independent 20 MHz channels • Show that shared 40 MHz is at least as efficient as 2 independent 20 MHz channels • List additional advantages and disadvantages of 40 MHz operation Robert Stacey, Intel

  3. PHY rates: 40 MHz Vs 20 MHz (normal GI) 40 MHz rate is slightly more than 2x the 20 MHz rate (2.08x) because of guard band tone filling Robert Stacey, Intel

  4. MAC efficiency assuming 3ms TXOP limit, 10% PER, A-MPDU aggregation (from [1]) MAC efficiency (simulated) Robert Stacey, Intel

  5. MAC efficiency • Sequence overhead is the same irrespective of whether 20 or 40 MHz is used • For equal number of spatial streams, preamble lengths are the same • SIFS, backoff, BA duration etc. are the same • For equivalent efficiency, a 40 MHz sequence needs twice the number of MSDUs per aggregate • There is a 64 MSDU limit on block ack window • At very high PHY rates, this affects efficiency (see plot) • This can be overcome with a combination of A-MPDU and A-MSDU aggregation • Aggregate up to 7 1500B MSDUs to form a 7.5KB A-MSDU and then form A-MPDU • Conclusion: MAC efficiency is same for both 20 MHz and 40 MHz operation Robert Stacey, Intel

  6. Channel access efficiency Backoff STA1 Data DataDataData Data DataDataData • With 1 STA accessing channel: • Average access delay is CWmin/2 slots • With 2 STAs accessing channel: • Access is gained by one of the two STAs in less than CWmin/2 slots • But occasional collisions increase access delay STA2 ACK BA ACK BA Collision Backoff Backoff STA1 Data DataDataData Data ACK BA STA2 TXOP STA3 Data DataDataData Data ACK BA STA4 Access delay Robert Stacey, Intel

  7. Channel access efficiency (simulated) • Access delay = time between successful (non-colliding) channel accesses • Includes time lost to collision and exponential backoff • Assumes there is a collision detect exchange at beginning of TXOP (i.e. Data/ACK or RTS/CTS) • Collision duration (2 or more STAs transmitting) is assumed to be 10 slots (approx 90uS) • Conclusion is not significantly different with higher collision penalty • With default CWmin=15, channel access is more efficient with 2 STAs than for 1 STA Robert Stacey, Intel

  8. Peak single stream: 450 packets per second Peak dual stream: <900 packets per second  Unlikely (but not impossible) that the peaks coincide and sum Statistical multiplexing gain Sum of the 2 flows 2 individual video flows Robert Stacey, Intel

  9. Statistical multiplexing gain • For independent random variables X and Y, the variance of the sum is the sum of their variance: • Standard deviation is • i.e. less than the sum • We don’t eliminate the maximums (peaks), just make them occur much less frequently • i.e. Xmax + Ymax occurs much less frequently than Xmax or Ymax individually Robert Stacey, Intel

  10. 40 MHz operation efficiency • We expect shared 40 MHz to be more efficient independent 20 MHz • Slightly higher PHY data rate (4% gain) • Slightly more efficient channel access (<1% gain) • The overall gain, however, is very slight: ~5% • In addition, there is some gain from statistical multiplexing • Standard deviation of the sum of two VBR streams is less than the sum of the standard deviation of the individual streams Robert Stacey, Intel

  11. Other advantages/disadvantages • Advantage: • Robustness: the more throughput headroom available, the easier it is to build and maintain a buffer against interference • Single flow benefits from having entire 40 MHz available • Dual flow benefits from statistical multiplexing gain and, to a small degree, increased efficiency • Disadvantage: • Interference that only affects one video stream may have an impact on the other due to increased use channel use from retransmissions Robert Stacey, Intel

  12. Throughput & Buffering Throughput headroom determines how quickly the receive buffer is filled or refilled throughput Throughput headroom User latency tolerance Sustained throughput time Buffer depth determines the length and degree of throughput degradation that can be tolerated Start streaming Start playout Robert Stacey, Intel

  13. References • E. Perahia, R. Stacey, Next Generation Wireless LANs: Throughput, Robustness and Reliability in 802.11n, Cambridge University Press, September 2008 Robert Stacey, Intel

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