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Enhancing CCA for Non-Primary Channels in 802.11n and 802.11ac

This paper by Youhan Kim of Atheros discusses the importance of enhanced Clear Channel Assessment (CCA) performance for non-primary channels in wireless communication standards 802.11n and 802.11ac. The proposed CCA using guard interval correlation is evaluated through simulation results, highlighting the need for improved CCA in wider bandwidth scenarios and mid-packet CCA for non-primary channels.

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Enhancing CCA for Non-Primary Channels in 802.11n and 802.11ac

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  1. Enhanced CCA for Non-Primary ChannelsUsing Guard Interval Authors: Date: 2010-01-06 Youhan Kim, Atheros

  2. Summary • Need for improved CCA performance for non-primary channels was well documented in [1] and CCA using guard interval correlation was proposed. • We echo the need for such enhanced CCA, and provide additional simulation results to demonstrate the effectiveness of CCA using guard interval. Youhan Kim, Atheros

  3. CCA for Secondary Channel in 11n • Only ED CCA with sensitivity of -62 dBm required on secondary channel • C.f. primary channel • SD CCA with sensitivity of -82 dBm • Virtual carrier sense • Higher probability of collision on secondary channel than on primary channel Youhan Kim, Atheros

  4. Wider Bandwidth in 11ac • 11ac allows transmission of signals with wider bandwidth than 11n [2] • 80 MHz and/or wider • While there is only one non-primary (secondary) channel in 11n (40 MHz), there will be many more non-primary channels in 11ac • E.g. 3 non-primary 20 MHz channels for a BSS using 80 MHz • Probability of collision in at least one of the non-primary channels is higher than it was in 11n • CCA performance enhancement for non-primary channels important in 11ac Youhan Kim, Atheros

  5. Mid-Packet CCA for Non-Primary Channel • Difficult to detect preamble in unused non-primary channel(s) during one’s transmission/reception • For STA1/STA2 to detect preamble of red packet • STA1: Need to support simultaneous TX and RX • STA2: Need to support parallel RX • CCA for non-primary channel should be able to detect data portion of packets STA3 to STA4 (BSS2) BSS1 Non-Primary 20 MHz PLCP Data Non-Primary 20 MHz Non-Primary 20 MHz PLCP Data Primary 20 MHz STA1 to STA2 (BSS1) Youhan Kim, Atheros

  6. CCA Using Guard Interval Long GI Symbol Detection Average over N symbols 4 us spacing | | > ? OR 2 Moving Average 0.4 us Average over N symbols 4 us spacing 1 / X Delay 3.2 us and conjugate | |2 Moving Average 0.4 us Average over N symbols 3.6 us spacing | | > ? | |2 Average over N symbols 3.6 us spacing 1 / X Short GI Symbol Detection Youhan Kim, Atheros

  7. Simulation Setup (1/2) • Number of samples per simulation run • Recall 11n: Can transmit 40 MHz PPDU if the secondary channel was clear for PIFS (25 us) • Each simulation run will utilize25 us worth of samples • Number of symbols (peaks)in PIFS • Short GI: 6.94 symbols in 25 us • Long GI: 6.25 symbols in 25 us • At most 6 symbols can be averagedfor GI correlation in PIFS Youhan Kim, Atheros

  8. Simulation Setup (2/2) • CCA per 20 MHz channel assumed • Simulation results are for one 20 MHz channel • Only simulation results using short GI packets shown • For a given multipath profile, detection performance of short GI packets is worse than that of long GI packets Youhan Kim, Atheros

  9. Threshold, Number of Symbols to Average • (=0.7, Nsym,avg=2), (=0.6, Nsym,avg=3)  Pmiss and Pfalse both ≤ 1% Ch B, 1x1, -82 dBm Ch D, 1x1, -82 dBm Youhan Kim, Atheros

  10. Comparison to Primary Channel CCA • Pmiss similar to or better than HT-SIG error rate of primary channel Ch B, 1x1 Ch D, 1x1 Youhan Kim, Atheros

  11. Effects of TX Beamforming • Performance under random TX beamforming is not worse than the case of no TX beamforming 3x1, Ch B 3x1, =0.6, Ch D Youhan Kim, Atheros

  12. Conclusion • Wider bandwidth in 11ac requires improved CCA performance in non-primary channels • CCA using guard interval correlation is a simple and effective means to enhance the mid-packet CCA performance • Pmiss similar to HT-SIG CRC error rate in primary channel • Performs similar to no TX beamforming case even for packets beamformed to another user Youhan Kim, Atheros

  13. References • [1] Hart, B. and Rangarajan, R., Legacy Coexistence – A Better Way?, IEEE 802.11-07/3001r0, January 2008 • [2] Stacey, R, et. al., Proposed Specification Frame Work for TGac, IEEE 802.11-09/0992r2, Sep. 24, 2009 Youhan Kim, Atheros

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