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Simulation of 20/40 MHz CCA Unfairness

Simulation of 20/40 MHz CCA Unfairness. Authors:. Date: 2007-11-19. Comment. We have defined CCA for 20/40 MHz operation, but not performed a detailed analysis of its effects. How should we analyze this? Analyze a very simple “home” scenario

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Simulation of 20/40 MHz CCA Unfairness

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  1. Simulation of 20/40 MHz CCA Unfairness • Authors: • Date: 2007-11-19 Hart (Cisco Systems)

  2. Comment Hart (Cisco Systems)

  3. We have defined CCA for 20/40 MHz operation, but not performed a detailed analysis of its effects • How should we analyze this? • Analyze a very simple “home” scenario • Two BSSs: one in a 20mx30m lot, another BSS in a nearby 20mx30m lot • Assume one BSS is 40MHz; one BSS is 20 MHz in the secondary Hart (Cisco Systems)

  4. D3.0 CCA rules on the secondary 20.3.21.5.2 Clear channel assessment (CCA) sensitivity in 40 MHz …The receiver of a 20/40 MHz STA with the operating channel width set to 40 MHz shall provide CCA on both the primary and secondary channels. … When the primary channel is idle, the receiver shall hold the 20 MHz secondary channel CCA signal busy for any signal at or above –62 dBm in the 20 MHz secondary channel. The receiver shall hold both the 20 MHz primary channel CCA and the 20 MHz secondary channel CCA busy for any signal present in both the primary and secondary channel that is at or above –62 dBm in the primary channel and at or above –62 dBm in the secondary channel. Hart (Cisco Systems)

  5. Two lots, two BSSs, six links, 12 RSSIs • Randomly place devices throughout each lot • Calculate RSSIs with random shadowing • Analyze who is unfair to whom Hart (Cisco Systems)

  6. Calculate RSSIs via the standard pathloss model • RSSI = P + Gtx + Grx + PL@1m - 10*PLE*log10(d) + N(0,7) • P = PowerInSecondary = 40MHz ? 12 : 15 dBm • Gtx = IsAP ? 2 : 0 • Grx = IsAP ? 2 : 0 • PL@1m = -46.9 at 5 GHz • PLE = 3.5 • D = distance in m • N(0,7) = 7dB of log-normal shadowing • Reciprocity is enforced Hart (Cisco Systems)

  7. Analyze each scenario for unfairness • Assume a NF and a minimum BSS SINR • Scenario is admissible if min RSSI on AB, BA, CD, DC > SINR+NF • AB BSS can be unfair to CD BSS via 4 links (AC, AD, BC, BD) • If AB BSS is unfair to CD BSS via AC, then add 25%. • Ditto AD, BC, BD • CD BSS can be unfair to AB BSS via 4 links (CA, DA, CD, DB) • If CD BSS is unfair to AB BSS via CA, then subtract 25%. • Ditto DA, CB, DB • Summary: • Unfairness of 100% means AB BSS is unfair to CD BSS on all 4 links • Unfairness of -100% means CD BSS is unfair to AB BSS on all 4 links • Unfairness of 0% means no unfairness Hart (Cisco Systems)

  8. Definition of Unfairness • Sustained, harmful unfairness • Node A is unfair to node C if: • A does not defer when C transmits • The SINR at node B when nodes A and C are transmitting is greater than the SINR threshold • C does defer when A transmits OR the SINR at node D when nodes C and A are transmitting is less than the SINR threshold • That is, A can happily transmit to B without penalty and without regard to C, but either C cannot transmit or C’s transmissions are collided with by A Hart (Cisco Systems)

  9. Simple Example of 100% Unfairness • AB is -50 dBm • CD is -80 dBm • AC, AD, BC, BD are -80 dBm • AB never defer to CD • AB experiences a SINR of 30 dB, so AB never backoff • Any A or B transmission destroys any C or D transmission (0 dB SINR) Hart (Cisco Systems)

  10. Simulation Notes • This is a PHY-level simulation, showing how the PHY can present poor information to the MAC • The impact on throughput and delay depends on traffic patterns • Needs a MAC simulation to resolve • Yet, if there is dense traffic on the 40 MHz BSS, and 100% unfair locations, then throughput on the secondary will be essentially zero. • 2000 simulation runs at each data point, so the effects of topology/location and shadowing are well averaged out • What is left is the CCA rule and any TX power imbalance Hart (Cisco Systems)

  11. Up to 24% unfairness • Up to 24% unfair locations in adjacent lots • Unfair locations are common for up to 15-20 dB SINR • Same 20/40 power due to FCC limits or battery limits • Power in secondary of 40 MHz device is less than 20 MHz devices hence mild unfairness at large range Hart (Cisco Systems)

  12. Did we cherry pick this example? • Try a second example: • Assume BSSs are entirely inside houses, and houses occupy half the lot Hart (Cisco Systems)

  13. In fact, the within-home-BSS problem is even worse • Up to 37% unfair locations in adjacent lots • Unfair locations are common for all SINRs • Same 20/40 power assumed • Power in secondary of 40 MHz device is less than 20 MHz devices hence mild unfairness at large range Hart (Cisco Systems)

  14. Within same-room BSSs is worse still, with up to 72% unfairness • E.g. media server, AP and display in a 4x5m room • Up to 72% unfair locations • Unfair locations are common for all SINRs • Unfair locations are common out to first (20m) and second (40m) adjacent lots • Typical failure mechanism: AB and CD could transmit to each other happily (-40dBm), yet CD sees AB at -75dBm & defers; and so AB can monopolize the channel Hart (Cisco Systems)

  15. Enterprise BSSs need channel planning + the DFS channels to avoid unfairness • E.g. One 40 MHz AP in a conference room; how quickly can that secondary channel be reused? • Assume channel planning, 400m2 per AP; AP at mid-BSA • For 1 floor, channel planning can mostly stop unfairness • For 3-floors, channel planning needs the DFS channels to reasonably limit unfairness Hart (Cisco Systems)

  16. Previous analysis is not inconsistent with these results • 06/608r1 indicates that no CCA on secondary is a bad idea • 06/608r1 achieves acceptable sharing when CCA-ED on the secondary is added • The CCA-ED threshold is not listed, but it seems to be low enough that CCA is being asserted • Therefore 06/608r1 does not seem to be simulating the scenario in this submission. • 06/608r1 does not appear to have anything to say when a 20 MHz BSS on the secondary does not trigger CCA in the 20/40 MHz BSS • If anything 06/608r1 reiterates the point of this submission: bad things happen when CCA on the secondary is non-existent (or ineffective) Hart (Cisco Systems)

  17. Three Alternatives 1. Major / Significant Impact on Legacy Operation in Ext Ch 2. Slight Impact on Legacy Operation in Ext Ch 3. Minimal Impact on Legacy Operation in Ext Ch • No CCA on extension channel • Mandatory independentCCA on both Control and extension channel (ED CCA for ext ch) • Mandatory to avoid 40MHz transmissions when ext ch CCA is busy • Mandatory independent CCA on both Control and extension channel (ED CCA for ext ch) • Mandatory to avoid 40MHz transmissions when ext ch CCA is busy • Independent back-off counters for both Control and Ext Channels • Relatively Fair Sharing of Medium • Including back off counter for ext ch provides some improvements in favor of legacy networks over alt. 2 • Dramatic negative impact on throughput of both HT and Legacy devices ED = Energy Detect, CCA = Clear Channel Assessment Hart (Cisco Systems)

  18. Lessons • Implementers should strongly consider not placing their 40 MHz secondary on top of another’s 20 MHz BSS • Implementers should implement a better CCA on the secondary channel • Should be accompanied by some modest incentive for implementers • But what? 5GHz basically has no relax-able requirements • Therefore just add new language in the standard Hart (Cisco Systems)

  19. Questions? • ? Hart (Cisco Systems)

  20. Strawpoll • Do you agree that the existing weak secondary CCA requirements are a problem that we should try to improve without affecting legacy compliance? • Y • N • A Hart (Cisco Systems)

  21. Candidate Comment Resolutions • In 20.3.21.5.2, insert: • The receiver [shall | should | may] additionally implement an enhanced CCA on the 20 MHz secondary channel. The enhanced CCA satisfies one or both of the following: • - When the primary channel is idle, the start of a valid 20 MHz HT signal in the secondary channel at a receive level equal to or greater than the minimum modulation and coding rate sensitivity of –82 dBm causes the PHY to set PHY-CCA.indicate(BUSY, {secondary}) with a probability > 90% within 4 μs. • - When the primary channel is idle, the receiver shall hold the 20 MHz secondary channel CCA signal busy for any signal at or above –72 dBm in the 20 MHz secondary channel. Hart (Cisco Systems)

  22. Strawpoll • Do you prefer “shall”, “should”, “may” in the previous text, or no text at all? Vote for 1 or more • Shall • Should • May • No Text Change • Abstain Hart (Cisco Systems)

  23. Backup Slides Hart (Cisco Systems)

  24. What About Excess Inter-Property Pathloss? • Same as slide 13, but adding 10 dB inter-property loss • Minimal changes in unfairness • Unfairness appears at closer distances Hart (Cisco Systems)

  25. Preliminary Measured Results • A 20/40 MHz BSS in a shield box (top right), and a 20 MHz BSS on the secondary in another shield box (bottom right), connected via a variable attenuator • APs do sensitive preamble CCA both P+S (beyond the standard) + insensitive CCA-ED • 20/40 client CCA follows the std • Inter-BSS atten increases L-to-R • When CCA-ED in use (L), it’s all good. Once the CCA-ED doesn’t get triggered (mid), thruput degrades nearly 50%. Hart (Cisco Systems)

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