Simulation of 20/40 MHz CCA Unfairness

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 • Same 20/40 power assumed 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. Lessons • Implementers should strongly consider not placing their 40 MHz secondary on top of another’s 20 MHz BSS • Implementers should strongly consider implementing DFS • We should investigate a better CCA • Any 5 GHz VHT effort should start with a stronger basis for coexistence • See 07/3001r0 (to be presented in VHT) Hart (Cisco Systems)

17. Questions? • ? Hart (Cisco Systems)

18. Strawpoll • Would you support further investigation into an improved CCA method, if it is parallel to existing methods, optional, testable and accompanied by some modest incentive for implementers? • Yes • No • Abstain Hart (Cisco Systems)

19. Backup Slides Hart (Cisco Systems)

20. Need 40 MHz devices on DFS channels in Europe/USA • 5GHz 40 MHz devices should be performing DFS: • Europe has only 80 MHz of non-DFS spectrum • USA has 80 MHz of non-DFS spectrum plus 80 MHz of outdoor-optimized spectrum • E.g. A 40 MHz AP coming to a mature 20 MHz environment struggles to find 40 MHz that does not significantly affect existing BSSs Hart (Cisco Systems)

21. 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)