1. Simulation of 20/40 MHz CCA Unfairness
September 2006
doc.: IEEE /1458r0
January 2008
Simulation of 20/40 MHz CCA Unfairness
Date:
Authors:
Hart (Cisco Systems)
Joonsuk Kim, Broadcom Corp.

2. Comment January 2008 Hart (Cisco Systems) CID Commenter LB Draft
Clause Number(C)
Page(C)
Line(C)
Type of Comment
Part of No Vote
Comment
Proposed Change
5362
Hart, Brian
115 3
205 59 T N
We have failed to produce a 20/40 coexistence scheme that is both fair to legacy devices yet enables 40 MHz operation where such operation does not substantially degrade fairness. With the current spec, many cases of asymmetric channel access (i.e. asymmetric hidden nodes) can be pointed to. This is because presently there is no virtual carrier sense on the secondary, weak physical carrier sense on the secondary, weak requirements on starting a 20/40 BSS at 5 GHz, and no in-service scanning requirements on the 20/40 BSS at 5 GHz. If a legacy user finds their legacy WiFi experience is severely degraded by 11n devices and there is no way to help the user except by saying "manually change your channel away from the secondary of a busy 5 GHz BSS as soon you manually detect that that a busy BSS has chosen to camp next to your channel" then we haven't done a very good job. s
Make degrading a legacy user's WiFi experience the last choice: revisit more stringent CCA thresholds on the secondary, stronger rules for starting a 20/40 BSS, stronger rules for in-service scanning. Reward devices that are fairer: for instance allow relaxed scanning rules for devices that have sensitive secondary CCA thresholds.
Hart (Cisco Systems)

3. January 2008
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
January 2008
D3.0 CCA rules on the secondary
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
January 2008
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
January 2008
Calculate RSSIs via the standard pathloss model
RSSI = P + Gtx + Grx *PLE*log10(d) + N(0,7)
P = PowerInSecondary = 40MHz ? 12 : 15 dBm
Gtx = IsAP ? 2 : 0
Grx = IsAP ? 2 : 0
= 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
January 2008
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
January 2008
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
January 2008
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. January 2008
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 January 2008
Up to 24% unfair locations in adjacent lots
Unfair locations are common for up to 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?
January 2008
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
January 2008
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
January 2008
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. January 2008
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
January 2008
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 January 2008
1. Major / Significant Impact on Legacy Operation in Ext Ch
Slight Impact on Legacy Operation in Ext Ch
3. Minimal Impact on Legacy Operation in Ext Ch
No CCA on extension channel
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
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
Dramatic negative impact on throughput of both HT and Legacy devices
Relatively Fair Sharing of Medium
Including back off counter for ext ch provides some improvements in favor of legacy networks over alt. 2
ED = Energy Detect, CCA = Clear Channel Assessment
Hart (Cisco Systems)

18. January 2008
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. January 2008
Questions? ?
Hart (Cisco Systems)

20. January 2008
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
January 2008
Candidate Comment Resolutions
In , 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. January 2008
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. January 2008
Backup Slides
Hart (Cisco Systems)

24. What About Excess Inter-Property Pathloss?
January 2008
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
January 2008
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)