1. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 1 Energy Detect CCA Threshold Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at.http:// ieee802.org/guides/bylaws/sb-bylaws.pdfstuart.kerry@philips.compatcom@ieee.org Date: 2006-11-14 Authors:

2. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 2 Background – 1571r5 1571r5 calls for a post packet detection energy based CCA (PD CCA) threshold in the following cases: 1.A non-GF capable device detects a GF packet 2.Any device detects a packet with a reserved HT-SIG indication The current version of this document has TBD for the exact thresholds to use

3. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 3 Traditional ED CCA Threshold Traditionally, the energy detect (preamble missed) CCA threshold has been 20dB above the minimum sensitivity threshold (-62 dBm) This was maintained in most of 802.11n (-60dBm) –Missed preamble CCA –Adjacent channel CCA before sending 40 MHz packet An idea has been floated to have non-GF devices defer to GF packets based on a post detection energy based CCA threshold equivalent to the minimum sensitivity (-80 dBm) Unfortunately a very low PD CCA threshold will be undesirable in the presence of adjacent channel interference

4. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 4 Tx Spectral Mask and CCA Only 20dB isolation required at edge of adjacent channel Only 28 dB isolation required at the center of adjacent channel 40 dB isolation for alternate and greater offsets (legacy devices)

5. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 5 Spectral Leakage and Raw Power CCA Spectral leakage is composed of distortion products. It appears as raw power and does not look like an 802.11 preamble. If put raw power CCA at minimum sensitivity level –~28dB CCA isolation from adjacent channel –~40 dB CCA isolation from legacy alternate and greater. If put raw power CCA at 20 dB above sensitivity level –~ 48dB CCA isolation adjacent –~60 dB CCA isolation to alternate and greater No easy fix: –Cannot filter out the leakage – it is already within the desired channel –Improving Tx spectral mask costs Tx power AND power efficiency We observe far higher channel busy times in presence of adjacent channel interference when the raw power CCA is lowered to the minimum sensitivity level

6. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 6 General Adjacent Channel Interference Example For example above, STA1 can receive from AP1 (20 dB SNR) If CCA ED threshold is below -70dBm, STA1 cannot transmit to AP1 If CCA ED threshold is above -70dBm, STA1 can transmit to AP1 If ED CCA threshold is high enough, the two networks operate without interference If ED CCA threshold is too low, the networks share airtime as if they were on the same channel STA1AP1STA2AP2 -50dBm-70dBm Channel 1 Channel 6 Tx mask leakage into channel 1

7. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 7 GF and Interference Example STA1 is a non-GF STA, using -80dBm post detection ED CCA AP1 sends a short GF packet (presumably to a different STA in the network) STA1 switches to the low PD CCA threshold If AP2 transmits while STA1 is receiving the GF packet, STA1 will not transmit until AP2’s transmission ends This is true if AP2 begins transmission before or while STA1 receives the the GF packet STA1AP1STA2AP2 -50dBm-70dBm Channel 1Channel 6 Tx mask leakage into channel 1

8. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 8 PD CCA Issues Does not depend on subtle timing between the networks. Happens if the transmission in the adjacent channel starts before or during the GF packet If traffic load in the adjacent channel is heavy, the hang happens on virtually every GF packet If there are multiple adjacent networks, the length of the “hang” can be longer than a maximum length packet – must see a break in all adjacent channel traffic If there is continuous (or near continuous) interference or noise above the threshold level, the non-GF station never gets to transmit again…

9. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 9 ED CCA Blocking Range We recommend leaving CCA at -68dBm or higher to allow ~10 meter AP separation to work well

10. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 10 What Is Going On? Most would agree that having a low energy detect (missed preamble) CCA threshold all the time is unworkable It would seem that since the PD CCA doesn’t kick in until you have seen a valid preamble you would gain a benefit However, it only does two things: –You don’t enter the “bad” CCA state until the first GF packet comes by –You get back out of it after all noise/interference in the area has dropped below the threshold Unfortunately, it still isn’t workable

11. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 11 Low ED CCA Threshold Not Needed – Within BSS Current draft specifies sufficient behavior to insure mixed networks of GF and non-GF devices work correctly: –9.14.2: “All STAs in the BSS shall protect Green Field PPDUs when there is at least one non-HT or non-GF STA associated with this BSS.” MAC protection is the correct choice –Necessary to communicate NAV when devices do not support all MCS/feature combinations –Necessary in order to allow future use of reserved HT-SIG indications –Necessary for the protection of legacy devices Given MAC protection, the new low ED CCA threshold is not needed

12. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 12 Low ED CCA Threshold Not Needed – OBSS In 11g, protection was not required when there were legacy devices (11b) in an OBSS –ED CCA was left 20dB above the sensitivity limit In 11n, protection is not required when there are legacy devices (11a/g) in an OBSS –11n is trusting ED CCA at 20dB above the sensitivity limit to be sufficient to protect legacy devices from GF packets in OBSS case –If current CCA limit doesn’t work for OBSS case, will need GF OBSS protection anyway

13. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 13 Conclusion We recommend the post detection CCA levels in 1571r5 to be set to -68dBm or greater This value will preserve the independence of networks operating on adjacent channels

14. doc.: IEEE 802.11/06-1790r1 Submission November 2006 Bill McFarland, Atheros Communications, Inc.Slide 14 Straw Poll Can you accept replacing the TBDs in document 1571r5 with -68dBm?