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Energy Detect CCA Threshold
November 2006 Energy Detect CCA Threshold Date: Authors: Notice: This document has been prepared to assist IEEE 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 Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures < ieee802.org/guides/bylaws/sb-bylaws.pdf>, 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 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at Bill McFarland, Atheros Communications, Inc. Bill McFarland, Atheros Communication, Inc.
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November 2006 Background – 1571r4 1571r4 calls for an energy detect (ED/raw power) CCA threshold equivalent to the minimum sensitivity level when: A non-GF capable device detects a GF packet Any device detects a packet with a reserved HT-SIG indication The minimum sensitivity level in n is currently specified as -80dBm Based on #2, everyone would have to implement the new ED CCA Bill McFarland, Atheros Communications, Inc.
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Traditional ED CCA Threshold
November 2006 Traditional ED CCA Threshold Traditionally, the ED CCA threshold has been 20dB above the minimum sensitivity threshold (-62 dBm) This was maintained in most of n (-60dBm) Missed preamble CCA Adjacent channel CCA before sending 40 MHz packet The new, much lower CCA threshold (-80dBm) will be undesirable in the presence of adjacent channel interference Bill McFarland, Atheros Communications, Inc.
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Tx Spectral Mask and CCA
November 2006 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) Bill McFarland, Atheros Communications, Inc.
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Spectral Leakage and Raw Power CCA
November 2006 Spectral Leakage and Raw Power CCA Spectral leakage is composed of distortion products. It appears as raw power and does not look like an 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 Bill McFarland, Atheros Communications, Inc.
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Adjacent Channel Interference Example
November 2006 Adjacent Channel Interference Example Tx mask leakage into channel 1 -50dBm -70dBm AP1 STA1 AP2 STA2 Channel 1 Channel 6 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 Bill McFarland, Atheros Communications, Inc.
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GF and Interference Example
November 2006 GF and Interference Example Tx mask leakage into channel 1 -50dBm -70dBm AP1 STA1 AP2 STA2 Channel 1 Channel 6 STA1 is a non-GF STA AP1 sends a short GF packet (presumably to a different STA in the network) STA1 triggers low CCA threshold STA1 is now hung (cannot Tx) for as long as AP2 is transmitting Networks once again share access time, whenever a GF packet is sent Bill McFarland, Atheros Communications, Inc.
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November 2006 ED CCA Blocking Range We recommend leaving CCA at -68dBm or higher to allow ~10 meter AP separation to work well Bill McFarland, Atheros Communications, Inc.
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Low ED CCA Threshold Not Needed – Within BSS
November 2006 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 MAC protection alleviates need for non-GF devices to decode GF HT-SIG and calculate TXTIME for all possible MCS/feature combinations Given MAC protection, the new low ED CCA threshold is not needed Bill McFarland, Atheros Communications, Inc. Bill McFarland, Atheros Communication, Inc.
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Low ED CCA Threshold Not Needed – OBSS
November 2006 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 Networks seem to have survived… In 11n, protection is not required when there are legacy devices (11a/g) in an OBSS 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 Bill McFarland, Atheros Communications, Inc.
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November 2006 Conclusion We should increase the ED CCA levels in 1571r4 if the ED CCA to -68dBm or greater The change is simple, and does not degrade performance The change will preserve the independence of adjacent channel networks Bill McFarland, Atheros Communications, Inc.
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