1. Energy Detect CCA Threshold
November 2006
Energy Detect CCA Threshold
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Bill McFarland, Atheros Communications, Inc.
Bill McFarland, Atheros Communication, Inc.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.