1. TGn-LB115-CID 5097
Date:
Authors:

2. Abstract
Resolution to LB115 CID 5097 related to 40 MHz STAs Exemption from scanning

3. Current status TGn Draft 3.0 provides the following
Devices that are associated as 40 MHz devices are required to scan to help the AP in determining the environment if it is safe to operate the 20/40 BSS
STAs provide “20/40 BSS Intolerant Channel Report element” that contains a list of channels on which a STA has found conditions that disallow the use of a 20/40 MHz BSS
STAs in the network are exempted from scanning if the STA meets the following condition:
the total duration of transmitted MSDUs and received unicast MSDUs during the previous Xus does not exceed Y% of Xus.

4. Issues with existing mechanism
STAs in the network are the ones that inform the AP about the status of BSS on the same channel or adjacent (allowed channels in the regulatory domain) channels and if STAs are exempt from scanning it might result in scenarios where 20/40 BSS operation is effecting 20 MHz STAs in the system (because AP doesn’t have the required information)
If all the 20/40 MHz STAs in the network are required to scan (because of lower threshold value) then it drains the battery power of portable devices

5. Signalling mechanism AP STA AP Existing Signalling mechanism STA
In 20/40 BSS Coex element add a new bit to signal
“Request for scanning exemption” flag
0 = no request for exemption
1 = request for exemption based on activity
20/40 BSS Coex element
ACK
ACK
Response from AP – 20/40 BSS Coex element with the field “Scanning Exemption Grant” flag set to
0 = no exemption
1 = exempt from scanning based on activity threshold
ACK AP
STA
Existing Signalling mechanism AP
STA
Proposed Signalling mechanism
Scanning exemption at STA :
A STA that chooses to be exempt from scanning based on activity threshold should be explicitly allowed by the AP to be exempt from scanning as per “Proposed Signalling mechanism” shown above
STAs that are allowed to be exempt from scanning by the AP can be forced to scan at a later time irrespective of their activity if the last received 20/40 BSS Coex element by the STA has the “Response for Scanning Grant” flag set to “0”

6. Setting the “Scanning Exemption Grant” field
If the request for scanning exemption is from a STA which is the only STA that has signalled the un-availability of a specific channel for 20/40 MHz use then the the STA is not allowed to be exempt from scanning
If there are multiple STAs that have indicated that the channel is not usable then :
If all the STAs that have indicated that a specific channel is not usable for 20/40 MHz operation and have all requested for “Exemption from Scanning” then the AP will pick atleast one STA that have indicated unavailability on a specific channel to continue scanning
If from the STAs that have indicated un-availability of a specific channel there is atleast one STA that has not requested for exemption from scanning then the AP can allow all the STAs that have requested exemption from scanning to be exempt from scanning