VHT BSS Channel Selection

1. VHT BSS Channel Selection Date: 2011-11-03 Slide 1 Slide 1

2. Motivation • It is an AP’s design choice to select a primary channel that can maximize the throughput of its BSS • However, a poorly designed AP may randomly select a primary channel, or always select a fixed channel as its primary channel, disregarding the OBSS conditions • A bad primary channel selection not only hurts the AP itself, but also hurts the performance of adjacent BSS’s, e.g. downgrades some adjacent BSS’s as 20MHz BSS

3. Transmission mode limitation In a 20/40/80 MHz BSS, even if the Secondary40 is idle, if the Secondary20 is busy, only the primary channel can be used In a 20/40/80/160 MHz BSS, even if the Secondary80 is idle, if the Secondary40 is busy, only the Primary40 can be used

4. Observations on primary/secondary channel access • Once a secondary channel is occupied by an OBSS as a primary channel, it is very hard for a STA to access the secondary channel even if the OBSS has medium load • The STA can grab the secondary channel only if the transmission gap in the primary channel “synchronizes” with that in the secondary channel

5. Avoid Secondary20/40 channel • If a new BSS uses the Secondary20 channel of an existing 80/160MHz BSS as its primary channel, both BSS’s block each other’s secondary20 channels, therefore also blocking each other’s secondary40/80 channels • A new BSS shall avoid aligning its primary channel with the secondary20 channel of an existing 80/160MHz BSS if no beacon is detected in the secondary40 channel • For the similar reason, a new BSS should also avoid overlapping its primary channel with the secondary40 channel of an existing 160MHz BSS

6. 40 MHz OBSS • Is it OK then to allow a new BSS to align its primary channel to a secondary channel of an existing 40MHz BSS or align its primary channel to a secondary channel of an existing 80/160MHz BSS when beacons are detected in the secondary40 MHz channel? ― Not preferred • In 11ac, although CCA threshold in secondary channels are improved, there is still 10dBm difference between the primary channel CCA and secondary channel CCA • It is good to preserve 40MHz chunk as the basic BW unit for HT/VHT BSS’s and drive HT/VHT OBSS’s to converge toward this direction

7. 40 MHz OBSS – cont. • HT AP is not allowed to align its primary/secondary channels to the secondary/primary channels of an existing 40MHz BSS • Assume a VHT AP starting a 20/40/80 MHz BSS and only HT STAs associates to the AP • So practically this is hard to distinguish from an HT 20/40 MHz BSS • The VHT AP could use the VHT Operating Mode Notification frame to drop its bandwidth to 40 MHz to make the similarities even closer. • The HT STAs associated with the VHT AP do not perform mid-packet detection and have 20dBm primary/secondary CCA difference; and can cause exactly the same unfair channel access issues as in an HT BSS if the primary channel is not aligned • The VHT AP is a VHT STA that is also an HT STA. From 11n, an HT AP would not be able to start a 20/40 BSS with its primary on the secondary of a second AP’s 40 MHz channel. • Would a VHT AP starting a 20/40/80 MHz BSS that allows 40 MHz HT STAs to associate be subject to the rules for an HT AP starting a 20/40 MHz BSS?

8. Proposed VHT primary channel selection rules • If an AP starts a VHT BSS that occupies some or all channels of any existing BSSs, the AP may select a primary channel of the new VHT BSS that is identical to the primary channel of any one of the exsting BSSs. • If the AP chooses to select a primary channel of the new VHT BSS from among the channels on which no beacons are detectedduring the OBSS scans, the selected primary channel • 1) shall not be identical to the secondary 20MHz channel of any existing BSSs, and • 2) should not be overlapped with the secondary 40 MHz channel of any existing 160 MHz BSSs

9. Examples (1)

10. Examples (2)