[STDS-802-11-TGBN] Regarding DSO for 20MHz-only non-APs

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Hi Rakesh, Manish,

Thanks for the offline discussions.

I want to reiterate that inclusion of 20MHz-only STAs is not a prominent use case for DSO. There were multiple aspects that have been highlighted already:

If you recall, 80MHz bandwidth APs and 20 MHz-only clients have existed since 802.11ac. The reason why DSO was so convincing in 802.11bn was because the operating bandwidths of the commonly available APs (viz. 320MHz or 160MHz) were distinctly higher than that of the max bandwidths of the commonly available clients (160MHz or 80MHz). Because of this, in many cases the AP’s operating bandwidth would end up being unused. So, DSO as a feature was proposed to ensure the operating bandwidth of the AP gets fully utilized using the 160/80MHz clients thereby, resulting in better system performance which in turn would result in better per-user performance. 20MHz clients cannot help any AP utilize its full operating bandwidth unless all of the following are true:

They are available in large numbers
All combinations of switching positions are supported by both the 20MHz clients and the APs
There is sufficient traffic for them to utilize the full operating bandwidth
Higher bandwidth clients are completely absent

Additionally, due to DPS, there is no incentive for higher bandwidth clients to operate with 20MHz.
Multiple PHY challenges regarding reception and transmission of 242-tone RUs from 20MHz-only clients like interference, sounding, beamforming, etc which pose significant scheduling constraints on the AP while allocating them in a wider bandwidth PPDU.

However, I also acknowledge your position on DSO support for 20MHz-only devices. Therefore, I think the next best solution would be to allow 20MHz DSO as a part of a 40MHz PPDU where the secondary 20MHz subchannel is the DSO subband. In a previous email, you have argued regarding how 20MHz DSO should be supported as a part of 160MHz or 320MHz PPDUs and with switching positions aligned with the 80MHz in 320MHz DSO switching positions. I want to point out some considerations regarding that approach:

The most preferred DSO configurations are 80in160MHz and 160in320MHz. The obvious and the only DSO subband positions for these 2 configurations are 80S in a 160MHz PPDU and 160S in a 320MHz PPDU respectively. Based on these examples, the configuration for 20MHz DSO should be 20in40MHz with the DSO subband located in 20S of the 40MHz PPDU.
One additional approved DSO configuration is 80in320MHz. This is a special case where cognizant of non-AP implementation constraints which might want the DSO subband to align with the subband containing the NPCA PC. The NPCA PC is always present in 160S of a 320MHz operating bandwidth. Hence, there is incentive here to allow 80MHz DSO switching positions other than 80S. Do 20MHz-only IOT devices also envision implementing NPCA for which they need these special switching positions?

Even for 80in320 DSO with multiple switching positions, we are still discussing how to solve the problem of assigning an 80MHz DSO subband to a non-AP so that it addresses non-AP constraints as well as the need for AP flexibility. The problem is compounded if it is extended to 20MHz-only STAs

Implementing the comb like 20MHz DSO switching positions requires the presence of other 80MHz, 160MHz and 320MHz clients anyway so that the PPDU transmitted by the AP is not fragmented
The comb-like structure also complicates the AP scheduling even further where it must try to account for the reception constraints on 242-tone RUs for multiple non-APs in the same PPDU
Multiplexing 20MHz and higher bandwidth RUs also compounds the constraint of having only Downlink transmissions and not Uplink.

All in all, it appears to us that 20in40MHz DSO with the DSO subband aligned with 20S of a 40MHz PPDU is the most natural compromise for 20MHz DSO. It will show clear gains when there are only 2 20MHz-only DSO clients associated with an AP. Let us accept this solution and obviate the complications of choosing and assigning DSO switching positions and performance degradation for 20MHz DSO. This configuration will also ensure a much higher probability of practical deployment of 20MHz DSO rather than remaining a standards-defined theoretical option.

Regards,

Sindhu

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