Hi Yan,
Thanks for your presentation and the discussion in the TGax call this morning.
Regardless of the outcome of the proposed change, it appears there is an issue with the description of the Beamformee STS capability field:
This field describes the number of “streams” a STA can receive, either as part of a sounding frame or as part of MU-MIMO frame.
The purpose of this field appears to be to indicate the (total) number of streams a STA supports in terms of
preamble processing, not for data processing. Indeed, the stream/MCS capabilities for data processing are separately indicated in the “Supported HE-MCS And NSS Set” field.
When it comes to preamble processing, the only way the number streams affects the preamble is through the number of HE-LTF symbols. In 11ac, the number of streams (be it sounding streams or total number of streams in an MU packet) maps
uniquely to a number of VHT-LTF symbols (see Table 21-13). This is not the case for 11ax MU PPDUs, which can lead to issues. For instance, if a STA supports processing of up to 4 HE-LTF, it would probably indicate 4 as Beamformee STS capability. However, such
a STA may fail to receive even a single stream if it was sent using 8 HE-LTF symbols.
To be correct in all cases, it looks like the Beamformee STS field should talk about the number of HE-LTF symbols more directly (rather than talking about it indirectly through the number of streams).
A quick solution could be to add the following note to the Beamformee STS capability field:
NOTE: the maximum number of space-time streams for which support is indicated can not be sent using more HE-LTF symbols than the corresponding entry in Table 21-13.
FYI – Table 21-13 is given by:
I believe that might even fully address the issue you were trying to resolve. If you feel it doesn’t, we can discuss further. Either way, I believe it would be good to provide this clarification about the interpretation of the Beamformee
STS field.
Regards,
Sigurd
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