Jeongki,
You asked:
slightly updated question.
How can the AP allow the untriggered UL when MU EDCA timer is non-zero value? Do you have an explicit signaling?
The NPCA text allows:
a) both untriggered and triggered on the NPCA channel, when AP does not enable MUEDCA, and the AP chooses to also send triggers
b) only untriggered on the NPCA channel - MUEDCA not enabled, with the AP chooses to not send triggers
c) only triggered on the NPCA channel - MUEDCA enabled by the AP
Your question is asking, because MUEDCA can only be enabled for both main and NPCA, but not individually, then is it possible to allow NPCA untriggered operation in the case when the AP wants to enable MUEDCA on the main channel?
The answer is "no", this is not possible, but the relevant prelude to your question is:
Does it matter?
That is, if the AP wants to disallow untriggered UL on the main channel, then why does it want to allow untriggered UL on the NPCA channel?
If the AP wants to disallow untriggered UL on the main channel this is probably because it wants to control access and it makes sense that if it is controlling access on the main channel, that it also wants to do this on the NPCA channel.
Futhermore, the nature of the development of NPCA was such that, as expressed in an earlier email in response to other questions, that many people desired a restriction for NPCA to allow only triggered UL, for reasons given in that other email.
As such, the MUEDCA solution allowed an easy way to provide for this mode of NPCA operation.
No one seemed to be asking for a mode wherein untriggered was allowed in the primary channel while untriggered would be allowed in the NPCA channel.
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