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Hi everyone,
I’m Gwangho Lee from KNUT.
I want to continue the discussion on my contribution presented on February 13 (DCN: 24/1885r2, “NPCA Hidden Node Problem”) and clarify a few points.
Before addressing the Q&A from the conference call, I first would like to clarify some points that may not have been fully explained during my presentation.
The contribution is for the case where NPCA STA receives both RTS and CTS frames transmitted in an OBSS. The issue of the current PDT text is that even when an NPCA STA receives both RTS and CTS, it determines whether to perform NPCA based on the bandwidth of the RTS frame only.
If a VHT STA that supports Dynamic Bandwidth Operation transmits an RTS, the subsequent CTS can be transmitted with a narrower bandwidth than the bandwidth of the RTS frame transmission. This means that while the RTS may occupy the NPCA PCH, the CTS may not occupy the NPCA PCH which leads to the waste of an NPCA opportunity.
To resolve this issue, the NPCA STA needs to check the bandwidth of the CTS frame transmission which can be done using the bandwidth information already available in the baseline spec.
RTS/CTS Exchange in VHT STA
When a VHT STA transmits an RTS using a non-HT (duplicated) PPDU format, the PHY inserts the CH_BANDWIDTH_IN_NON_HT value into specific bits (bits 6 and 7 out of the first 7 bits) of the Scrambling sequence, indicating the bandwidth at which the RTS is transmitted.
Additionally, if the recipient of the RTS is a VHT STA, the sender must set the TA in the MAC header to the bandwidth signaling TA (mandatory requirement).
The PHY of the VHT STA, set as the RA in the RTS, interprets bits 6 and 7 of the Scrambling sequence as CH_BANDWIDTH_IN_NON_HT upon receiving the RTS and delivers this information to the MAC as an RXVECTOR. If the TA of the received RTS is the bandwidth signaling TA, the MAC considers the CH_BANDWIDTH_IN_NON_HT value valid and recognizes it as the RTS transmission bandwidth.
The STA that receives this RTS can then transmit the CTS using the same or a narrower bandwidth as indicated by the RTS's CH_BANDWIDTH_IN_NON_HT value (dynamic bandwidth operation). The bandwidth information of the CTS is also conveyed using specific bits (bits 6 and 7 of the first 7 bits) of the Scrambling sequence, just as in the RTS.
As a result, the RTS transmitter can determine the bandwidth of the CTS by decoding the specific bits in the Scrambling sequence upon receiving the CTS.
Implications for NPCA STA
It is important to note that the CH_BANDWIDTH_IN_NON_HT value is included in the CTS as part of the RTS/CTS exchange process. If an NPCA STA, which is a third party not participating in the RTS/CTS exchange, can utilize this bandwidth information from the CTS, it will be able to determine the bandwidth at which the CTS was transmitted.
Of course, not all CTS frames necessarily contain bandwidth information. For instance, in a case where only a CTS is received without the corresponding RTS, if the CTS frame contains bandwidth information cannot be determined for sure. NPCA switching is not allowed in such a case by the accepted Motion (#164).
To ensure whether a CTS contains bandwidth information, whether the preceding RTS’s TA was set as the bandwidth signaling TA is a prerequisite.
In summary, if an NPCA STA receives both RTS and CTS and verifies that the RTS’s TA is the bandwidth signaling TA, the CTS contains bandwidth information. If the determined bandwidth of the CTS frame transmission does not occupy the NPCA PCH, NPCA switching can be performed accordingly.
Below, I will provide the detailed answers to the questions raised during the conference call.
Q&A
Q1: Are MU-RTS cases also considered?
A1: There was a mistake in the SP text. This contribution does not consider MU situations. Main focus is the basic RTS/CTS exchange case. The revised SP text, without (MU) is uploaded on the server (24/1885r3).
Revised as follows:
• Do you agree to include the following into the 11bn SFD?
• When an NPCA STA receives a RTS frame and a CTS frame in non-HT (duplicate) PPDU, identification of the channel occupied by received frames is determined by examining the bandwidth of the non-HT (duplicate) PPDU containing the CTS frame
Q2: Is the decision based solely on the CTS bandwidth information? (Is the RTS not decoded?)
A2: No, as I explained above, both the RTS and CTS frames should be received and decoded by the NPCA STA.
If you have any further questions or comments, please feel free to let me know.
Thank you,
Gwangho Lee
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