1. Calibration using NDP
Vincenzo Scarpa

2. NDP rules
Section (D1.07) defines two simple rules about the use of NDP. The intent is to make the behavior at the transmitter/receiver always predictable:
A STA that transmits an NDP shall start the transmission of the NDP as follows:
a SIFS interval after sending a +HTC frame that does not require an immediate response with the NDP Announcement field set to 1
a SIFS interval after successfully receiving a correctly formed and addressed immediate response to a +HTC frame that requires an immediate response with the NDP Announcement field set to 1
On the receiver side this means that:
if a STA receives a +HTC frame with NDP set and Normal ACK policy, it expects an NDP within a SIFS from the response (i.e after the ACK)
if a STA receives a +HTC frame with NDP set and No ACK policy, it expects an NDP within a SIFS from the current received frame

3. Issue related to NDP rules
In the procedure in Figure-n55 (section ) STA A doesn't respect the rule 2):
Even if the frame with Calibration Position=1 (i.e. Cal Start) has the NDP flag set and requires Immediate ACK, the NDP is not sent by STA A within a SIFS from the ACK

4. RDG rules Section 9.14.3 says:
"After transmitting a PPDU containing one or more +HTC MPDUs in which the RDG/More PPDU field set to 0, the RD responder shall not transmit any more PPDUs within the current response burst.“

5. Issues related to RDG
In the procedure in Figure-n55 STA B sets the RDG/More PPDU bit to zero in its ACK frame, and according to the rule described above, it shouldn’t be allowed to send any other PPDU (i.e. NDP from STA B is not allowed)
Why to require an optional feature (i.e. RDG) to implement another optional feature (i.e. Calibration)? It seems that we are adding an unnecessary complexity

6. A slightly new approach
We suggest to re-consider the definition of sounding frame in case of NDP:
At least from a MAC perspective, a +HTC frame with NDP and No-ACK policy followed by an NDP frame within a SIFS is a "single" frame (obviously this implies that the Duration/ID value in the +HTC frame must cover the following NDP)
According to this new definition, we don't need to use RDG since the response from STA B in Figure-n55 is not a response burst any more but just a single MAC frame
STA A can now use SIFS instead of PIFS (there is no need for RDG error recovery) thus respecting the rule 2) in section
The frame exchange sequence doesn’t need to be changed and the overall procedure is simplified

7. New proposal for Calibration using NDP
Non NDP + HTC
NDP announcement bit = 1
ACK Policy: Normal
Calibration Position = 1
TRQ = 1 (i.e. sounding request)
No need for RDG
Non NDP + HTC
ACK Policy: No ACK
Calibration Position = 3
QoS Null + HTC
Calibration Start
NDP
QoS Null + HTC
Sounding Complete AP
SIFS
SIFS
SIFS
SIFS
ACK + HTC
NDP
QoS Null + HTC MA
CSI Feedback
STA
Non NDP + HTC
Calibration Position = 2
NDP announcement bit = 1
Non NDP + HTC
MA = 1 (i.e. Management Action frame)
At MAC level this is a single response frame