Meaning
Physical layer control frames contain no data payload and act solely as a reference signal for channel estimation. In wireless networks, the ndp frame carries long training fields that allow receivers to measure the phase and amplitude shifts of the propagation channel. Because there is no MAC service data unit in this frame, the transceiver can allocate the entire transmission to these pilot patterns.
This structure allows the receiver to compute the matrix of channel coefficients.
Packet Structure
The preamble contains the standard training fields followed by several long training fields that match the number of active transmit antennas. This design ensures that each spatial path is measured independently by the receiver during the ndp frame transmission. The absence of a payload field terminates the frame immediately after these training fields.
Calibration Role
Access points use this standardized training signal to run calibration routines that correct hardware mismatches between the transmit and receive paths. Measuring the ndp frame allows the system to adjust for internal phase imbalances across the RF chains. This calibration improves the accuracy of subsequent steering actions.
Overhead Profile
This frame design reduces transmission time by omitting the payload portion of the packet. The quick transmission of the ndp frame minimizes the airtime used for channel sounding.
