Sounding is supported by ePMP 3000 access point (AP) for F300 series subscriber modules (SMs).
Sounding protocol is based on IEEE Std 802.11-2016, sec. 10.34.5.
Beamforming sounding
Transmit beamforming and downlink MU-MIMO require knowledge of the channel state to compute a steering matrix that is applied to the transmitted signal to optimize reception at one or more receivers. Note that channel is measured end-to-end including AP TX path, antennas, channel, and SM RX path. To some extent beamforming compensates HW variations.
The AP transmitting using the steering matrix is called the beamformer, and a SM for which reception is optimized is called a beamformee. An explicit feedback mechanism is used where the beamformee directly measures the channel from the training symbols transmitted by the beamformer and sends back a transformed estimate of the channel state to the beamformer. The beamformer then uses this estimate, perhaps combining estimates from multiple beamformees, to derive the steering matrix.
Beamforming sounding is performed periodically with a default period of 1s. That allows to compensate slow channel variations. To minimize system overhead beamforming sounding is blocked for inactive SMs.
Azimuth detection
For the purpose of MU-MIMO grouping AP determines SM azimuth relative to its sector antenna. The beamforming feedback is post-processed to determine the relative azimuth.
The azimuth is detected periodically with a default period of 60s. That allows to serve nomadic SMs.