During a VHT Transmit Beamforming sounding exchange, the beamformee transmits a Compressed Beamforming frame to the beamformer. What is communicated…

In the 802.11ac (VHT) explicit transmit beamforming (TxBF) sounding procedure, the beamformer sends a Null Data Packet (NDP) to the beamformee. The beamformee uses this NDP to measure the characteristics of the MIMO channel. Based on this measurement, it calculates the channel state information (CSI). This CSI is then encapsulated within a VHT Compressed Beamforming frame and transmitted back to the beamformer. The specific name for this CSI data structure, as defined by the IEEE 802.11 standard, is the Feedback Matrix. The beamformer then uses this matrix to compute its own steering matrix for subsequent directed transmissions.

A. Steering Matrix: This is calculated and used by the beamformer after receiving the Feedback Matrix from the beamformee.

B. Beamforming Matrix: This is a generic term; "Feedback Matrix" is the specific, standardized name for the information sent by the beamformee.

D. Beamformee Matrix: This is not a standard term used within the IEEE 802.11 specifications for the beamforming process.

1. IEEE Std 802.11™-2020 (Revision of IEEE Std 802.11-2016), Section 9.35.2, "Explicit feedback beamforming," Paragraph 2: "The beamformer initiates a sounding feedback sequence by transmitting a sounding PPDU. The beamformee then sends a VHT Compressed Beamforming frame or an HE Compressed Beamforming frame containing a feedback matrix that describes the channel."

2. Perahia, E., & Stacey, R. (2013). Next Generation Wireless LANs: 802.11n and 802.11ac. Cambridge University Press. Chapter 10, Section 10.4.2, "Explicit Feedback," page 258: "The beamformee then computes the CSI and sends it back to the beamformer in a compressed beamforming frame... The beamformer uses the CSI feedback to compute the steering matrix V."

3. IEEE Std 802.11™-2020 (Revision of IEEE Std 802.11-2016), Section 21.4, "VHT MIMO Control field," Paragraph 1: "The VHT MIMO Control field is present in the VHT Compressed Beamforming frame... It contains feedback information for single user or multi-user beamforming." This section further details the components that make up the feedback matrix.