The 802.11ac standard simplified Transmit beamforming and removed implicit Transmit beamforming completely. It also mandated the sounding method (NDP) and feedback mechanism. Explicit Feedback was selected as the only feedback method. Furthermore, the only allowed explicit feedback type was compressed -V beamforming weights.

The 802.11ac  Transmit Beamforming is widely deployed for it simplified the implementation and limited the number of different Transmit Beamforming methods. The VHT sounding protocol is greatly simplified from its 802.11n precursor. The 802.11ac VHT-Sounding protocol is discussed here as it is widely deployed. For 802.11n transmit beamforming methods – refer the book (Next Generation Wireless LANs by Eldad Perhaia and Robert Stacey)

In 802.1ac, the Transmit feedback weights should be obtained in the same TXOP (Transmit Opportunity). The delayed feedback that was allowed in 802.11n is not supported

VHT Transmit Protocol

The VHT Sounding Protocol is shown below.

Fig Courtesy: Next Generation Wireless LANs – Eldad Perhaia and Robert Stacey

• The VHT Sounding protocol starts with a VHT NDP Sounding announcement frame.
• The announcement frame indicates that the next frame that is to be received is the NDP frame
• After receiving the VHT NDP frame, the Receiver is supposed to respond with the VHT Compressed Beamforming matrix in SIFS duration

Tx-vector and Rx-vector