On the Design of Beamforming Feedback for Wi-Fi Sensing

In recent years, channel state information (CSI) has been widely used for Wi-Fi sensing. Most of the existing Wi-Fi sensing researches are conducted based on the CSI matrix. It is noted that since the IEEE 802.11ac standard, beamforming weights feedback will be the only feedback type supported in the mainstream Wi-Fi standards. This letter aims to investigate Wi-Fi sensing based on beamforming weights feedback, where two feedback schemes namely Scheme 1 and Scheme 2 are analyzed and discussed. In Scheme 1, the transmit beamforming matrix (i.e., beamforming weight) is obtained by directly performing eigenvalue decomposition (EVD) on the channel autocorrelation matrix. As for Scheme 2, the transmit beamforming matrix is obtained based on performing EVD on the conjugate transpose of the channel autocorrelation matrix. The analysis shows that Scheme 1 retains the angle of departure (AOD), together with Doppler differences and time delay differences between different paths, while Scheme 2 can retain the entire AOD, Doppler, and time delay information. The effects of imperfect systems on sensing performance in practice are also discussed in this letter.