  Passive Wi-Fi-based radars (PWRs) are devices that enable the localization of
targets using Wi-Fi signals of opportunity transmitted by an access point.
Unlike active radars that optimize their transmitted waveform for localization,
PWRs align with the 802.11 amendments. Specifically, during the channel
sounding session preceding a multi-user multiple-input multiple-output downlink
transmission, an access point isotropically transmits a null data packet (NDP)
with a known preamble. From these known symbols, client user equipments derive
their channel state information and transmit an unencrypted beamforming
feedback (BFF) back to the access point. The BFF comprises the right singular
matrix of the channel and the corresponding stream gain for each subcarrier,
which allows the computation of a beamforming matrix at the access point. In a
classical PWR processing, only the preamble symbols from the NDP are exploited
during the channel sounding session. In this study, we investigate multiple
target localization by a PWR exploiting hybrid information sources. On one
hand, the joint angle-of-departure and angle-of-arrival evaluated from the NDP.
On another hand, the line-of-sight angle-of-departures inferred from the BFFs.
The processing steps at the PWR are defined and an optimal hybrid fusion rule
is derived in the maximum likelihood framework. Monte-Carlo simulations assess
the enhanced accuracy of the proposed combination method compared to classical
PWR processing based solely on the NDP, and compare the localisation
performance between client and non-client targets.
