Impact of MU-MIMO on Passive Wi-Fi Sensing: Threat or Opportunity?

Passive Radars are devices that make use of existing communication signals for wireless channel sensing. On the other hand, Wi-Fi, standardized under IEEE 802.11, has become the main gateway that connects devices to the internet. Recently, IEEE established the WLAN Sensing Task Group whose purpose is to study the feasibility of Wi-Fi-based environment sensing within the 802.11 framework, by combining radar and localization technologies. In the meantime, Multi-User Multiple-Input Multiple-Output (MU-MIMO) technology is introduced to the Wi-Fi standard. It is designed to improve the spatial efficiency of the wireless channel by simultaneously transmitting directive Wi-Fi signals to users. This paper aims at quantifying the impact of MU-MIMO signals on Passive Wi-Fi-based Radar-like sensing. First, based on the position of the client devices and the channel geometry, the radiation pattern of the AP is derived. While the wireless channel is illuminated by directive radio waves, the magnitude of the Poynting vector is obtained at a local point target, which then reflects the incident radio waves. Finally, the signal power seen by a sensing device is computed under the influence of a multipath channel. Our numerical analyses focus on an urban street, and we show that MU-MIMO can be seen as; i) an opportunity, since the vicinity of client devices are better illuminated, or ii) a threat, since the remaining parts of the street do not receive sufficient amount of power for channel sensing applications.