Anti CSI-Based Passive Sensing with Private Precoding: A Design, Privacy and Communication Performance Study

Channel state information (CSI) as essential knowledge in communications has recently been exploited by WiFi and Internet-of-Things (IoT) devices to achieve sensing such as human activity and event detection. This passive sensing paradigm in WiFi/IoT systems poses a new privacy threat since CSI is easily accessible by user/IoT devices (UDs), but it is hard to know whether a UD analyzes CSI to extract environmental information and to prevent it from doing so. This paper presents a theoretical study of how a WiFi/IoT access point (AP) can privately precode signals, coined anti-sensing precoding, to impact a UD's CSIbased sensing upon AP-UD communication. To draw insights into environmental information privacy, we model and derive UD's sensing performance metrics in closed form. Two possible cases in which a UD has prior or online CSI knowledge as a basis for sensing decisions are studied. The optimal anti-sensing precoding that minimizes UD's correct decision probability in each case is derived and analyzed. Further, the influences of the precoding on UD's communication performance are discussed. In simulations, we validate our theoretical results and reveal the benefit and cost of the precoding to anti CSI-based passive sensing.