WiCrossing: The First Fresnel Zone Crossing Detection Using Commodity WiFi Devices

Detecting whether a target crosses a certain zone can enable various applications, such as intrusion detection and visitor counting. Traditional methods usually use cameras or infrared sensors for detection. These methods may raise privacy concerns or limit the detection range. Unlike these methods, by exploiting channel state information (CSI) obtained from a commodity WiFi device, WiFi-based methods have great detection potential due to their nonintrusive, privacy-preserving nature and wider coverage area. Although many existing WiFi-based methods can detect whether a target crosses the line of sight (LOS) of the WiFi transceiver pair, these methods usually require a high-sampling rate (such as 1000 Hz), which can severely impact WiFi communication. Moreover, these methods cannot precisely detect whether the target crosses the first Fresnel zone (FFZ). In this article, we propose WiCrossing, which can accurately detect whether a target crosses the FFZ of a pair of WiFi transceivers, even under low-sampling-rate conditions. We divide the crossing FFZ behavior into three stages: 1) entering the FFZ; 2) crossing the LOS path; and 3) exiting the FFZ. By using the frequency diversity of CSI, we design a metric to discriminate whether the target enters or exits the FFZ. In order to achieve accurate detection under low-sampling-rate conditions and mitigate the multipath effects, we propose an effective metric construction method that can extract features that are only relevant to the moving target, and we propose an algorithm to obtain the crossing LOS indicator which can robustly detect whether the target crosses the LOS in different environments even under low-sampling-rate conditions (such as 50 Hz). We implement WiCrossing with commercial WiFi devices, and extensive experiments demonstrate that WiCrossing can achieve a high true positive rate (TPR) and a low false positive rate (FPR) under various evaluation conditions.