A Novel Device-Free Tracking System Using WiFi: Turning Fading Channel From Foe to Friend

Since that phase error exists in Channel State Information (CSI) measured by commodity WiFi device, the existing device-free tracking systems, based on Doppler extracted from CSI, suffer from the ambiguity of moving direction, which results in tracking error. Therefore, in this paper, we propose the design and evaluation of a device-free tracking system to solve this challenge. The contributions of this paper are listed as follows. Firstly, we creatively propose to use signal fading phenomenon, which is considered to be enemy to communication system, to estimate moving direction of the target. Secondly, we introduce ray tracing model to analyze the influence of moving target on received signal power and then build the profiles of received power of two cases, including the target moving towards and away from the link between transmitter and receiver. Thirdly, we rely on short-time Fourier transform (STFT) to process CSI, and then extract the power of Doppler in a time window. Then, Derivative Dynamic Time Warping (DDTW) is utilized to identify the moving direction. Consequently, the target can be tracked by moving speed and direction. We implement the proposed system on the commodity WiFi device and the experimental results show that it can achieve median tracking error 0.84m.