Phase based Indoor Real-Time Tracking of Mobile UHF RFID tags

Indoor real-time location system (RTLS) has attracted more and more attention due to the growing demands on location-based services (LBSs). Many localization algorithms have been developed, but most of them are either expensive to implement or suffering from low accuracy because of various impacts, e.g. Occlusion, multipath, and device heterogeneity. This paper presents a novel tracking method based on the measured phase values. Our method could provide real-time tracking of mobile RFID tags using commercial off the shelf(COTS) devices without comprising the accuracy. The basic idea to realize tracking is that the trajectory can be approximated by a series of line segments. Between two consecutive time steps, the tag's velocity can be assumed to be constant. The instant speed at every line segments can be estimated by differentials of the phase measurements. The starting point of the trajectory is calculated using received signal strength (RSSI) through least squares. Then the trajectory can be recursively inferred from the integral of velocity. The performance of our methods is studied in simulations and experimentally verified in our lab.