EMoD: Efficient Motion Detection of Device-free Objects Using Passive RFID Tags

Efficient and accurate tracking of device-free objects is critical for anti-intrusion systems. Prior solutions for device-free object tracking are mainly based on costly sensing infrastructures, resulting in barriers to practical applications. In this paper, we propose an accurate and efficient motion detection system, named EMoD, to track device-free objects based on cheap passive RFID tags. EMoD is the first RFID system that can estimate the moving direction as well as the current location of a device-free object by measuring critical power variation sequences of passive tags. Compared with previous solutions, the unique advantage of EMoD, i.e., the capability to estimate moving directions, enables object tracking using a much sparser tag deployment. We contribute to both theory and practice of this phenomenon by presenting the interference model that precisely explains it and using extensive experiments to validate it. We design a practical EMoD based intrusion detection system and implement a prototype by commercial off-the-shelf (COTS) RFID reader and tags. The real-world experiments results show that EMoD is effective in tracking the trajectory of moving object in various environments.