See Through Walls with COTS RFID System!

Through-wall tracking has gained a lot of attentions in civilian ap-plications recently. Many applications would benefit from such device-free tracking, e.g. elderly people surveillance, intruder de-tection, gaming, etc. In this work, we present a system, named Tadar, for tracking moving objects without instrumenting them us-ing COTS RFID readers and tags. It works even through walls and behind closed doors. It aims to enable a see-through-wall technol-ogy that is low-cost, compact, and accessible to civilian purpose. In traditional RFID systems, tags modulate their IDs on the backscat-ter signals, which is vulnerable to the interferences from the am-bient reflections. Unlike past work, which considers such vulner-ability as detrimental, our design exploits it to detect surrounding objects even through walls. Specifically, we attach a group of RFID tags on the outer wall and logically convert them into an antenna array, receiving the signals reflected off moving objects. This paper introduces two main innovations. First, it shows how to eliminate the flash (e.g. the stronger reflections off walls) and extract the reflections from the backscatter signals. Second, it shows how to track the moving object based on HMM (Hidden Markov Model) and its reflections. To the best of our knowledge, we are the first to implement a through-wall tracking using the COTS RFID sys-tems. Empirical measurements with a prototype show that Tadar can detect objects behind 5′ ′ hollow wall and 8′ ′ concrete wall, and achieve median tracking errors of 7.8cm and 20cm in the X and Y dimensions. 1