  Without requiring objects to carry any transceiver, device-free based object
tracking provides a promising solution for many localization and tracking
systems to monitor non-cooperative objects such as intruders. However, existing
device-free solutions mainly use sensors and active RFID tags, which are much
more expensive compared to passive tags. In this paper, we propose a novel
motion detection and tracking method using passive RFID tags, named Twins. The
method leverages a newly observed phenomenon called critical state caused by
interference among passive tags. We contribute to both theory and practice of
such phenomenon by presenting a new interference model that perfectly explains
this phenomenon and using extensive experiments to validate it. We design a
practical Twins based intrusion detection scheme and implement a real prototype
with commercial off-the-shelf reader and tags. The results show that Twins is
effective in detecting the moving object, with low location error of 0.75m in
average.
