Using low-power Wi-Fi to track moving humans — even behind walls | MIT

"To do this, the system uses two transmit antennas and a single receiver. The two antennas transmit almost identical signals, except that the signal from the second receiver is the inverse of the first. As a result, the two signals interfere with each other in such a way as to cancel each other out. Since any static objects that the signals hit — including the wall — create identical reflections, they too are cancelled out by this nulling effect. “So, if the person moves behind the wall, all reflections from static objects are cancelled out, and the only thing registered by the device is the moving human.”

http://web.mit.edu/press/2013/new-system-uses-low-power-wi-fi-signal-to-track-moving-humans.html