Wi-Vi system uses Wi-Fi to see through walls

Researchers at MIT's Computer Science and Artificial Intelligence Laboratory have developed what could become low-cost, X-ray vision. The system, known as "Wi-Vi," is based on a concept similar to radar and sonar imaging, but rather than using high-power signals, this tech uses reflected Wi-Fi signals to track the movement of people behind walls and closed doors.
Journal
http://people.csail.mit.edu/fadel/papers/wivi-paper.pdf
When a Wi-Fi signal is transmitted at a wall, a portion of that signal penetrates through and reflects off any humans that happen to be moving around in the other room. Since only a tiny fraction of the signal passes through the wall, with the rest being reflected, the researchers had to devise a technology that could could cancel out the arbitrary reflections, and keep only those reflecting from moving human bodies.
Dina Katabi, a professor in MIT’s Department of Electrical Engineering and Computer Science, and her graduate student Fadel Adib have tuned a system that uses two transmission antennas and a single receiver. The two antennas transmit almost identical signals, except the second antenna's signal is the inverse of the first, resulting in interference.
This interference causes the signals to cancel each other out. Since any static objects that the signals hit create identical reflections, they are also cancelled out by this effect. Only the reflections that change between the two signals, like moving bodies on the other side of the wall, arrive back at the receiver, allowing the system to track the moving people.
Adib says, "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."
Previous attempts to see through walls in this manner have done so using an array of spaced antennas, which capture the signal reflected off of moving people in the room. Such systems, though effective, would be too cumbersome and expensive for use in a handheld device. By using just one receiver, the new system effectively measures the time it takes for signals to reflect, leading to a calculation of location.
Relying on low-cost Wi-Fi technology, the Wi-Vi system could be utilized in everything from disaster recovery to gaming. Because the device can detect action behind a wall, the system could be used as a gesture-based interface for controlling appliances or lighting.
Venkat Padmanabhan, a principal researcher at Microsoft Research, says the possibility of using Wi-Vi as a gesture-based interface that does not require a line of sight between the user and the device itself is perhaps its most interesting application of all.
Source: MIT