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System uses low power Wi-Fi signals to see through walls

Low cost X-ray vision could soon be on the way thanks to researchers at MIT, who have created a system that uses reflected Wi-Fi signals to track the movement of people behind walls and closed doors.

Dubbed Wi-Vi, the technology is based on a concept similar to radar and sonar imaging.

When a Wi-Fi signal is transmitted at a wall, a portion of the signal penetrates through it, reflecting off anyone on the other side.

However, since only a tiny fraction of the signal makes it through to the other room, the rest is reflected by the wall, or by other objects.

The researchers therefore had to devise a technology that could cancel out the arbitrary reflections, and keep only those reflecting from moving human bodies.

Lead researcher Dina Katabi, a professor in MIT's department of Electrical Engineering and Computer Science, decided to incorporate two transmit antennas and a single receiver into the system.

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. In this way, only those reflections that change between the two signals, such as those from a moving object, arrive back at the receiver.

"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," Katabi explained.

The researchers believe the Wi-Vi system could be utilised in everything from disaster recovery to gaming. Katabi says it could even be used as a gesture based interface for controlling appliances in the home such as lighting.

Laura Hopperton

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