Researchers See Through Walls With ‘Wi-Vi’

A technology that lets you see behind walls could soon be built in to your cell phone.
MIT professor Dina Katabi and graduate student Fadel Adib have announced Wi­Vi, a demonstration of a technology that uses Wi­Fi to allow a viewer to “see” a person moving behind a wall. (Wi­Vi stands for “Wi­Fi” and “vision.”)
Previous work demonstrated that the subtle reflections of wireless inter signals bouncing off a human could be used to track that person’s movements, but those previous experiments either required that a wireless router was already in the room of the person being tracked, or “a whole truck just to carry the radio,” said Katabi.
The new device uses the same wireless antenna as is found in a cell phone or laptop and could in theory one day be embedded in a phone. [See also “WiSee Detects Your Gestures Using WiFi.”]
The trick is canceling out all interfering signals – Wi-Fi doesn’t just bounce off humans, but also walls, floors, and furniture. And those signals are 10,000 to 100,000 times more powerful than the reflections off a human body.
Katabi’s wi­vi sends out two wireless signals, one of which is the inverse of the other. In what Katabi calls “interference nulling,” the two signals cancel each other out unless they hit a moving target – such as a human.
“To silence the noise, we change the structure of the Wi-Fi signal so all the undesired reflections cancel,” she said.
The device is meant to be portable so, for example, a person worried that someone was hiding in the bushes could do a quick scan for her personal safety.

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