New technology can track movement behind walls

Scientists are developing a new technology that could give us 'X-ray' vision with the ability to track moving humans hiding in closed rooms or behind walls.
Researchers at Massachusetts Institute of Technology's Computer Science and Artificial Intelligence Laboratory are using low-cost wi-fi technology to develop the system that spots movement of people in rooms or behind walls.
"We wanted to create a device that is low-power, portable and simple enough for anyone to use, to give people the ability to see through walls and closed doors," said Dina Katabi, a professor in MIT's Department of Electrical Engineering and Computer Science.

Researchers have long attempted to build a device capable of seeing people through walls. However, previous efforts to develop such a system have involved the use of expensive and bulky radar technology that uses a part of the electromagnetic spectrum only available to the military.
The new system, called "Wi-Vi", is based on a concept similar to radar and sonar imaging. But in contrast to radar and sonar, it transmits a low-power wi-fi signal and uses its reflections to track moving humans. It can do so even if the humans are in closed rooms or hiding behind a wall.
As a wi-fi signal is transmitted at a wall, a portion of the signal penetrates through it, reflecting off any humans on the other side.

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

"So we had to come up with a technology that could cancel out all these other reflections, and keep only those from the moving human body," Katabi said.
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.

In this way, only those reflections that change between the two signals, such as those from a moving object, arrive back at the receiver," researcher Fadel Adib said.
"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," Adib added.
Once the system has cancelled out all of the reflections from static objects, it can then concentrate on tracking the person as he or she moves around the room.
Wi-Vi will be presented at the Sigcomm conference in Hong Kong in August.