Wi-Fi

Seeing through walls

Print edition : July 26, 2013

The MIT research may soon allow us to have X-ray vision like Superman. Photo: Christine Daniloff/MIT

NOW X-ray vision is no longer sci-fi, thanks to researchers at the Massachusetts Institute of Technology’s Computer Science and Artificial Intelligence Laboratory.

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 available only to the military. Now a system being developed by Dina Katabi of MIT’s Department of Electrical Engineering and Computer Science and her graduate student Fadel Adib could give all of us the ability to spot people in different rooms using low-cost Wi-Fi technology.

The 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 it and reflects 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 says.

To do this, the system uses two transmit antennas and a single receiver. The 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 walls —create identical reflections, they too are cancelled out. 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.”

The Wi-Vi uses just one receiver to track the person. As the person moves through the room, his or her distance from the receiver changes, meaning the time it takes for the reflected signal to make its way back to the receiver changes too. The system then uses this information to calculate where the person is at any one time.

Wi-Vi could be used to help search-and-rescue teams to find survivors trapped in rubble after an earthquake, say, or to allow police officers to identify the number and movement of criminals within a building to avoid walking into an ambush. It could also be used as a personal safety device.

R. Ramachandran

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