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Wi-Fi Signals Configured as Multi-Room Motion Detector

Dina Katabi (Massachusetts Institute of Technology)

Dina Katabi (Massachusetts Institute of Technology)

Computer scientists at Massachusetts Institute of Technology developed a technique for harnessing Wi-Fi signals to track people’s movements in different rooms. Dina Katabi, a computer science professor at MIT, and graduate student Fadel Adib will discuss their research in August at the ACM Sigcomm conference in Hong Kong.

Katabi and Adib use low-power Wi-Fi signals to reflect and track movement in a similar way to radar and sonar, much higher-powered methods. Wi-Fi signals can penetrate walls, but only a small part of a Wi-Fi signal gets through and reflects off humans on the other side, while most of the signal is reflected back from the wall. The technology developed by the MIT researchers can discriminate between the reflected signals from the humans and the signals bouncing off the walls.

The MIT system, called Wi-Vi, has two transmitter antennas and one receiver. The two transmitters send out almost identical signals, except one signal is the inverse of the other, and designed to interfere with and cancel out each other. When these signal pairs encounter a static object, such as a wall, they create identical reflections and, like the original signals, the reflections cancel out each other.

With moving objects, however, the reflected signals are not identical and thus are not cancelled out and can be detected. As a person moves through a room, the distance changes from that person to the single receiver, which means the time needed for the signal to be reflected back to the receiver also changes. Wi-Vi uses these timing differences from the reflections to spot a moving object and track its movements.

Previous attempts to adapt Wi-Fi for motion detection used multiple antennas, which made the systems more costly and too complex for handheld devices. Katabi says the technology could be built into personal safety monitors, or in search-and-rescue or law-enforcement systems to find people trapped or hiding in buildings.

Another application of Wi-Vi is to detect gestures or movements behind walls for gesture-based controls of lights, door openers, and thermostats. A related application is computer games that can now capture gestures, but only in a sight-line with the gaming device. Katabi notes that the ability to interact with a gaming console in another room would create the potential for more complex and interesting games.

Adib demonstrates Wi-Vi in the following brief video.

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