Low-power WiFi signal tracks movement, even behind walls

To figure out what’s on the other side of a wall or some other opaque barrier, you either need superpowers or a lot of expensive, heavy radar equipment. That might not be the case for long, though. Researchers at MIT’s Department of Electrical Engineering and Computer Science think they’ve sorted out a way to use inexpensive WiFi gear to detect objects and people behind a wall, or even in other rooms.
The technology, known as Wi-Vi, is being developed by professor Dina Katabi and grad student Fadel Adib. The goal is to make this sensor tech as cheap and compact as possible. Yes, gone are the days you have to lug around heavy radar equipment to see through walls. Well, it’s not as simple as that, but using WiFi signals could make Wi-Vi a commodity. It might even start showing up in consumer products that already have WiFi antennas.
Despite the way Wi-Vi is described by MIT, it’s not quite akin to a real x-ray. Wi-Vi is essentially a motion sensing technology that relies on the same principals as x-rays or radar. When an electromagnetic wave encounters an object, there is some amount of reflection. Wi-Vi is designed to detect that with WiFi signals. Wi-Vi is not getting an image of the other side of the wall.

The test rig designed by Katabi and Adib has three WiFi antennas — two are for transmission and one is for reception. The transmitting antennas are identical , except that they output inverse signals. This causes the waves to cancel each other out as they propagate outward from the device. Any static object the waves encounter will produce identical reflections, which are also cancelled out.
When something is moving, the null waves will be reflected differently, and the returning waves won’t be cancelled out. They are picked up by the receiver antenna, and translated into negative and positive signal changes on an output graph. A negative signal would indicate something moving away from the devices, and a positive one toward it.
So Wi-Vi could tell you if a single person was lurking (even very slowly) around behind a wall, but how sensitive is it? Using a computer program, the team realized it is possible to parse out more complicated readings from the receiver. In his testing, Adib found that Wi-Vi was able to reliably detect up to three separate people in another room 90% of the time. The level of sensitivity is very impressive considering these are low-energy WiFi signals. Small movements like shifting your weight, or raising a hand can be detected by Wi-Vi.
Wi-Vi opens up the possibility of gesture-based control from another room. Katabi and Adib designed a test protocol to see how accurately Wi-Vi could tell the difference between two different hand gestures. They found the system had a 100% success rate at distances of up to 6 meters through a wall. Forget the Kinect.
The MIT team believes this technology could be of immense use to search and rescue workers, law enforcement, and regular people who just want to know what’s coming up around the next bend in the road. The gesture sensitivity of Wi-Vi could also add to computer interface design and video game interactions.


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