Dina Katabi, a professor in MIT's Department of Electrical Engineering and Computer Science, and her graduate student Fadel Adib have developed a way to use low-cost Wi-Fi technology that allows tracking moving humans (or other moving objects) through walls and closed doors. They call the system, which uses reflections from the Wi-Fi signal in a way similar to radar, “Wi-Vi”. Katabi explained, “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.” The reflected signal, especially after passing through walls, is weak, so, Katabi said, “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,”
The system uses two transmit antennas. The signals are almost identical, with the exception that the signal from the second antenna is 180 degrees out of phase with the first. One receive antenna is used. Non-moving objects create identical reflections from the two antennas that are out of phase and cancel each other at the receive antenna as do the two direct signals. Adib explained, “So, if the person moves behind the wall, all reflections from static objects are canceled out, and the only thing registered by the device is the moving human.”
This technique is different than previous experiments using an array of spaced antennas and appears to be different than or perhaps an enhancement of the “WiSee” gesture sensing technology developed at the University of Washington I reported on last month which relies on the Doppler shift on reflections from moving objects.
Potential applications include disaster recovery, personal safety, and gaming. Katabi said, “If you are walking at night and you have the feeling that someone is following you, then you could use it to check if there is someone behind the fence or behind a corner. It could also be used as a gesture controlled interface, similar to WiSee. Venkat Padmanabhan, a principal researcher Microsoft Research, sees Wi-Vi being used as gesture-based interface that does not require a line of sight between the user and the device itself: “Such an interface could alter the face of gaming.”
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