Researchers at MIT’s Computer Science and Artificial Intelligence Laboratory set out to find an easier way to do this than with current bulky radar technology. They decided to use low-cost Wi-Fi technology.
“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.
The system, called "Wi-Vi," transmits a low-power Wi-Fi signal and, similar to radar and sonar imaging, uses reflections of the signal to track objects.
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. But most of the signal is 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 antennas to transmit and has one receiver. The second Wi-Fi signal is exactly the inverse of the first, and so when they bounce of a static object such as a wall, the signals cancel each other out.
Only those reflections that change between the two signals are sent to 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," said Katabi's graduate student, Fadel Adib.
Wi-Vi will presented at the Sigcomm conference in Hong Kong in August. Engineers believe the technology will be used for search-and-rescue operations, or to allow police to identify criminals or hostages within a building.
The device can also detect gestures or movements by a person standing behind a wall, such as a wave of the arm, Katabi said.
"Such an interface could alter the face of gaming," said Venkat Padmanabhan at Microsoft Research. He said the possibility of using Wi-Vi as a gesture-based interface that does not require a line of sight between the user and the device itself is its most interesting application.