Wi-Fi signal used to track moving humans — even behind walls
July 3, 2013
‘Researchers at MIT’s Computer Science and Artificial Intelligence Laboratory.are developing a system called “Wi-Vi” that transmits a low-power Wi-Fi signal and uses its reflections to track moving humans — even if they are in closed rooms or hiding behind a wall.
How it works
The researchers borrowed a technique called inverse synthetic aperture radar (ISAR), which has been used for mapping the surfaces of the Earth and other planets. ISAR uses the movement of the target to emulate an antenna array.
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. 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
To do this, the system uses two transmit antennas and a single receiver. The two 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 wall — create identical reflections, they too are cancelled out by this nulling effect. In this way, only those reflections that change between the two signals, such as those from a moving object, arrive back at the receiver.
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.
Possible uses in disaster recovery, personal safety, gaming
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, says Dina Katabi, a professor in MIT’s Department of Electrical Engineering and Computer Science. “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.”
The device can also detect gestures or movements by a person standing behind a wall, such as a wave of the arm, Katabi says. This would allow it to be used as a gesture-based interface for controlling lighting or appliances within the home, such as turning off the lights in another room with a wave of the arm.
Venkat Padmanabhan, a principal researcher at Microsoft Research, says 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 perhaps its most interesting application of all. “Such an interface could alter the face of gaming,” he says.
Unlike today’s interactive gaming devices, where users must stay in front of the console and its camera at all times, users could still interact with the system while in another room, for example. This could open up the possibility of more complex and interesting games, Katabi says.
UPDATE July 3, 2013 — For further information, please see the open-access ACM SIGCOMM paper (also shown in References below). [Also added setup diagram above.]