Household Radar Can See Through Walls and Knows How You’re Feeling

Modern wireless tech isn’t just for communications. It can also sense a person’s breathing and heart rate, even gauge emotions

Hidden Figures:These images of human figures were obtained using radio waves. Each column contains two images of the same person. 

When I was a boy,I secretly hoped I would discover latent within me some amazing superpower—say, X-ray vision or the ability to read people’s minds. Lots of kids have such dreams. But even my fertile young brain couldn’t imagine that one day I would help transform such superpowers into reality. Nor could I conceive of the possibility that I would demonstrate these abilities to the president of the United States. And yet two decades later, that’s exactly what happened.

There was, of course, no magic or science fiction involved, just new algorithms and clever engineering, using wireless technology that senses the reflections of radio waves emanating from a nearby transmitter. The approach my MIT colleagues and I are pursuing relies on inexpensive equipment that is easy to install—no more difficult than setting up a Wi-Fi router.

These results are now being applied in the real world, helping medical researchers and clinicians to better gauge the progression of diseases that affect gait and mobility. And devices based on this technology will soon become commercially available. In the future, they could be used for monitoring elderly people at home, sending an alert if someone has fallen. Unlike users of today’s medical alert systems, the people being monitored won’t have to wear a radio-equipped bracelet or pendant. The technology could also be used to monitor the breathing and heartbeat of newborns, without having to put sensors in contact with the infants’ fragile skin.

You’re probably wondering how this radio-based sensing technology works. If so, read on, and I will explain by telling the story of how we managed to push our system to increasing levels of sensitivity and sophistication.

It all started in 2012,shortly after I became a graduate student at MIT. My faculty advisor, Dina Katabi, and I were working on a way to allow Wi-Fi networks to carry data faster. We mounted a Wi-Fi device on a mobile robot and let the robot navigate itself to the spot in the room where data throughput was highest. Every once in a while, our throughput numbers would mysteriously plummet. Eventually we realized that when someone was walking in the adjacent hallway, the person’s presence disrupted the wireless signals in our room.

We should have seen this coming. Wireless communication systems are notoriously vulnerable to electromagnetic noise and interference, which engineers work hard to combat. But seeing these effects firsthand got us thinking along completely different lines about our research. We wondered whether this “noise,” caused by passersby, could serve as a new source of information about the nearby environment. Could we take a Wi-Fi device, point it at a wall, and see on a computer screen how someone behind the wall was moving?

That should be possible, we figured. After all, walls don’t block wireless signals. You can get a Wi-Fi connection even when the router is in another room. And if there’s a person on the other side of a wall, the wireless signal you send out on this side will reflect off his or her body. Naturally, after the signal traverses the wall, gets reflected back, and crosses through the wall again, it will be very attenuated. But if we could somehow register these minute reflections, we would, in some sense, be able to see through the wall.

Using radio waves to detect what’s on the other side of a wall has been done before, but with sophisticated radar equipment and expensive antenna arrays. We wanted to use equipment not much different from the kind you’d use to create a Wi-Fi local area network in your home.

Elliptical Reasoning About Location

The time needed for a radio signal to travel from a transmitting antenna to a reflective object (here, a person) and back to a receiving antenna can be used to constrain the position of that object. Knowing the travel time between one such pair of antennas, you can determine that the reflector is located somewhere on an ellipse that has the two antennas at its foci (an ellipse being the set of points for which the distances to the two foci sum to a constant value) [top]. With two such pairs of antennas, you can better pin down the location of the reflector, which must lie at the intersection of two ellipses [middle]. With even more antenna pairs, it’s possible to work out where two or more reflecting objects are located. The hot colors in the lower diagrams [bottom] show the location of two people in a room, as seen from above.

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