The idea is pretty simple: take two transmitters and one receiver. Each transmitter sends out a signal that is precisely 180 degrees out of phase with the other, so the two cancel each other out and the receiving antenna “hears” nothing.
But put any moving object in the area, and it reflects the signals. The signals don’t cancel out, and where once there was no radio “noise” at all, there is now radio energy coming off of the moving object or person. A still object also reflects radio waves, but the time it takes for a wave to bounce back to the receiver stays the same and the reflections will still cancel out.
The invention, being developed by Dina Katabi, an electrical engineering professor, and her graduate student Fadel Adib, is called Wi-Vi. The two will present it at the Sigcomm conference in Hong Kong this August.
There are several uses for this technology — a small handheld detector could find people buried under tons of rubble, showing rescue workers where to look. Or police could use it to see if there is someone inside a room.
Wi-Vi differs from traditional X-ray or terahertz wave systems. In that case a beam of radio waves is sent to an object that reflects them back. This kind of detection is just like what your eyes do – seeing reflected light. (Radio waves just happen to be in a different part of the spectrum).
It’s also a twist on a previous attempt at using Wi-Fi routers to see through walls by taking advantage of the signals they emit. Wi-Vi doesn’t require that a router be in place.
The advantage here is that this works with radio frequencies that penetrate walls relatively easily, at least for short distances. The wavelengths are also short, so the antenna doesn’t need to be very large. It also needs just one receiving antenna, so it can be fit onto a hand-held device.