We've seen terahertz cameras that can look through walls and X-ray
scanners that fit in the palm of your hand.
Now, scientists at MIT have developed a way to track movement through walls
using Wi-Fi signals.
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, now radio energy is emanating off 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 — which Dina Katabi, an electrical
engineering professor, and her graduate student Fadel Adib are developing — 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).
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.
And the reason it uses Wi-Fi type signals is that they
aren’t reserved for the military, and are open to use by any device with less
need to get approvals from regulators.
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