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Wi-Vi system uses Wi-Fi to see through walls


July 1, 2013

A new Wi-Fi-based "Wi-Vi" system can track people moving behind walls (Image: Christine Daniloff/MIT)

A new Wi-Fi-based "Wi-Vi" system can track people moving behind walls (Image: Christine Daniloff/MIT)

Researchers at MIT's Computer Science and Artificial Intelligence Laboratory have developed what could become low-cost, X-ray vision. The system, known as "Wi-Vi," is based on a concept similar to radar and sonar imaging, but rather than using high-power signals, this tech uses reflected Wi-Fi signals to track the movement of people behind walls and closed doors.

When a Wi-Fi signal is transmitted at a wall, a portion of that signal penetrates through and reflects off any humans that happen to be moving around in the other room. Since only a tiny fraction of the signal passes through the wall, with the rest being reflected, the researchers had to devise a technology that could could cancel out the arbitrary reflections, and keep only those reflecting from moving human bodies.

Dina Katabi, a professor in MIT’s Department of Electrical Engineering and Computer Science, and her graduate student Fadel Adib have tuned a system that uses two transmission antennas and a single receiver. The two antennas transmit almost identical signals, except the second antenna's signal is the inverse of the first, resulting in interference.

This interference causes the signals to cancel each other out. Since any static objects that the signals hit create identical reflections, they are also cancelled out by this effect. Only the reflections that change between the two signals, like moving bodies on the other side of the wall, arrive back at the receiver, allowing the system to track the moving people.

Adib says, "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."

Previous attempts to see through walls in this manner have done so using an array of spaced antennas, which capture the signal reflected off of moving people in the room. Such systems, though effective, would be too cumbersome and expensive for use in a handheld device. By using just one receiver, the new system effectively measures the time it takes for signals to reflect, leading to a calculation of location.

Relying on low-cost Wi-Fi technology, the Wi-Vi system could be utilized in everything from disaster recovery to gaming. Because the device can detect action behind a wall, the system could be used as a gesture-based interface for controlling appliances or lighting.

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.

Source: MIT


That's kinda creepy! Say your neighbour comes over to your house and plants one of these antennae somewhere, he would be able to know which room you were in!


I'm waiting on a hand-held, portable device that can scan the human body, and identify any irregularities, with no harmful side effects. (kinda like StarTrek stuff).... It's coming...


Nowhere to hide.

Bruce H. Anderson

I have one question. Do you have a warrant? I'm sure this may have a very limited constitutional useful purposes but more than likely used for nefarious purposes and continued breakdown of our fourth amendment and constitution in general. The science is cool but how it will be used surely won't be.


Oh, I can see the Dept. of Homeland Security just eating this up.


Wow low-cost interferometry for all. 8*) Once again we must trade some privacy for the many good things it brings.


Que perigo isso não! Qual o propósito verdadeiro desta tecnologia, fala sério....

Paulo Monteiro

TechHead -- as I read this no planting of an antenna in your property would be required. Just two transmitters of an inverted signal which could be on his/her property. As I think about it you might even be able to use a typical cable broadband connections which are shared amoungst a block of subscribers, as your transmitters. That is you could probably induce multiple wifi routers on your street to emit packets and with a little bit of DSP use those packets to probe the whole neighborhood. . . .

Mr. Jim

Should sell like hotcakes to P.I.s everywhere! "Here is the recording Judge, subject enters, meets plaintiff, proceeds to... well you get the picture"

The Skud

This technology onboard a uav yeah thats the next step i fear


Everyone looks at the negatives. Here are a few useful ways this tool could be used:

A home/office security system. Set it before you leave the house/office and in combination between your various wireless devices and access point, have real-time movement detection system in your house, negating the need to have dedicated sensors installed.

Use tracking system to guide wifi signal similar to the way the Dlink smartbeam technology works, but one that actually moves in real-time with the user, so your 'AP' has a lock on you to give the best signal.

Each frequency provides its own reflection profile. If the system could sweep across the 2.4 and 5.4 GHz range, such a system could improve its resolution and possibly discriminate between materials. Ie - poor man's metal detector.

Based on known coefficients of absorption and reflection, use a similar sweep to assess the wall material, and/ or materials beyond the wall.


Retaining privacy is simply a matter of an appropriate Faraday cage incorporated into the external walls of your residence.

Only one transmitter is needed with a two-antenna array with 180 degree phasing between the two antennas -- which more than likely would probably be narrow-beam directional. If a single frequency is used the phasing can be incorporated into the physical structure of the antenna system. If spread-spectrum signalling is used the phasing would have to be accomplished electronically.

There is no practical way to control indeterminate latency neighborhood Wi-Fi emitters to accomplish this scenario. First is the problem of coordinating the transmissions in time. Second, you would have to know the exact 3D coordinates of every transmitter and then calculate their phasing to reinforce within the space of interest.


Re: DeepThought

Would be curious what separation is required between the transmitter and receiver to make this feature work with one AP on a set frequency ie - most access points these days that support 11.n have two or more antennas for echo cancellation to achieve higher bandwidth.

If this system did not need much separation between antennas, and each antenna was capable of independent Tx/Rx, then you would definitely know what geographic position the first antenna is relative to the second. Antenna Tx and Rx gain, orientation, elevation and declination can all be entered in by the user.


I moan because my yard thick stone house walls stop my WiFi from working. Hmmm, maybe they are not so bad after all.

Doug MacLeod
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