Low-power Wi-Fi signal tracks movement—even behind walls

Jun 28, 2013
Credit: Cristine Daniloff/MIT

The comic-book hero Superman uses his X-ray vision to spot bad guys lurking behind walls and other objects. Now we could all have X-ray vision, thanks to researchers at MIT's Computer Science and Artificial Intelligence Laboratory.

Researchers have long attempted to build a device capable of seeing people through walls. However, previous efforts to develop such a system have involved the use of expensive and bulky that uses a part of the only available to the military.

Now a system being developed by Dina Katabi, a professor in MIT's Department of Electrical Engineering and Computer Science, and her graduate student Fadel Adib, could give all of us the ability to spot people in different rooms using low-cost Wi-Fi technology. "We wanted to create a device that is low-power, portable and simple enough for anyone to use, to give people the ability to see through walls and closed doors," Katabi says.

The system, called "Wi-Vi," is based on a concept similar to radar and sonar imaging. But in contrast to radar and sonar, it transmits a low-power Wi-Fi signal and uses its reflections to track moving humans. It can do so even if the humans are in closed rooms or hiding behind a wall.

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. "So we had to come up with a technology that could cancel out all these other reflections, and keep only those from the moving human body," Katabi says.

Motion detector

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, 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."

Once the system has cancelled out all of the reflections from static objects, it can then concentrate on tracking the person as he or she moves around the room. Most previous attempts to track moving targets through walls have done so using an array of spaced antennas, which each capture the signal reflected off a person moving through the environment. But this would be too expensive and bulky for use in a handheld device.

So instead Wi-Vi uses just one 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, being presented at the Sigcomm conference in Hong Kong in August, 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, Katabi says: "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.

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.

Explore further: Entrepreneur builds a sleek ship, but will anyone buy it?

Related Stories

See-through-wall surveillance with WiFi shown at UCL

Aug 03, 2012

(Phys.org) -- A surveillance device that uses WiFi radio waves has been devised to see through walls to detect, in military and surveillance parlance, moving personnel targets. The device serves as a radar prototype designed ...

Wi-Fi signals can see through walls

Oct 05, 2009

(PhysOrg.com) -- Researchers at the University of Utah, USA, have discovered that variations in signal strengths in wireless networks can be used to "see" movements of people on the other side of walls or ...

After Newtown: A new use for a weapons-detecting radar?

Mar 26, 2013

In the weeks after the Connecticut school shooting, as the nation puzzled over how it happened and what might prevent it from happening again, Kamal Sarabandi was listening to the news. Talk turned to giving ...

Recommended for you

Off-world manufacturing is a go with space printer

Dec 20, 2014

On Friday, the BBC reported on a NASA email exchange with a space station which involved astronauts on the International Space Station using their 3-D printer to make a wrench from instructions sent up in ...

First drone in Nevada test program crashes in demo

Dec 19, 2014

A drone testing program in Nevada is off to a bumpy start after the first unmanned aircraft authorized to fly without Federal Aviation Administration supervision crashed during a ceremony in Boulder City.

User comments : 7

Adjust slider to filter visible comments by rank

Display comments: newest first

Matthewwa25
2.2 / 5 (10) Jun 28, 2013
This is really cool!
CapitalismPrevails
2.1 / 5 (11) Jun 28, 2013
I bet the NSA would interested in this...
Grallen
5 / 5 (5) Jun 28, 2013
Suddenly I feel my walls might need a Faraday cage.
Stephen_Crowley
2.8 / 5 (9) Jun 29, 2013
This is not in the least bit cool, the nanny state already has enough tools of oppression
Cave_Man
1.6 / 5 (7) Jun 29, 2013
Seriously, how will I ever masturbate with impunity again!
Open-minded Individual
1.9 / 5 (7) Jun 29, 2013
Yea, this definitely won't be misused...

Meanwhile...

SECRECY NEWS
from the FAS Project on Government Secrecy
Volume 2010, Issue No. 84
October 21, 2010

INVENTION SECRECY STILL GOING STRONG

There were 5,135 inventions that were under secrecy orders at the end of Fiscal Year 2010, the U.S. Patent and Trademark Office reported last week.

"The current list of technology areas that is used to screen patent applications for possible restriction under the Invention Secrecy Act is not publicly available and has been denied under the Freedom of Information Act. (An appeal is pending.) But a previous list dated 1971 and obtained by researcher Michael Ravnitzky.

the 1971 list indicates that patents for solar photovoltaic generators were subject to review and possible restriction if the photovoltaics were more than 20% efficient. Energy conversion systems were likewise subject to review and possible restriction if they offered conversion efficiencies "in excess of 70-80%."
Open-minded Individual
1.6 / 5 (7) Jun 29, 2013
...One may fairly ask if disclosure of such technologies could really have been "detrimental to the national security," or whether the opposite would be closer to the truth. One may further ask what comparable advances in technology may be subject to restriction and non-disclosure today. But no answers are forthcoming, and the invention secrecy system persists with no discernible external review."

Just wanted to bring that up since it's conveniently never discussed. Wouldn't want to talk about how the U.S. Patent Office is used to suppressed energy technology and maintain a monopoly over energy by the oil, nuclear power, natural gas, coal and electric companies.
we wouldn't want to put them out of business, it might decrease the cost of living and increase the standard of living...

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.