Tuesday, November 3, 2015

Techy Tuesday - "Seeing Through the Walls" Using WiFi as Radar

MIT's Computer Science and Artificial Intelligence Lab, CSAIL, reports results using frequencies near the 5 GHz WiFi band to image people through walls
Since 2013, CSAIL researchers have been developing technologies that use wireless signals to track human motion. The team has shown that it can detect gestures and body movements as subtle as the rise and fall of a person’s chest from the other side of a house, allowing a mother to monitor a baby’s breathing or a firefighter to determine if there are survivors inside a burning building.

Next up? Seeing a person’s silhouette and even distinguishing between individuals.

In a paper accepted to the SIGGRAPH Asia conference taking place next month, the team presents a new technology called RF Capture that picks up wireless reflections off the human body to see the silhouette of a human standing behind a wall.
In other words, from the opposite side of a building, RF Capture can determine where you are, who you are, and even which hand you are moving.  (RF Capture paper - pdf warning)  Who you are?  They claim the ability to identify the right person out of 15 with a 90% success rate.

While the surveillance state and privacy aspects of this are obvious, MIT's press release chooses to barely mention them; for instance, preferring to mention a firefighter being able to determine if there are survivors in a burning building and more family-friendly things like that child monitoring example. 
“We’re working to turn this technology into an in-home device that can call 911 if it detects that a family member has fallen unconscious,” says Katabi, director of the Wireless@MIT center. “You could also imagine it being used to operate your lights and TVs, or to adjust your heating by monitoring where you are in the house.”
Like most radar, the software is the heart of it.  The system operates by sending out these "5 to 8 GHz" radio waves and getting reflections from the subjects.  It does a lot of processing of those reflections, and they're up-front about the weaknesses of the system in their conclusions:
1. It assumes that the subject starts by walking towards the device, hence allowing RF-Capture to obtain consecutive RF snapshots that expose his body parts. Future systems should expand this model to a more general class of human motion and activities. [emphasis added - SiG]

2. The current method captures the human figure by stitching consecutive snapshots, and hence cannot perform fine-grained full skeletal tracking across time. Future work may consider combining information across multiple RF-Capture sensors to refine the tracking capability.

3. Our implementation adopts a simple model of the human body for segmentation and skeletal stitching. Future work can explore more advanced models to capture finer-grained human skeleta.
Interesting idea.  Wall-penetrating imaging using WiFi (probably because so much equipment is available to experiment with) are not new, and while this is crude, there's clearly promise here.  This particular device is really just a lab curiosity, and I don't think it's very useful at this level.  Still, it's worth being aware of and thinking about.



  1. Time we all start putting copper mesh in our walls.....

  2. Instead of Faraday-caging our walls it might be easier (and cheaper) to simply have a "broken" Wi-Fi- range extender that drowns out the signal. I wonder how much power it would take.

  3. Like FLIR this is overrated and over hyped NSA "eye of Mordor"/ "WE SEE ALL" propaganda. ---Ray

  4. I feel certain your favorite "Uncle" has already developed millimeter-wave radar and/or other technology to penetrate some structures (stick-built homes would seem likely to be the most vulnerable) and locate individuals within that structure. Not something _every_ SWAT team will have handy, but available when needed by "Uncle", no doubt. As costs come down on technology like this, it may become in more common use.

    (This technology was featured in Hollywood films over twenty years ago - though probably not in actual existence at that time - so I'd be terribly surprised if it isn't now in use, if just not talked about. Like "Stingray", etc.)

  5. Thinking about this some more, I'll pose this: SiG, you're in the biz - what kind of equipment would it take to detect the fedz are scanning your structure? I'd guess anything that shows a jump above normal background in the 5-8 kHz range.

  6. Here is one article on some of what is openly available and in use. I'd bet my next month's retirement check that NSA and other alphabet agencies have bigger (smaller?) and better devices in use already.


  7. This system, around 5 GHz, is incapable of fine details the way it's implemented. Generally, radars have a resolution that depends on the wavelength, the physical size of the signal they're processing. With a wavelength of a little over 2", it's not going to be able to separate objects that are much less than 1/4 of that, half in inch.

    Any sort of metal screen, aluminum, brass or copper (probably in order of cost) would mess it up.

    As for detecting its transmissions with something off the shelf, that depends on the frequency it's operating on. Something like this system, might be found with WiFi signal finders, although something like the cheap police radar detectors wouldn't be that hard to build.

    There's some things here that might be fun to do another column on.

  8. Is the terahertz wave imaging still active/

  9. @alien

    That probably wouldn't work.

    The paper says it's using signals near the 5GHz WiFi band, not in the 5GHz WiFi band.