

Wi-Vi: See Through Walls with Wi-Fi Signals - WestCoastJustin
http://people.csail.mit.edu/fadel/wivi/

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stephengillie
Previous discussion:

[https://news.ycombinator.com/item?id=5824286](https://news.ycombinator.com/item?id=5824286)

[https://news.ycombinator.com/item?id=4334264](https://news.ycombinator.com/item?id=4334264)

I'm still waiting for someone to make an app that does this -- would it
require linking 2 phones?

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mirashii
From what I recall when I skimmed the original paper, you would need more
general hardware than the wifi chipset in a phone. Actually, that this says it
uses wifi at all is a bit misleading. It happens to work on the same frequency
bands that wifi uses, but their experiments did not actually use wifi hardware
at all and actually used USRPs. Getting off the shelf wifi hardware to do the
same would require firmware and drivers that allow raw access to the radio
signals, which no cards I know of come even close to.

Additionally for their experiments they needed at least five antenna, so even
if it were going to happen with phones you'd probably need more than just two.

~~~
stephengillie
So it's like how Xbee radios use the Wifi radio frequency range, but use their
own protocols and are actually 1-2 MHz off from Wifi?

Xbee hardware can be co-opted into Wifi transmission by an Arduino or more
powerful processor: [https://github.com/cjbearman/xbee-wifi-spi-
arduino](https://github.com/cjbearman/xbee-wifi-spi-arduino)

I wonder if Xbee devices would be more suitable to this task.

~~~
pudquick
Actually that driver is specifically for 802.11 (wifi) XBee modules like
these: [http://www.digikey.com/product-highlights/us/en/digi-
interna...](http://www.digikey.com/product-highlights/us/en/digi-
international-xbee-wi-fi/108)

It does not cause 2.4Ghz 802.15.4 XBee modules to act like a normal wifi
(802.11) chipset.

Just an FYI.

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femto
Combating reflections (hence multipath) from moving objects is precisely the
reason Wireless LANs use orthogonal frequency division multiplexing
(implemented with an FFT). The environment leaves its imprint on the received
signal and depending on your point of view: communications or radar, that
imprint is either information to be retrieved (radar) or eliminated
(communications).

If you look at one of the early WLAN papers [1] (I'm an author), Figure 1(b)
shows a frequency selective fade, meaning there is a dip, or null, in the
received signal and some of the frequencies are missing. The way we created
that null was by a person walking through the room. With a change of emphasis
in the signal processing, a WLAN is a very good person detector.

[1]
[http://wireless.ece.ufl.edu/twong/Preprints/00566198.pdf](http://wireless.ece.ufl.edu/twong/Preprints/00566198.pdf)

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kyle_martin1
RF and EE here. This is nothing new. Or at least a motivated junior/senior in
college would stumble upon this simple application of wave propagation theory
pretty early on.

~~~
jffry
This is notable because it's an actual implemented system. Yes it works in
theory, on the exact same principle that a radar speed gun works, but these
researchers have shown that it is possible to implement in the 2.4GHz band.
That's the first step to making it smaller and cheaper.

~~~
kyle_martin1
If you think this hasn't been cheaply implemented before then you're behind
research in wave propagation brother. Search on IEEE if you're interested.

~~~
throwawaykf02
I know there has been tons of see-thru-the-walls work done over many years...
But that used UWB and required specialized equipment and antennas, IIRC,
whereas this uses "commodity" USRPs. Could you provide examples of the type of
work you're talking about?

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zalzane
This looks like it would make an excellent spying tool, if there isn't already
a similar device available.

~~~
zw123456
Great, now the NSA not only can snoop on my emails but use my wifi hotspot to
watch me too.

~~~
Spooky23
Given that secure buildings typically lack windows or have RF shielding on the
windows, I'm sure this sort of capability has been around for a long time.

Example:
[https://www.astic.com/sdfilms.php](https://www.astic.com/sdfilms.php)

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qrybam
I can see this being used for security. Imagine replacing passive IR motion
detectors in the home with this!

~~~
jffry
The only advantage I can think of for this is that it could be concealed in
the wall, where a passive IR motion detector requires a clear field-of-view of
a room. The downside is that this is actively emitting microwave radiation, so
it would be easy to detect, while a passive IR sensor would not be emitting
any discernible radiation (assuming its internal electronics are properly
shielded)

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mrcactu5
why can it detect humans and not other moving objects?

knowing how many people on the other side of the wall will change our sense of
privacy

~~~
jffry
The linked article and video never said that it wouldn't work on non-human
moving objects, just that it _would_ work on moving humans (as it is a
wireless relative-motion detector)

It is possible that this tech is especially effective at tracking humans due
to the fact that we're bags of mostly water and the specific frequency used
interacts with water more than other nearby frequencies. (I'm not a microwave
engineer, so I don't know this for sure)

~~~
kyle_martin1
You are correct. We can determine what an object is made out of or at least
behaves like based on the reflected EM waves. Every piece of matter has an
associated electric permittivity and permeability. These can, and often do,
change with temperature and the other environmental variables.

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nether
seems like something the CIA science directorate could have come up with, but
never released to the public

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dobbsbob
Put a SICP tent in your room (:

