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.
Actually, this is similar work but by different people. The links you posted are about work at University of Washington, whereas the OP is work done at MIT.
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.
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.
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.
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?
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)
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)
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.
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.
https://news.ycombinator.com/item?id=5824286
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?