A team of researchers has devised a novel use for WiFi that could potentially save lives or add to the collection of methods that can be used to snoop on you.
This is another of those chance observations that turned out to be of potentially practical value. Most WiFi devices have a RSS - Received Signal Strength - detector which can be read by software. Someone noticed that when a human stands in front of a WiFi unit then the RSS changed as they breathed in or out - only a small change but any change can be the start of a detection system.
The next step was to monitor the RSS output of a WiFi point in the same room as a human subject and perform a Fourier analysis on it to give a spectrum. When this was plotted it showed a marked peak at the breathing rate of the occupant.
The research group then investigated using a single or multiple WiFi units to detect breathing and to estimate the breathing rate. They discovered that while a single WiFi unit did respond to breathing it wasn't reliable enough to use for detection or rate estimation. However with a number of units not only could the breathing be reliably detected it could also get an accurate estimate of the breathing rate and even the amplitude of breathing.
However, for the moment at least, don't hold your breath for an app that will detect someone's breathing in a room using nothing but your standard WiFi. The experiments used 20 WiFi devices, a subject that was lying on a bed, and the devices were arranged close to the bed. Under these conditions, the detection method was sensitive to the subject's movements but it isn't immediately generalizable to domestic use.
On the other hand, the same team has already shown how a WiFi network signal can be used to discover which rooms in a house are occupied by the movements and now, with breathing detection, they might be able to extend this to locating people who aren't moving.
To work the system up into something that might be capable of detecting breathing using nothing more than a domestic WiFi network would need more data, i.e. readings over a longer time interval, and a more sophisticated data analysis mechanism to recover the signal from the RSS data.
It all just goes to demonstrate that it is difficult to go unnoticed in a world of technology.
Monitoring Breathing via Signal Strength in Wireless Networks arXiv:1109.3898v1
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