  This paper shows experimentally that standard wireless networks which measure
received signal strength (RSS) can be used to reliably detect human breathing
and estimate the breathing rate, an application we call "BreathTaking". We show
that although an individual link cannot reliably detect breathing, the
collective spectral content of a network of devices reliably indicates the
presence and rate of breathing. We present a maximum likelihood estimator (MLE)
of breathing rate, amplitude, and phase, which uses the RSS data from many
links simultaneously. We show experimental results which demonstrate that
reliable detection and frequency estimation is possible with 30 seconds of
data, within 0.3 breaths per minute (bpm) RMS error. Use of directional
antennas is shown to improve robustness to motion near the network.
