  Non-contact vital sign monitoring has many advantages over conventional
methods in being comfortable, unobtrusive and without any risk of spreading
infection. The use of millimeter-wave (mmWave) radars is one of the most
promising approaches that enable contact-less monitoring of vital signs. Novel
low-power implementations of this technology promise to enable vital sign
sensing in embedded, battery-operated devices. The nature of these new
low-power sensors exacerbates the challenges of accurate and robust vital sign
monitoring and especially the problem of heart-rate tracking. This work focuses
on the investigation and characterization of three Frequency Modulated
Continuous Wave (FMCW) low-power radars with different carrier frequencies of
24 GHz, 60 GHz and 120 GHz. The evaluation platforms were first tested on
phantom models that emulated human bodies to accurately evaluate the baseline
noise, error in range estimation, and error in displacement estimation.
Additionally, the systems were also used to collect data from three human
subjects to gauge the feasibility of identifying heartbeat peaks and breathing
peaks with simple and lightweight algorithms that could potentially run in
low-power embedded processors. The investigation revealed that the 24 GHz radar
has the highest baseline noise level, 0.04mm at 0{\deg} angle of incidence, and
an error in range estimation of 3.45 +- 1.88 cm at a distance of 60 cm. At the
same distance, the 60 GHz and the 120 GHz radar system shows the least noise
level, 0.0lmm at 0{\deg} angle of incidence, and error in range estimation 0.64
+- 0.01 cm and 0.04 +- 0.0 cm respectively. Additionally, tests on humans
showed that all three radar systems were able to identify heart and breathing
activity but the 120 GHz radar system outperformed the other two.
