Comprehensive Vital Sign Detection using a Wrist Wearable Nonlinear Target and a 5.8-GHz ISM Band Intermodulation Radar

Nonlinear components can be used to generate third order intermodulation frequencies. Taking advantage of this phenomenon the movement of a nonlinear target can be monitored and distinguished from background clutter. This paper discusses the use of an intermodulation radar, operating in the 5.8-GHz ISM band, and a passive nonlinear tag, located on the wrist, to monitor human vital signs and more specifically to improve remote monitoring of the heart rate. The non-ideal background clutter rejection of an intermodulation radar allowed for the respiration rate to also be monitored. Since the movement of the chest reflects the two individual tones from the transmitter and not the third order intermodulation frequency, for which the receiver is tuned, the respiratory frequency component has a scaled amplitude in comparison to what is seen with the use of a Doppler radar. This prevents the heart rate from being indistinguishable from the harmonics of the respiration.