Stepped-Frequency Continuous-Wave Radar with Self-Injection-Locking Technology for Monitoring Multiple Human Vital Signs

This article proposes a single-conversion stepped-frequency continuous-wave (SCSFCW) radar that combines a stepped-frequency continuous-wave (SFCW) radar and a self-injection-locked (SIL) radar to benefit from the range resolution and the Doppler sensitivity of the two radars. An 8.5-9.5-GHz prototype SCSFCW radar system that comprises a subharmonic up/down converter with a 3-3.5-GHz stepped chirp local-oscillator (LO) signal and a 2.5-GHz SIL IF signal was developed to monitor the vital signs, i.e., respiration rate (RR) and heart rate (HR), of multiple humans. The coherence and range of the developed system were significantly enhanced by using a low pulse repetition frequency (PRF). In the experiment, the minimum distinguishable radial spacing between the vibrating frequencies of the metal plates that are not azimuthally overlapped with one another corresponds to a theoretical range resolution of 15 cm. However, owing to scattering by the human body, the minimum radial spacing for distinguishing between the vital signs of the individuals is three times than that for distinguishing between the metal plates in a similar experimental setup. Accordingly, the monitoring of up to three human vital signs using the developed system was demonstrated with a range-vital-Doppler map.