Frequency- and Self-Injection-Locked Radar With Adaptive Phase Tracking for High-Sensitivity Vital Sign Monitoring

Noncontact vital sign sensing is conventionally conducted using Doppler radars, among which self-injection-locked (SIL) radars offer enhanced sensitivity. However, the perfor mance of SIL radars degrades under certain injection phase conditions, and conventional SIL radars require bulky delay lines for frequency demodulation. Therefore, this study designed a frequency- and self-injection-locked (FSIL) radar architecture that integrates a frequency-locked loop (FLL) with an SIL. The FLL dynamically tracks the phase of the subject under test (SUT), ensuring optimal injection conditions and eliminating the need for external delay elements. Moreover, a phase-shifter-based static clutter cancellation mechanism is incorporated into the designed architecture to improve the radar’s vital sign detection capability. A 2.3-GHz prototype of the designed FSIL radar was developed and experimentally validated. With a transmit power of 10 dBm, the developed prototype detected a minimum vibration amplitude of 0.1 mm at a range of 3 m. The proposed architecture exhibits superior phase-tracking and sensing capabilities under strong clutter, confirming its suitability for high-sensitivity vital sign sensing applications.