In-Vehicle Multiple Passengers Respiration Monitoring Based on Surface-Circuit Metasurface Tags Using Time-Division FMCW Radar.

Since respiration rate is a crucial indicator of the health state, respiration monitoring for a long duration is critical for assessing the physical health of in-vehicle passengers, especially in the cases of seniors, patients, or children. Microwave radar sensor has been shown to be suitable for detecting and monitoring respiration rates without any physical contact. However, there are two main challenges to overcome for the in-vehicle scenarios: the respiration monitoring of multiple closely spaced subjects, and the impact of strong stationary clutters and multipath reflection from in-vehicle environments. To tackle these challenges, a time-division multiple tags (TDMT) based frequency-modulated continuous wave (FMCW) radar is proposed to detect the phase changes caused by the movements of the human chests, which are captured by novel surface-circuit metasurfaces (SCM) tags attached to the seat belts worn by passengers. On the one hand, the TDMT scheme can separate respiration signals from two closely spaced passengers. On the other hand, the innovative SCM tags can simultaneously accomplish spatial filtering, retro-reflection, and polarization rotation to combat the strong clutters and multipath reflection in in-vehicle environments. The miniaturized radar prototype and tags are analyzed and implemented. Its accurate and robust performance is demonstrated with extensive experiments conducted under real in-vehicle scenarios.