Time-Division Multiplexing MIMO Radar System With Self-Injection-Locking for Image Hotspot-Based Monitoring of Multiple Human Vital Signs

This article presents a time-division multiplexing (TDM) multiple-input-multiple-output (MIMO) radar system that differs from existing ones by incorporating a heterodyne self-injection-locking (HSIL) architecture for enhanced Doppler sensitivity. The system operates in the unlicensed 6-GHz band and produces the 3-D images of multiple individuals, enabling the identification of the hotspot on a person's chest where the maximum echo amplitude occurs. This approach allows for the accurate and reliable measurement of vital signs, despite variations in body posture and orientation. Quantitatively, the system can detect a minimum amplitude of vibration of 15~\mu \text{m} for a 10\times10 cm metal plate placed 1.5 m away. A Doppler-weighted super-resolution technique helps to distinguish between the images that are generated by two metal plates of equal size and separated by less than the theoretical range and angular resolution limit of 15 cm and 14°, respectively. Consequently, the system effectively detected the vital signs of three subjects with varying body postures and orientations by locating the hotspots in their respective images, demonstrating its potential for practical applications.