UC Davis

Remote vital sign detection has gained popular interests from researchers to conduct through-clothing and non-contact measurements of the chest wall motion induced by the heart beat and the respiration of the human subjects. Although many architectures have achieved the vital sign detection with decent sensitivity, the Doppler radar system is widely adopted by researchers due to its simple architecture, perfect phase coherence and its cost effectiveness. On the other hand, improvements on the Doppler radar system are imperatively needed on the topics of radar system sensitivity for increased detection robustness, spatial resolution for multi-subject detection and linear displacement demodulation. This dissertation is devoted to improve the Doppler radar system through comprehensive study on the coupling effect during the Doppler radar system.

In this dissertation, we first discussed the development on the system integration of a dual-PLL low-IF Doppler radar system using PLL with a shared input reference. Followed by this, a compact module design has been achieved for constructing the first 6x6 Doppler MIMO radar system. This Doppler MIMO radar has been measured with a detection angle accuracy within 1.5 degrees, which helps to accurately allocate the motion-sensitive area of the subject to increase the detection sensitivity. The 18-degree spatial resolution enables the device to distinguish between vital sign signals from two different subjects measured at the same distance of 1.9 m away.

Next, we analyzed the coupling effect contributing to the residual phase noise in the dual-PLL low-IF system architecture and validated it through controlled experiments. Followed by this analysis, an integrated automatic RF cancellation module has been developed using microcontrollers along with the designed cancellation algorithm. This module has been tested under both in-air and through-wall vital sign detection scenarios, resulting in an effective SNR increase of more than 15 dB with a less-than 2 seconds of implementation time.

Lastly, the coupling effect has been studied on the modulation scheme associated with the remote target motion. Instead of the phase modulation due to the Doppler effect, the coupling effect also results in amplitude modulation at the received waveform. Our work investigated such effect through both theoretical derivations and interpretations in an IQ plot. Based on this finding, a new system architecture has been proposed for remote displacement sensing. The developed new architecture demonstrated a superior performance without using the mixer component, which greatly improves the power utilization efficiency for remote displacement sensing.