Benefits of Coherent Low-IF for Vital Signs Monitoring Using Doppler Radar

Homodyne receiver systems have been used extensively in wireless life signs monitoring applications. The main advantage of a homodyne system is range correlation resulting in cancellation of the oscillator phase noise. However, direct down-conversion to dc and the subsequent baseband amplification circuit will introduce additional flicker noise to the signal. Physiological signals have significant content around dc that will make them susceptible to 1/f noise. A coherent low-IF system is applied to solve this problem. This architecture has the range correlation benefits of the homodyne system, while minimizing the baseband flicker noise. Measurements on a mechanical target and a human subject demonstrate a signal-to-noise ratio improvement of 7 dB, which can increase the range of operation by 50%. Measurements on a human subject have demonstrated low-IF heart rate detection with a root-mean-square error of less than 0.8 beats/min at a distance of almost 3 m with transmit power of 0.1 mW, whereas direct conversion architecture output completely failed in this case.