Switch-Based Low Intermediate Frequency System of a Vital Sign Radar for Simultaneous Multitarget and Multidirectional Detection

This paper proposes a radio frequency (RF) switch-based radar system for simultaneously detecting multitarget and multidirectional vital signs. Compared with traditional continuous wave (CW) radars that encounter mixed and distorted phases due to multiple reflections, frequency modulation CW (FMCW) radars have a suitable range resolution and relatively simple system architecture. Moreover, FMCW radars have the inherent capabilities of conducting multiple target monitoring and providing distance information. Based on our proposed algorithm, the vital signs of two targets at different distances from the antenna can be detected simultaneously even closely separated within the resolution bin limitation by simply using the proposed algorithm upon an FMCW radar to extract the vital signs separately. Moreover, instead of using multiple radar systems, a single transceiver can detect the vital signs from multiple directions and multiple targets by adding an RF switch as a multiplexer. This RF switch provides the mixing function. Therefore, a low intermediate frequency (IF) architecture was proposed by using the proposed RF switch-based radar system. This architecture simultaneously conducts multidirectional vital sign detection; thus, the signal-to-noise ratio (SNR) can be improved by appropriately selecting the IF value to reduce the flicker noise. The heart rate (HR) demodulation in a baseband is also challenging due to the low SNR and the respiratory rate (RR) harmonic for FMCW vital sign radars. Therefore, we applied an ensemble empirical mode decomposition algorithm to extract the intrinsic mode functions of RR and HR. Experimental results revealed that the proposed algorithm can significantly improve the SNR and accuracy. The vital sign errors of detecting multiple targets are less than 3% on average.