Accurate Extraction of Heartbeat Intervals with Near-Field Coherent Sensing

Continuous monitoring of vital signs including heart rates, blood pressures, respiration rates and breath efforts is critical for eHealth. Current approaches are limited in accuracy, convenience and sensing capabilities. Some methods such as electrocardiogram (ECG) is difficult for long-term applications due to the requirement of direct skin contact, which is uncomfortable to wearers, restricts body motion and disrupts circadian rhythms. Meanwhile accurate measurement of the inter-heartbeat timing features is important for wellness monitoring and human emotion studies. In this paper, we introduce a new method to modulate the external and internal mechanical motion of the wearer body directly onto multiplexed radio frequency (RF) signals by near- field coherent sensing (NCS). To minimize the deployment and maintenance cost of NCS vital-sign monitoring, passive RF identification (RFID) tags can be integrated into garments at the chest and wrist areas. The unique ID of each tag helps discriminate its signal against interferences from ambient interferences and other tags. NCS utilizes both amplitude and phase of the electromagnetic field to couple the motion, which significantly increases the signal quality in comparison to methods based solely on signal strength. The tag backscattering signal is then processed with spectral equalization to amplify the high frequency components to recover not only the waveform details originally submerged in the low-frequency components but also the sharp peaks for precise heartbeat intervals with improved peak detection certainty. The derived heartbeat interval shows improved stability in comparison with synchronous ECG.