An Adaptive Template Matching Method for Noncontact Cardiac Inter-Beat Interval Detection Using a 120 GHz FMCW Radar

Heart rate variability (HRV), the variance in time between heartbeats, has shown great potential in detecting autonomic nervous system function and diagnosing certain cardiovascular diseases. Although it is feasible to measure HRV using millimeter-wave radar by noncontact detection of the cardiac inter-beat interval (IBI), accuracy and adaptability remain challenges. To achieve accurate and adaptive detection of IBI, a custom-designed 120 GHz frequency-modulated continuous wave (FMCW) radar sensor with a narrow-beam lens antenna was developed, along with a novel adaptive template matching algorithm to generate personalized heartbeat templates. These templates were cross-correlated with the signal to detect successive heartbeats accurately. Experimental results demonstrated a mean absolute error of 18 ms, a root mean square error of 24.4 ms, and a median error of 12.6 ms, with a 98.8% coverage rate of the heartbeat intervals in a normal environment, using the electrocardiogram (ECG) signal as a reference. Clinical tests on an atrial fibrillation patient also suggest that the proposed system could assist in diagnosing heart disease, making it a promising option for cardiac monitoring in both home and medical facility settings.