Chest-Worn Self-Injection-Locked Oscillator Tag for Monitoring Heart Rate Variability

This paper presents a 5.8 GHz wireless chest-worn sensor for monitoring heart rate variability (HRV). The sensor consists of a self-injection-locked oscillator (SILO) and a tag antenna. It transmits a radio-frequency (RF) signal that is frequency-modulated based on self-injection-locking by the physiological motion of the chest to a handheld receiver for demodulation to analyze HRV. The tag antenna is a stacked structure composed of a complementary slot ring resonator (CSRR) and a circularly polarized (CP) slot radiator, playing important roles as a near-field sensing tip and a far-field radiation antenna. As an experimental result, the comparison of the inter-beat interval (IBI) data between the proposed sensor system and an electrocardiogram (ECG) equipment for a wearer shows excellent agreement.