Wearable Vital Sign Sensor Using a Single-Input Multiple-Output Self-Injection-Locked Oscillator Tag

In this paper, to simultaneously address the advantages of wearable vital sign sensors such as high sensitivity, low power consumption and mobility, the proposed radar system is designed to be bistatic with a single-input multiple-output (SIMO) self-injection-locked oscillator (SILO) tag attached to the subject's chest and with a handheld receiver at a remote location. Embedded in the tag is a circularly polarized (CP) antenna for functioning the SILO and a polarization-diversity CP array antenna for transmitting the tag's output signal to the receiver for Doppler detection. Owing to polarization diversity, the tag exhibits strong immunity to the moving clutter caused by the receiver. In the experiment with a 2.4-GHz ISM band prototype, the tag delivered reliable cardiopulmonary information when there was a relative movement between the tag and the receiver.