A Microwave Passive Topology Based on Simultaneous Injection-Locking and Injection-Pulling for Passive Indoor Sensing Applications

The opportunistic use of ambient radio-frequency (RF) signals for e-healthcare, smart living, security, and IoT applications has been attracting significant attention over the last years. Researchers and engineers have already proposed various approaches to integrate wireless communication with remote sensing by passively collecting Wi-Fi 2.4-GHz frequency band signals in indoor environments. Most of the existing passive sensing methods demand complex digital signal processing algorithms and/or adaptations to existent radio topology. In this paper, a passive microwave topology based on simultaneous injection-locking and injection-pulling of a RF oscillator for indoor passive sensing applications is presented. The direct-path signals from a source of RF waves and the signals that are phase-modulated by the target's motion are captured, combined, and fed into the injection-locking port of an oscillator. Due to the highly selective injection-locking for the stronger direct-path signal, and the injection-pulling behavior for the weaker scattered signal, the phase shifts of the electromagnetic waves that bounce off a moving target can be recovered. Experimental results demonstrate the feasibility of the proposed technique for microwave passive vital signs monitoring.