Mutcouple-tag: A New Paradigm for Interface Design of RFID-based Sensing Mechanism via Mutual Inductance Coupling

RFID-based sensing provides battery-free and non-invasive solutions essential for applications such as human health monitoring and human-computer interaction. While sensor-assisted RFID circumvents material constraints via sensors interface wired connection, it remains constrained by bidirectional-effect sensor self-interference and the hardware overhead of customized impedance matching networks. In this work, we propose Mutcouple-tag, a material-independent, matching-network-free, and flexible sensor interface that enables COTS RFID tags for human-centered sensing such as vital sign monitoring. The key idea is to exploit mutual inductive coupling between the RFID tag's matching loops and sensor's loop for transmission. This coupling introduces impedance variations that modulate the signal's amplitude and phase. To address the problem of linear direct-connection designs yielding minimal backscatter modulation constrained by weak mutual coupling, we propose a diode-enhanced mutual inductance modulation architecture that leverages a nonlinear mechanism to significantly amplify modulation depth. To address the problem of eliminating multipath interference and electromagnetic transients, we design a denoising workflow leveraging tag-pair differencing and Wavelet Transform. Unlike direct-connection designs limited to signals > 6V, Mutcouple-tag captures micro-variations of sensor signal of 200-500mV, achieving a 30-fold amplification in modulation depth. Extensive experiments demonstrate the system's high-precision sensing, achieving correlation coefficients of 90.12% for pulse and 88.35% for FCG signals.