Detection of Passive Nonlinear RF Tag Micro-Motion Using Communication Signals of Opportunity

We present an approach for passive micro-motion tracking of nonlinear radio-frequency (RF) tags using signals of opportunity from commodity communications hardware. The approach is based on detecting nonlinear responses of diode-based RF tags and estimating the time-varying phase response using channel state information (CSI) obtained using standard 802.11 preambles. Many wireless communications systems use a set of continuous-wave tones for channel estimation that, when incident on the diode, generate intermodulation products. The proposed method of measuring tag micro-motions is based on time-frequency signatures of CSI information obtained from the retransmitted signal, offering a novel approach for estimating the time-varying response of the CSI principal components. This approach provides a measure of the micro-Doppler response of the tag, which can be used for motion tracking. The concept is experimentally validated by a 2.4 GHz tag and commercially-available ESP32 communications transceivers, demonstrating velocity estimation with an accuracy of 0.086 m/s.