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 generated by commodity communications hardware. The approach is based on detecting the nonlinear responses of simple diode-based RF tags and estimating the change in the response over time using channel state information (CSI) obtained using standard preambles in existing commercial wireless communications protocols. Many wireless communications systems use a set of continuous-wave tones for channel estimation that, when incident on the diode, generate intermodulation products near the fundamental frequencies. The retransmitted signal can be used for identification, as demonstrated in other works, or can be tracked to estimate the motions of the tag. The concept is experimentally validated by a system using a 2.4 GHz tag and commercially-available ESP32 communications transceivers, demonstrating velocity estimation with an accuracy of 0.086 m/s.