Millimeter accuracy passive tag ranging via second harmonics RF backscattering against body movement interference

Interference caused by unpredictable body motion is notorious in RF-based ranging and tracking. For passive tags, the strong self-leakage signal and large unwanted direct reflections encountered in conventional RFID transceivers make precision ranging even more difficult. To achieve fast, high accuracy ranging capability against body motion interference, we investigate a second harmonic backscattering solution in the opportunistic method of sequential-test heuristic multi-frequency continuous wave ranging (ST-HMFCW). We show that direct reflection and self-interference are greatly suppressed, so the received second harmonic phase variation is decoupled from the power-dependent phase distortion caused by large interference. ST-HMFCW further seeks opportunities in phase fluctuations caused by interférer motion with drastic performance improvement as bandwidth increases. Robust millimeter-accuracy ranging is achieved under strong body-motion interference with the broadband nonlinear transmission line (NLTL) backscatter tags. We present the theory, simulation and experimental prototypes to verify the effectiveness of the proposed ST-HMFCW ranging method.