Phased-Based Composite Ranging for Backscatter RF Tags: System Analysis and Measurements

A phase-based composite ranging system is investigated that provides accurate and absolute distance estimates for backscatter radio frequency (RF) tags by combining the range estimates of a dual-frequency, continuous-wave (DFCW) and a continuous-wave (CW) radar. The relatively low-resolution DFCW range estimate is used to resolve the wavelength ambiguity in the CW radar, and the disambiguated CW range estimate provides the final, high-resolution, accurate range estimate. Two data fusion methods for combining the DFCW and CW range estimates are proposed and their performance is compared based on measurement results with a backscatter RF tag system operating in the 5.8 GHz industrial, scientific, and medical (ISM) frequency band. Each data fusion method uses spatial averaging, as the RF tag is moved through space to reduce the overall system error. The efficacy of spatial averaging is investigated by showing that multipath and hardware imperfections cause oscillating errors in the phase used for ranging and the errors are significantly reduced through spatial averaging. It is shown that the proposed system can achieve millimeter-level ranging resolution in a typical indoor multipath environment while adhering to the bandwidth constraints of the 5.8 GHz ISM frequency band.