Pushing the Boundaries of High-Precision AoA Estimation With Enhanced Phase Estimation Protocol

The emergence of high-precision indoor backscatter tag tracking in GPS-deprived environments has advanced applications from virtual reality to factory automation. Despite this, the high-precision tracking range remains limited to just a few meters, restricting the use of backscatters to the vicinity of checkpoints in warehouses, even though they possess a communication range of 50 meters. We have identified that this limited localization range primarily originates from the butterfly effect in localization systems, where a slight phase measurement error gradually escalates into a substantial localization error. This paper introduces two innovative phase estimation protocols to address the intrinsic challenges in achieving high-accuracy phase estimation over long-distance communication. The first, CPE, resolves the π-ambiguity commonly encountered with commercial RFID readers. Building on this, CPE+ is designed to cancel flicker noise, neutral white noise, and restore spatial and temporal imbalances. Our experimental results demonstrate that CPE+ extends the range of accurate Angle of Arrival (AoA) estimation and centimeter-level localization from 8 meters to 15 meters in stationary scenarios. It maintains decimeter-level accuracy across the entire 50-meter communication range for CPE+ with two or more gateways. In dynamic scenarios, the error of CPE+ increases with tag speed, reaching a median localization error of 11.7 cm at 5 meters for tag speeds of 50 cm/s.