Leveraging Spatial Diversity for Ambiguity-Free Ultranarrowband Phase-Based 3-D Localization

Phase-based 3-D localization of radio-frequency (RF) markers has high sensitivity and accuracy. However, phase measurements suffer from oscillator phase noises, wavelength ambiguities, and multipath interferences. Additionally in the near field, antenna detuning and medium inhomogeneity render the phase-distance relation nonlinear and nonmonotonic and bring forth extra ambiguities, especially with obstructed Line-of-Sight (LoS). In this work, we present a novel precision localization framework which leverages spatially diverse redundant channels to resolve ambiguities without relying on broad bandwidth. First, measured differential phases were used to accurately retrieve differential distances from spline-fitted phase-distance curves. Then, distances from multiple channels were combined to generate 3-D location estimates. Finally, location ambiguities were removed by taking different channel subsets to identify one unambiguous location using spatial clustering. An experimental multiple-input–multiple-output (MIMO) network was implemented by a Universal Software Radio Peripheral (USRP) platform and harmonic RF markers to demonstrate millimeter-level 3-D localization at sub-1GHz carrier frequencies within heavy multipath ambient, simulating the condition inside building structures.