Accurate indoor ranging by broadband passive NLTL tags

Millimeter-precision, decameter-distance real-time indoor ranging capability is challenging due to multipath reflections in rich scattering environment. We present a solution by passive broadband nonlinear transmission line (NLTL) tags. Via harmonic backscattering, reader self-jamming and errors caused by direct reflections from ambient scatterers can be greatly reduced. By the broadband property of NLTL, a simple, robust method by multi-frequency continuous wave (MFCW) is formulated to resolve the phase integer ambiguity and to further improve ranging accuracy in the occasion of large phase errors. We present theoretical and simulation analyses, followed by experimental verification.