Magneto-Inductive Localization: Fundamentals of Passive Relaying and Load Switching

Magneto-inductive systems are extensively used for communication and wireless power transfer. However, they are equally advantageous for close range localization because they provide a high channel gain over short ranges and hardly suffer from non-line-of-sight conditions, synchronization problems, and interference. Their inherent near-field operation also introduces an exciting opportunity, which has hardly been explored to date in a localization context: The use of passive relays in the form of passive resonant coils with known locations. In this paper we systematically investigate the potential of passive relays for magneto-inductive localization with a circuit model which accounts for arbitrarily arranged system nodes. Based on the Cramér-Rao lower bound, we show that with carefully placed relay nodes the position root-mean-square error (RMSE) can be improved by an order of magnitude. However, in order to also benefit from randomly placed relays, we slightly extend their functionality by including the possibility to switch off their loads (binary load switching). With this extension we can exploit the presence of multiple randomly placed relays to improve the RMSE by orders of magnitude and also drastically extend the supportable localization range.