Passive ranging by low-directivity antennas with quality estimate

Compared to the high-directivity patch and horn antennas, miniaturized omni-directional antennas allow more flexible integration with portable devices because of their smaller size, lower cost, broader angle coverage and less phase center variation. However, locating passive tags with low-directivity antennas in indoor environment becomes even more challenging due to more multi-paths and weaker line-of-sight (LoS) path. We propose a simple solution by digital beamforming with two arrays of 2-element antennas in a broadband harmonic multi-static backscatter system. Millimeter-precision ranging in weak LoS environment is achieved in a broad and sparse frequency scheme with omni-directional antennas which offer peak and average gain of 0.5dBi and -1.9dBi respectively. Our system employs sensing frequencies that are separated wider than typical indoor coherence bandwidth and therefore, we show that ranging robustness can be further enhanced by estimating angles of arrival (AoA) gap, i.e., unreliable measurements from dominant multi-paths can be distinguished and rejected. We present the fundamental theory, simulation and experiments with a homodyne harmonic RFID reader in a rich scattering environment.