Experimental Evaluation of a UHF-MIMO RFID System for Positioning in Multipath Channels

This paper presents an experimental evaluation of an ultra-high-frequency (UHF) multiple-input-multiple-output (MIMO) radio-frequency identification (RFID) system for positioning. To this end, we propose a set of novel parametric maximum likelihood direct-positioning algorithms capable of exploiting the coherent measurements performed by closely-spaced antennas and simultaneously exploiting the non-coherent measurements by widely-spaced antennas. The radio channel indoors for relatively small bandwidth can be characterized by a line-of-sight component plus a multitude of so-called dense multipath components (DMC). The proposed algorithm framework is able to consider the DMC, enabling more accurate positioning. We present an experimental RFID testbed, capable of performing wideband measurements up to 50MHz bandwidth. This testbed is able to query the RFID tag at the UHF band while performing the position measurements in the 2.45 GHz band, supporting such wideband signal transmission. A dual-frequency RFID tag is presented, equipped with two antennas, scattering back synchronously at the UHF and 2.45 GHz bands. Utilizing the experimental UHF-MIMO RFID testbed we show that 80% of the position errors are smaller 0.15m if the DMC process is included in the algorithm.