Simultaneous Gen2 Inventory and Angle of Arrival Measurement of Backscatter Signals with Multiple Commodity SDRs

Simultaneous inventory and angle of arrival (AoA) measurement of UHF RF tags is strongly demanded in item searching applications. Existing high-accuracy AoA measurements of RF tags mainly employ a radio comprised of multiple antennas and a coherent receiver to produce synchronized multiple IQ streams. Such radios are realized as dedicated hardware and software, which is not suitable for prototyping new ideas. This paper proposes a method to realize simultaneous inventory and AoA measurement with two commodity software defined radios (SDRs) with four receiving antennas. The mutual phase offset between the two SDRs is canceled by sequentially selecting two antennas from the four antennas with two RF switches and physically switching the combination of these two antennas and the receivers with an RF switch. In this way, the SDR receiver phase calibration can be performed during the process of inventory without human intervention. AoA measurement with all four antenna elements can be realized only with two SDRs with a penalty on the AoA measurement agility. Spatial smoothing is applied to counter the multipath problem. The antenna switching is integrated into an SDR Gen2 interrogator developed on a dual RF-port USRP. The external RF switch is synchronously controlled along with Gen2 inventory progress using Gen2 protocol features called action and target. The accuracy of the phase offset cancellation is evaluated by conducting wired and wireless experiments with commercial Gen2 RF tags. The experiments reveal that the AoA measurement can be simultaneously performed with Gen2 inventory achieving within ±15 degree accuracy with four receiving antennas in an indoor environment when the fading is stable over the measurement duration.