Multichannel digital transmit beamforming experiments for RFID tag localization

The spatial localization of passive UHF (ultra-high frequency) radio-frequency identification (RFID) tags is an emerging technology that has reached a high level of interest in industry and research. Most solutions are based on classical receive beamforming techniques, like the angle-of-arrival estimation, that are prone to multipath and deliver low localization accuracies. Therefore, a new approach for estimating the position of a passive UHF RFID tag is introduced that uses a transmit beamforming technique called ´Angle-of-Activation´ (AoAct). It is based on manipulating the shape of the radiation pattern of the reader´s transmit antenna array. Based on a set of power levels and phases for the transmit signals of the array elements, the I and Q components of the complex baseband phasor are calculated. In comparison to analog techniques, this new approach delivers higher angular accuracy of the radiation patterns. The self-developed digital beamforming hardware is able to pivot the main lobe of a three- element antenna array in a range between -52° and 52° with a mean angular resolution of 0.02°. By utilizing the very sensitive response threshold of passive UHF RFID tags, the AoAct can be estimated wiht high accuracy. Experimental measurements deliver a mean AoAct accuracy of 1.2° in the anechoic chamber, 19° in a basic multipath environment, and 2.5° in a real-world multi-path environment in a range of ±45°.