Transmit beamforming for Angle-of-Activation (AoAct) estimation in passive UHF RFID systems

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 in the last few years. 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, we introduce a new approach for estimating the position of a passive UHF RFID tag by using a transmit beamforming technique that we call `Angle-of-Activation´ (AoAct). It is based on manipulating the shape of the radiation pattern of the reader´s transmit antenna array. To do this, the I and Q components of the complex baseband phasor are calculated based on a set of power levels and phases for the transmit signals of the array elements. In comparison to analog techniques that are widely used, this new approach delivers higher angular accuracy of the radiation patterns. Our 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 0.34°. By utilizing the very sensitive response threshold of passive UHF RFID tags, we are able to estimate the AoAct with a mean measured accuracy of 1.2° in the anechoic chamber, 1.9° in a basic multipath environment with one reflection, and 2.5° in a harsh multipath environment.