Localization of Passive UHF RFID Tags Using the AoAct Transmitter Beamforming Technique

The localization of passive ultra-high frequ- ency (UHF) radio frequency identification (RFID) tags is an emerging technology that has reached a high level of interest in industry and research. Most solutions use classical techniques, such as the angle-of-arrival or the received-signal-strength methods. As an alternative, we present a new approach for estimating the position of a passive UHF RFID tag by using a transmitter beamforming technique that we call angle-of-activation (AoAct). It is based on manipulating the shape of the radiation pattern of the interrogator's transmitter antenna array. By pivoting the main lobe of the pattern in the azimuth plane of the array and by utilizing the very sensitive response threshold of passive UHF RFID tags, we are able to estimate the tag direction. We use three-channel digital beamforming, where the phases and amplitudes of the antenna array's feeding radio frequency signals are controlled by the baseband I and Q components of the interrogator's transmitter signal. Our self-developed hardware pivots the main lobe of a three-element antenna array in a range of ±52° with a resolution of 0.02°. It senses the AoAct with a mean measured accuracy of 1.2° in the anechoic chamber, 1.9° in a basic multipath environment, and 2.5° in a real-world multipath environment. After estimating the AoAct of a tag from different locations, the tag position is triangulated. Experiments show that our AoAct system delivers a localization accuracy of 7 cm in a typical multipath environment.