Localization of Signal Pattern Based UHF RFID Sensor Tags

This letter presents a prototype system that combines sensing and localization capabilities based on the radio frequency identification (RFID) technology in the ultrahigh-frequency (UHF) band. The system detects empty water tanks by recognizing a predefined transponder (tag) signal pattern at the reader. The shape of this pattern is determined by a custom-built sensor tag antenna that interacts in a unique way with an off-the-shelf microchip and the water filling level. The localization of this sensor tag is achieved by a 2-D direction of arrival (DoA) estimation, exploiting a $2\times2$ antenna array. The measurement results compare well with electromagnetic simulations. It is shown that the detection of the sensing state is sufficiently robust to cope with different tank positions, even in multipath environments. Furthermore, the system is robust in its localization performance, although the sensing state influences the wireless channel.