Multifrequency-Based Range Estimation of RFID Tags

Radio frequency identification (RFID) is a rapidly developing wireless communication technology for electronically identifying, locating, and tracking products, assets, and personnel. RFID has become one of the primarily means to construct a real-time locating system (RTLS) that tracks and identifies the location of objects in real time using simple, inexpensive tags attached to or embedded in objects and readers that receive the wireless signals from these tags to determine their locations. Most RFID tag localization techniques heavily rely on precise estimation of the range between a reader and the tags. Traditionally, range information is obtained from the received signal strength indication (RSSI). Such approach is inaccurate, particularly in a complicated propagation environment. Recent development on phase difference of arrival (PDOA) allows coherent signal processing for improved range estimation performance. Exploiting multiple frequencies may further improve the range estimation performance. In this paper, we focus on multifrequency-based techniques to achieve several important advantages in the range estimation of passive or semi-passive RFID tags. The use of well designed multiple frequencies allows effective phase upwrapping and elimination of the range ambiguity problem which may be encountered in PDOA methods. In a complicated propagation environment, the mutifrequency-based techniques provide frequency diversity for robust range estimation when signals are highly faded at some frequencies. These advantages not only yield improved range estimation accuracy of RFID tags in various applications, but also enable robust range estimation in challenging scenarios.