Tag localization in passive ultra-high frequency radio frequency identification (UHF RFID) has become something of a holy grail since the system was introduced. First and foremost, it promises improved accuracy for inventory systems, which have to deal with false positive reads outside the intended read zone and the consequent corruption of inventory lists.

This thesis addresses the problem of passive UHF RFID tag localization, with a focus on range finding methods. It shows how typical system setups influence the propagation channel and why this creates a harsh environment for any type of localization. Based on the conclusions drawn in this channel analysis, tag localization approaches are investigated. These investigations cover narrowband through ultra-wideband localization approaches, with three methods discussed in detail: phase-based continuous-wave ranging, frequency-modulation continuous-wave ranging, and ultra-wideband impulse radar.

It is shown that robust and accurate localization of state-of-the-art UHF RFID transponders is possible only in controlled environments. Classification to large read zones such as portals or dock doors, on the other hand, is feasible in principle in the field as well. However, robust classification with low error rates still requires a high number of position estimates. This might be unfeasible in applications with high tag throughput.

Wideband and ultra-wideband methods offer considerably greater robustness and accuracy in typical fields of application for UHF RFID, but come at the cost of increased system complexity. It is shown how state-of-the-art passive RFID tags can be modified to support wideband and even ultra-wideband backscatter localization and also how these methods can be implemented in a reader. Critical system parameters and vulnerabilities with respect to structural and physical properties of UHF RFID installations are discussed in detail and possible countermeasures are provided.

Achievable accuracies depend on the environment, the application scenario, and the localization system in use. Provided that the line-of-sight to the tag is not blocked, errors for a single position estimate range from several tens of centimeters for ultra-wideband systems to several tens of meters for narrowband localization in typical UHF RFID environments. Increased accuracy and robustness with respect to blocked line-of-sights can be achieved by combining several estimates, e.g., by using tracking approaches.