Experimental test system for distance estimation of standardized passive UHF RFID systems

In this paper an experimental test setup for the signal-strength based distance estimation of passive radio-frequency identification (RFID) tags is proposed. The system shall use freely available ultra high frequency (UHF) tags, and shall require no modifications of the tags or the communication protocol. A test setup based on a software defined radio (SDR) system is implemented. However, first measurements with this simple setup show, that a reliable distance estimation is not possible. Due to interference effects, such as multi-path propagation, the signal strength is not mainly dominated by its distance dependency. Therefore, the test system is modified and a number of optimization approaches is proposed. The goal hereby is to reduce the impact of interference on the signal strength, so that the measured value exhibits a mainly distance dependent behavior. Besides frequency hopping, various forms of antenna diversity are discussed. Using these optimization techniques it's possible to implement a test system that allows a fairly reliable distance estimation under controlled laboratory conditions.