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In this paper an experimental test setup for the signal-strength based distance estimation of passive radiofrequency 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.