Received signal strength indication signature for passive UHF tags

Received signal strength indication (RSSI) has commonly been used for estimating the distance between a radio frequency identification (RFID) tag and its reader. However a plot of RSSI vs. frequency reveals a signature specific to the physical surrounding environment. In an attempt to understand how the environment impacts the RSSI signature, an experiment was performed in a 'real world' environment where the test setup was shifted incrementally though an open room to see how the RSSI signature changes. In another experiment, the RSSI signature was measured in an anechoic chamber with and without absorbers on the floor so the effect of the ground plane reflection on the RSSI signature could be isolated and examined. The RSSI signature was found to be too complex to pinpoint a single feature as being caused by the ground plane reflection; however it can be concluded that not only is the RSSI signature repeatable within an unchanged physical environment (and therefore not merely random noise), small changes in the environment result in small changes in the RSSI signature. These experiments demonstrate a potential improvement in fingerprinting and distance estimation, if frequency hopping is used to measure RSSI, by averaging RSSI over a frequency range.