Range Estimation and Performance Limits for UHF-RFID Backscatter Channels

The accuracy of time-of-flight-based ranging over ultra-high frequency (UHF) radio frequency identification (RFID) backscatter channels is fundamentally limited by the available bandwidth and highly dependent on the channel characteristics. Comprehensive wideband channel measurements are presented and analyzed with respect to parameters which influence the potential ranging performance. The Cramér Rao lower bound on time-of-flight-based ranging is evaluated and we study the influence of dense multipath on the bound. Based on a line-of-sight (LOS) plus dense multipath (DM) radio channel model, a multiple-input multiple-output (MIMO) ranging algorithm is developed, capable of iteratively estimating the LOS parameters and the statistics of the DM. The accuracy of the developed algorithm is compared to the performance bound. The results highlight the tradeoff between a higher bandwidth and spatial diversity for UHF RFID systems with respect to time-of-flight-based ranging. In a $\boldsymbol {2\times 2}$ MIMO setup, an accuracy of about 0.5m is achieved at a bandwidth of 50 MHz.