Study On Passive UHF RFID-Based Real Time Locating Systems

Real Time Locating System (RTLS) based on Ultra High Frequency (UHF) Radio Frequency Identification (RFID) is promising due to lots of advantages, and it has been becoming a research focus of indoor location technology recently. However, limited by low cost and low power of passive tags and narrow bandwidth of the system, UHF RFID reader can not locate the tags with high accuracy unless it has been improved drastically. Pseudo-random Number Ranging (PNR) method and multi-point location theory have been introduced into the system, and a passive UHF RFID-based RTLS has been proposed and validated in this dissertation.Firstly, signal transmission characteristics of the system are discussed, and a UHF RFID-based RTLS using PNR method and multi-point location technique is put forward. The feasibility, the difficulties and possible solutions of the proposed system are analyzed.Secondly, power transmission characteristics of UHF RFID are discussed and main system parameters are calculated. A Leaking Carrier Canceller (LCC) is adopted to suppress the large leakage due to insufficient isolation between the transmitter and the receiver. A LCC-based RF transceiver architecture is proposed.Thirdly, several key technologies of the system are analyzed, including the auto-correlation of ranging codes, the synchronization and the overall accuracy of the system. A 125-bit shortened m sequence is selected as the ranging code, and a synchronization control loop and a low-complexity correlator comprised of four parallel matched filters are designed. Three techniques of improving the accuracy are discussed.Fourthly, a UHF RFID-based real time locating reader is designed and analyzed, including a LCC, a RF transceiver and a Digital Signal Processing (DSP) block. The LCC and the transceiver have been modeled in ADS (Advanced Design System) and Matlab/Simulink for frequency-domain and time-domain performance analyses, and the DSP block has been co-simulated by Matlab and Modelsim. Simulated results testify the function of the whole system.Finally, a prototype of passive UHF RFID-based RTLS has been implemented, including a real time locating reader and two tag emulators. A detailed testing plan has been established to test the prototype and to validate the system. Measured results indicate that the prototype meet the requirements of RTLS, with the average accuracy 0.5 m and locating area 0.79 m2, which prove the correctness and reliability of the proposed system.