Analysis And Measurements To The Performance Of Passive UHF RFID Channels

High frequency (HF) radio frequency identification (RFID) technology has grown up and been widespread used. Ultra high frequency (UHF) RFID technology will also become more and more widely used with the developments of "Internet of things" and EPC global organization. UHF RFID uses the principles of radar rather than voltage transformers to model the radio frequency (RF) communications. The UHF tags are passive and the radio waves propagate randomly, which cause the demands of identifying huge number of tags simultaneously at high read rate become difficult for UHF RFID systems. A successful identification will come true only when the field strength of forward link is higher than the activation threshold of the tag, and the backscattered power is adequate to decode the data in the backward link. Hence it’s very important to research the UHF RFID communication links in order to improve the reliability of reading rates by using of measurements, analysis and modeling of the links.The research status of test technology in UHF RFID’s RF interfaces and propagation channels was introduced firstly. It’s very important to develop the protocols conformance test instruments and typical application test platforms. It’s also important to analysis and model the UHF RFID channels because the differences with conventional wireless communication systems. Then the main contents and key technologies were introduced and international test standards were analyzed. We developed a UHF RFID test platform. The details of the design plan and some test cases, such as protocols conformance and the third party sniffer, were showed as well. Based on the test platform, an automatic assembly test method to the blind spots of identification and propagation channels for the UHF RFID portal application was proposed, and the affecting factors were also analyzed.Wireless power transmission and backscatter modulation are core technologies for UHF RFID systems, which make the tags have simple structure and lower factory cost. Antenna, rectifier and modulator are vital parts of the tags’RF interface. Their circuit parameters determine the performance of forward activation link and backscatter link. The measurable important parameter of propagation links is tag’s radar cross section, through which the BER and activation range of UHF RFID can be evaluated. Consequently, based on the test platform, the thesis has analyzed the above contents, and proposed its measurement method.Due to the special properties of UHF RFID in carrier frequency, width of frequency band, data rates and application environments, the thesis has studied its numerical characteristic using four basic propagation mechanisms. Small scale and large scale empirical models were used to model the UHF RFID channels, and the test results of portal application were fit to log-normal model. The computation methods of model parameters and blind spots probability were also showed. A complete model was proposed including many major factors which have significant effects on the channels. Based on the test platform, the parameters of the proposed model can be measured, and then the real identification area of UHF RFID systems can be computed.Multi-antenna diversity reception and transmit technologies can improve identification rate and the performance in multipath fading channel for UHF RFID systems. The thesis has studied the performance for forward and backward channels of multi-antenna RFID systems. Firstly, a channel estimated method for ISO18000-6C multi-antenna RFID systems has been proposed. Then, based on the criteria of maximal signal-to-noise ratio combing, both the analytical expressions of forward activation range and backward identification range were derived. Considering some channel estimated errors and interferences from other readers, the bit error rate (BER) and identification range of backward link were studied. The expressions were testified through numerical simulating and tests at last.