Analysis And Estimation Of Performance For Passive Ultra-high Frequency Radio Frequency Identification Systems

Internet of Things (IOT) is a network of real-world objects which can be linkedby the internet and interacting through on-line services. With low-cost informationgathering and dissemination devices, such as two-dimension code, sensors andelectronic tags, IOT facilitates fast-paced interactions among the objects at any pointin time. Radio frequency identification (RFID) is one of the critical technologies ofIOT, which transfers data between a reader and a tag attached to an object with radiofrequency communication technology, and then complete the purpose of identification,localization, monitoring, tracking, etc. Due to its cost-effectiveness and ease of use,passive RFID technology in ultra high frequency (UHF) band is gaining more andmore interests from both scientific and industrial communities. In order to select thedevices best-suited and deploy the project of RFID system optimizedly in the realapplication, the accurate and cost-effective benchmark tools are desiderated for RFIDdesigners and users. Nevertheless, the rigorous characterizations of UHF RFID systemin terms of reading region and differential radar cross section cannot be performed bytraditional methods based on vector network analyzers. In view of the currentsituation above, a high performance RFID test platform is developed. Based on theplatform, the effects of polarization mismatch, mutual coupling and multipath fadingon RFID systems’ performance are analyzed and measured as well.Firstly, the basic theory, compositions and relevant standards of RFID aredescribed. By means of tracking and analyzing the development status, existing issuesand tendencies of RFID test technology, the test contents and environments requiredby a RFID tester are discussed. Utilizing the technology of software defined radio andvirtual instrument, an open RFID test platform, which based on NI boards, includingPCI-5640R, PXI-5610, PXI-5600, etc, is built with built-in RFID protocol stack andreal-time communications capability. The transmiting, receiving and analyzing forRFID signals can be performed by FPGA model embedded in PCI-5640R. Withreal-time processing and reconfigurable architecture capabilities of FPGA, theplatform can implement different functions for all the commonest standards and beexpended for the user-defined standards. Combining with the simulation environmentconstructed by the technology of PLC and OPC, the platform complete the test functions of air interface conformance, systems’ performance, sniffer, applicationscenarios, etc.Secondly, based on the principles of RFID technology and the theory of antennascattering, a link budgets model of UHF RFID in free space which consists of onereader and one tag is provided, and the impedance matching of a tag is discussed. Inpractical applications, the close displacement of RFID tags can be considered as anelectromagnetic interconnected system which causing mutual coupling effects amongantennas of tags and results in change in impedance matching condition. Expressionsof mutual impedance in dense environments are derived by using of the method oftwo-port network. Utilizing the parameters of power transmission coefficient andmodulation factor, the variation of system performance is discussed in theory andtested by three experiments which conducted in open indoor envi ronment. Themeasurement results show that the change rate of minimum power transmitted by thereader antenna is in a range from-7%to11.6%for two-tag scene and-10%to12.5%for two-plane arrangement.Thirdly, based on the theory of electromagnetic wave propagation and Friispropagation equation, a path loss model of UHF RFID is provided. The results ofsimulation and actual applications indicate that the small-scale fading of UHF RFIDhas deeper fades than that observed in the conventional transmitter-to-receiversystems. The Fresnel clearance and horizontal interval between reader antenna and tagare employed as dependent variables, and then the obstructing effect of the firstFresnel zone on path loss is discussed. Utilizing the methods of linear regr ession andminimum mean-square error, a dual-slope logarithm distance path loss model isproposed. The path loss of UHF RFID under different parameters is measured in openindoor environment. The measurement results indicate that RFID system experiencesless fading when the Fresnel clearance is1.5times higher than the first Fresnel radius.Compared with traditional logarithm distance path loss model, the standard deviationof the proposed model with two slopes decreases ten percent or more.Finally, integrated with the impact factors for various performances of UHFRFID System in practical applications, a complete propagation model in realenvironments is provided. Utilizing the parameter of recognition rate of target region(RRTR) defined in this paper, a systems’ performance evaluating method is presented.In practical applications, the impact factors cause blind zone, where a tag cannot beread by a reader. Meanwhile, when the radiation pattern of a single antenna isinadequate, the multiple-antenna configurations can be employed in spite of some blind zone additionally due to the interaction of the transmitted fields from theantennas. Based on the theories of electromagnetic propagation and coherentinterference suppression, two optimization methods including tag diversity and phaseswitch are proposed to get the maximization of RRTR. The RRTR under differentparameters are measured in open indoor environment with commodity hardware. Themeasurement results show that the value of RRTR can be enhanced by10%and7.6%with the two optimization methods respectively, and will be better with both.