Research And Application On The UHF Rfid Backscatter Signal Theory In Multi-Antenna Environment

In recent years,UHF radio frequency identification(RFID)technology based on backscat-ter theory has developed rapidly and has been gradually applied to various fields of produc-tion.The application of RFID technology in intelligent transportation,intelligent logistics,public safety and other fields has brought great convenience to people’s travel,vehicle man-agement,logistics automation,and supervision of medical consumables,making people’s lives more convenient and safer.In addition,the application of RFID technology in the retail in-dustry,manufacturing industry,book industry,smart city,national defense military and other fields is becoming more and more common,and it has become an important part of the coun-try’s future Internet of Things system.Although RFID technology is continuously expanding its application fields and its scope of application is becoming more and more extensive,the cur-rent RFID system still has problems such as short tag access distance and low access efficiency,which restricts the further development of RFID technology.In addition to the limited system transmission power,large backscatter communication channel fading,and high tag activation power threshold,a major problem is that the current RFID system uses a single antenna channel communication system,which causes the current back scattering system has the problems of low throughput and short access distance.As we all know,multi-antenna systems can enhance the throughput and reliability of communication channel by adding antenna channels at both ends of the communication system.Therefore,researchers in RFID systems have also begun to pay attention to the design and performance analysis of multi-antenna RFID systems in re-cent years.And some achievements have been made in the research of multi-antenna RFID system in 5GHz frequency band.But in the UHF frequency band,there has no in-depth study of on multi-antenna RFID systems.Therefore,this paper conducts comprehensively research on the application of multi-antenna technology in UHF RFID systems(800MHz～900MHz),proposes corresponding detection algorithms and access algorithms,and conducts theoretical analysis,simulation and physical verification of the performance of these algorithms,realizing the innovation of theory and application of the multi-antenna system on UHF RFID systems.The research of this paper mainly includes the following aspects:A method for estimating the number of tags in high-dimensional space suitable for multi-antenna backscattering systems is proposed.This method is applicable for scenarios where the system only needs to know the number of RFID tags within its access range,but does not need to inquire the detailed information of the items recorded inside the tags.The method first arranges the complex signal vectors received by multiple receiving channels of the multi-antenna backscatter system to construct a high-dimensional signal space.Then,the classical density-based clustering analysis algorithm is used to cluster the signal samples in the high-dimensional space,and the number of signal clusters after clustering is counted.Finally,the relationship between the number of signal clusters and the number of tags is used to give an estimate of the number of currently accessed tags.Different from the common multi-antenna selection algorithm or the maximum combination ratio algorithm,this method not only makes full use of the information of different path signals received by multiple antennas,but also avoids the channel estimation procedure required in the maximum combination ratio algorithm.It not only has low implementation complexity,but also can estimate the number of tags accessed by the current system in a short time.Simulations and physical experiments prove that the estimation performance of this method is better than existing methods in the case of low signal-to-noise ratio,and it has a good application prospect in practice.A tag”anti-collision”algorithm based on wide linear minimum mean square error is pro-posed.This method takes advantage of the characteristic that the RFID signal in most UHF backscattering systems is a non-circular signal,and uses wide linear signal processing technol-ogy to make full use of the second-order non-circular statistical information contained in the signal,which improves the detection performance of the UHF backscattering systems on mul-tiple mixed non-circular signals.The ability to detect mixed backscattered signals reduces the impact of”tag collision”phenomenon on system throughput.Numerical simulation and physi-cal experiments results show that the wide linear detector using the non-circular characteristics of the backscatter signal has performance advantages over the traditional linear detector,and it has been successfully applied to the practical multi-antenna UHF backscatter system with satisfactory results.A multi-antenna backscatter system(NOMA-RFID)based on non-orthogonal access the-ory is proposed.The advantage of the channel capacity of the NOMA-RFID system over the current RFID system based on time division multiple access has been theoretically proved;a power allocation scheme of the NOMA-RFID system is given in the scenarios of the system channel capacity maximum and the tag access fairness maximum;a adaptive power allocation algorithm based on the number of un-accessed tags is presented,which achieves a good compro-mise between tag access fairness and the system total capacity;the expression of the channel capacity of the NOMA-RFID system in the case of non-circular signals is given,and the in-fluence of the non-circular characteristics of the backscatter signal on the total capacity of the system is discussed;a system capacity allocation scheme based on adjustable non-circular sig-nals is proposed;The system capacity of the NOMA-RFID system with the non-circular signal is greater than the one using the circular signal when there is error propagation in the system has been theoretically proved for the first time,which reveals the potential of the non-circular signal in the NOMA-RFID system;the performance of a multi-antenna NOMA-RFID system based on the WLMMSE-SIC detector is analyzed,and both the distribution function of the SINR of the detection signal,and the closed expressions of the outage probability and the symbol error rate of the tag are given.The statistical characteristics of the signal-to-interference and noise ratio(SINR)of the output signal of the wide linear minimum mean square error(WLMMSE)detector under arbi-trary antenna configuration are analyzed.Besides,the performance of the uplink multi-antenna communication system based on the WLMMSE detector are also analyzed.It is proved for the first time that the SINR expression of the output signal of the WLMMSE MIMO system con-taining N_ttransmitting antennas and N_rreceiving antennas can be expressed as the sum of N_tGamma random variables with different parameters.The approximate closed analytical expres-sion of the probability density function(PDF)of the SINR distribution under arbitrary antenna configuration are given.The approximate expression is concise and clear,and in most cases,the results given by the approximate expression are in good agreement with the simulation results.Simulation results also show that the approximate PDF expression is not only suitable for the performance analysis of small-scale MIMO systems,but also for the performance analysis of large-scale MIMO systems.In addition,the PDF expression of the derived SINR is used to an-alyze the performance of the WLMMSE MIMO system under any antenna configuration.The obtained results explain some simulations results in the tag”anti-collision”algorithm based on the wide linear model very well.The theoretical results also provide a reference for the analysis on other MIMO communication systems based on wide linear processing.The wide linear minimum mean square error detection algorithm based on serial interfer-ence cancellation(WLMMSE-SIC)is analyzed。The PDF function of the SINR of the output signal of each layer of the WLMMSE-SIC detector with N_ttransmitting and N_rreceiving an-tennas under the condition of error-free propagation is given.The closed analytical expressions of the outage probability,bit error rate,and diversity gain of each layer of the WLMMSE-SIC MIMO system are also given.In addition,the impact of error propagation on the performance of the WLMMSE-SIC detector is discussed,and the closed analytical expressions for the symbol error rate and outage probability of each layer of the WLMMSE-SIC MIMO system under the condition of error propagation are given.Considering that the optimal ordering algorithm has a greater impact on the performance of the WLMMSE-SIC detector,we also analyze the SINR distribution of the output signal of the WLMMSE-OSIC detector based on the SINR ordering of the detection signal.Finally,the outage probability of the WLMMSE-OSIC MIMO system and the performance expression of the symbol error rate are given.