Research On Carrier Phase Measurement Technology In UHF RFID Positioning System

Due to its high precision and low cost,RFID technology is gaining more and more applications in indoor positioning.However,the main function of the traditional RFID system is to identify the target,not to position targe;in addition,the ranging method based on the arrival time and the received signal strength is susceptible to interference,the accuracy of the instrument is high and the implementation is difficult..Therefore,in this thesis,based on the arrival phase difference method,a carrier phase measurement system is designed,to measure the arrival phase of the tag and analyze and study the non-ideal factors.This thesis is based on NI's software radio platform to achieve a fast and high-precision carrier phase measurement system.Aiming at the non-ideal factors such as DC offset and I/Q mismatch generated by the reader,the DC offset elimination method and the I/Q mismatch factor detection method are proposed,and then establish a delay-phase model to correct the measured phase.According to the characteristics of the tag return signal,the carrier phase measurement system is designed based on the I/Q digital baseband signal,is composed of the filter module and the carrier phase measurement module.The filtering module extracts and filters the digital baseband signal.To meet the protocol requirement of 40 k Hz to 640 k Hz return data rate,in this thesis,a configurable extraction rate filter composed of a CIC filter and a half-band filter are designed to reduce the data rate,and the FIR filter filters out the out-of-band noise.The carrier phase measurement module calculates the phase,including the threshold module,the eliminate DC module and the improved CORDIC algorithm module.Aiming at the deficiency of the traditional CORDIC algorithm,which occupies large resources and cannot cover the whole circumference with the scope of convergence domain,the rotation angle is optimized based on it.and the range of the CORDIC algorithm is extended to 0 to 360 degrees.Through analysis,the error is less than 0.02 degrees.The experiment on carrier phase measurement system shows that the relationship between phase and distance is in line with expectations,and the error from the theoretical value is within 1 cm.The experiments on the factors affecting the measured phase shows that the phase measured by the two tags are basically the same with the change of distance;when the signal-to-noise ratio is greater than 34 d B,the phase change has little effect on the phase accuracy;the rotation direction of the tag has a great influence on the phase,and the degree of influence on the phase of different tags is different.In practical applications,the tag with less influence on the rotation direction should be selected according to the application.