| At present,the global navigation satellite system(GNSS)has always been one of the most important means to achieve location and navigat ion services.However,the satellite has some inherent shortcomings,including poor penetration,interference from the surrounding environment,poor anti-interference ability,and possible attack on the enemy's network,etc.Recently,satellite navigation systems are constantly being disturbed and deceived by human activities,making GNSS increasingly unreliable.Therefore,the method of using the opportunity signal,such as LTE signal,without satellite,has attracted more and more attention.This method can avoid the occurrence of interference and deception.How to synchronize the signal accurately and improve the positioning accuracy is the difficulty of this problem.The opportunity location system based on the traditional LTE receiving terminal can only carry out continuous measurement of the main service cell.For the adjacent area,the continuity of measurement and location can not be guaranteed only when the event occurs.In addition,the signal transmitted by the LTE base station distinguishes the antenna port,and the traditional terminal can not distinguish the port.In view of the shortcomings of the traditional terminal,this paper designs and implements the LTE downlink synchronization and measurement receiving terminal by using USRP and LabVIEW.In addition,the correlation algorithm of opportunity location is realized by using the measured data.First of all,this paper introduces the status of mobile communication and the development of LTE,as well as the advantage of using the opportunity signal to locate a place.It introduces the key technology of the physical layer of LTE signal,including the frame format and the synchronous signal,which lays the foundation for the reception synchronization and location of the signal.Then the key technologies and algorithms of LTE downlink synchronization and measurement are studied and analyzed.At the transmitter of the LTE signal,the generation principle and properties of two synchronous signals,which are the Primary Synchroniz ation Signal(PSS)and the Secondary Synchronization Signal(SSS),are introduced.The details of the generation of Cell-specific Reference Signals(CRS)and their important role in measuring Reference Signal Received Power(RSRP)are explained in detail.It studies how to map the above three types of signals to the resource grid through resource mapping.The role and types of the Cyclic Prefix(CP)in the LTE system are introduced.Finally,how to convert a signal from frequency domain to time domain by OFDM modulation is described.At the LTE receiver,the synchronization algorithms of the two synchronization signals are analyzed and compared.The relationship between the cell ID number and the synchronization signal,the CP type detection algorithm,the OFDM demodulation method and the RSRP measurement algorithm are studied.Based on USRP and LabVIEW platform,the prototype implementation of transmitter and receiver of LTE synchronization system is completed.T he primary synchronization signal is synchronized using cross-correlation timing synchronization algorithm according to its good correlation,and the use of the symmetry of the synchronous signal has successed to reduce the amount of calculations by half.The secondary synchronization signal is synchronized by using the generation rule in the inverse solution.From the synchronization signal,the cell ID number can be extracted to complete the cell search.In addition,LTE downlink synchronization also requires CP type detection of the received signal in order to remove the CP,and OFDM demodulation to convert the time domain signal to the frequency domain.Signals are transmitted and received using the USRP.LabVIEW is used to processes the signals.After the synchronization is completed,cell-specific reference signals can be measured.A variety of signal features is used to analyze and compare the localization methods.Combined with the characteristics of the LTE signal used in this paper,we choose the RSRP values obtained at different locations to establish a fingerprint map,and then use the KNN algorithm in position fingerprinting to locate the target,the performance of the positioning algorithm was tested outdoors. |
