Research On Synchronization Algorithm Of Spread Spectrum Code For LEO Satellite Receiver

With the development of satellite communication technology,low earth orbit(LEO)satellite system has been paid more and more attention.Compared with geostationary orbit(GEO)satellites,LEO satellite system has very good development prospects in national economy,national defense construction and personal communication because of its low orbit,short transmission delay,low path loss and strong anti-destructive ability.While the CDMA technology has strong anti-noise performance,flexible bandwidth allocation and strong confidentiality advantages,so is particularly suitable for multiple access,military communications and location search applications.Therefore,the receiver of multi-user LEO system based on CDMA technology is studied in this thesis.Signal capture and tracking analysis is critical to designing an achievable LEO satellite CDMA receiver.The following two considerations must be taken into account for CDMA transmission over LEO satellite channe ls: signal attenuation on LEO satellite and relatively large Doppler Shift.Firstly,from system design perspectives,designs the downlink communication system based on LEO satellite spread spectrum communication.Then,based on the transmission characteri stics of the satellite mobile channel,the formation of the weak signal and characteristics of Doppler Effect of receiver are analyzed.Doppler Effects are divided into carrier Doppler and code doppler,research these three parts for the pseudo-code synchronization system respectively.After that,introduce the structure and signal model of CDMA communication system.On this basis,this dissertation generally divides the pseudo-code acquisition algorithm into three coping strategies: For the low SNR applications,to study several conventional accumulations capture algorithms which can effectively improve the signal to noise ratio of detection by the receiver.For effects of carrier Doppler,study algorithms of partial integration,including PMT-FFT algorithm and DBZP algorithm.For the code Doppler Effect and the low SNR application existing simultaneously research muti-loop accumulation algorithm,and propose the scheme for Doppler first derivative existing.Muti-loop accumulation algorithm can improve performance of the capture probability nearly 9 times.In addition,combing the resource consumption characteristics of capture of Hadamad sequences Golay and Hadamard sequences,they are selected as codes of the P-SCH and S-SCH channels,and the scheme of LEO satellites using primary synchronization and secondary synchronization channels to capture is proposed.Adaptive threshold of constant false alarm algorithm in this system can improve the performance of pseudo-code synchronization.What's more multi-dwell time decision and margin header check can further reduce false alarm probability of the synchronization system.Combined with simulation analysis of mathematical model and performance of PLL,and structural characteristics of co-frequency between P-CPICH and DPDCH channel,the tracking scheme of LEO satellite is proposed with P-CPICH channel,using non-coherent delay-locked loop.Finally,through Simulink to modele and simulate the system of design,the results show that the above pseudo-code synchronization scheme can basically solve the problem of great doppler and weak signal.