Research On Direct-expansion MSK Signal Full Digital Despreading Receiver

Spread spectrum technology has a strong anti-intercept,multi-user random location ability and anti-jamming performance.Minimum shift keying(MSK)modulation technology has constant envelope,quick side-lobe attenuation,high spectrum efficiency and is not sensitive to nonlinear distortion.DSSS MSK communication system combines the advantages of direct sequence spread spectrum(DSSS)technology and MSK modulation technology,and has been widely applied in a lot of military and civil fields.Therefore,this thesis carrys out research on the intermediate frequency digital despreading receiver and algorithms of DSSS MSK signal.This paper introduces parallel and serial generation methods of DSSS MSK signal.In the serial method,a design method of the conversion filter based on convex optimization algorithm is proposed to transform DSSS BPSK signal into approximate DSSS MSK signal.In the work of code acquisition,in view of the problem that the Doppler offset will affect correlation peak,a code acquisition algorithm based on delay correlation is proposed to eliminate the influence of Doppler offset.In consideration that the delay correlation algorithm cannot capture code phase error and Doppler frequency offset at the same time,a code,Doppler joint acquisition method based on Doppler compensation is put forward.This algorithm improves the Doppler offset estimation range.But the estimation accuracy and estimation range of this algorithm is limited by computing complexity.In order to find an acquisition algorithm that gives consideration to estimation accuracy,estimation range and computing complexity at the same time,the paper proposes a joint acquisition algorithm based on compressed sensing(CS)by bringing the theory of CS into code acquisition for the first time.Simulation results illustrated that the proposed CS algorithm can capture code phase error and Doppler frequency offset at the same time,and offers optimal performance while maintaining low complexity.For DSSS MSK signal tracking,the paper uses full digital early-delay locked loop and Costas loop for code tracking and carrier phase tracking respectively,and determines the optimal loop parameters via Matlab simulations.Finally,by comparing the error performance of the DSSS MSK despreading receiver which is designed in this paper and the conventional receiver,it is concluded that the receiver designed in this paper can avoid the SNR loss,and is more suitable for low SNR environment.