Research On Direct Spread Spectrum MSK IF All - Digital Receiver And Its Algorithm

DS-SS MSK communication system combines the technology of spread spectrum communication with Minimum shift keying modulation, has the advantages of low capture probability, strong anti-interference ability, constant envelope, high spectrum efficiency, energy concentration, insensitivity to nonlinear distortion. So that it is widely used in tactical data link, civil aviation air-ground data link, guided missile command transmission, broadband radar data transmission and so on. Accordingly, it makes sense to research on the intermediate frequency digital receiver and algorithms of DS-SS MSK.The thesis introduces the basic theory of direct sequence spread spectrum communication and discusses the serial/parallel generation methods of DS-SS MSK firstly. The performances of match-filter-based algorithm and FFT-based algorithm in frequency domain for pseudo-noise code acquisition are analyzed. In consideration of the effect of Doppler shift to the performance of pseudo-noise code acquisition, the ML-FFT algorithm based on maximum likelihood theory and FFT is proposed to implemente the acquisition and compensation of Doppler shift before pseudo-noise code acquisition. According to the characteristics of DS-SS MSK signal, a new method for receiving is proposed, the DS-SS MSK signal is structured into an approximate expansion of DS-BPSK signal so that the mature acquisition algorithms of DS-SS BPSK can be used for DS-SS MSK. In consideration of the limited acquisition range of ML-FFT, the algorithm is unavailable in high-dynamic environment. Besides, the time consuming and implementation complexity are high if the acquisitions of pseudo-noise code and Doppler shift are implemented respectively. Aiming at the problem, the two-dimensional acquisition algorithm based on the Doppler compensation and FFT is proposed to acquire the pseudo-noise code and Doppler shift simultaneously.The algorithm shortens the acquisition time and expands the acquisition range of Doppler shift greatly, can adapt to high-dynamic environment preferably. The early-late delay locked loop is designed for code tracking and the loop’s parameters are determined via simulation. Costas loop is designed for carrier phase tracking and the feasibility is verified via Matlab simulations. In the end, the coherent demodulation algorithm is adopted to demodulate the signal after despreading, and the analysis of error performance is given.