Research On Demodulation Technology Of Direct Sequence Spread Spectrum

With combination of MSK-Minimum shift keying modulation and spread spectrum communication, the system has the advantages of both MSK signal and spread spectrum technology including low capture probability, good security and strong anti-interference ability, so that it is widely used in tactical data link, guided missile command transmission, civil aviation air-ground data link, satellite communication and so on. Accordingly, it makes sense to research on spread spectrum MSK modulation and demodulation.The characteristics of the pseudo-random sequences is studied and gold sequence with good self-correlation and co-correlation characteristic is selected as spreading code in this paper. Direct sequence spread spectrum serial MSK modulation is proposed. Doing the derivations of DS-SS MSK signal linear expression and design the DS-SS MSK modulation, despreading and demodulation scheme, then verify the feasibility and performance of the designed modem system via Matlab simulation. Acquisition and tracking of DS-MSK in high-dynamic environment are studied in detail. Some enhancement on base of the conventional match-filter-based code acquisition is present in this paper, two-stage acquisition is used to achieve a balance between acquisition time and hardware complexity, double differentially coherent pseudo-noise acquisition is used to improve the performance of acquisition. In consideration of the Doppler shift in high-dynamic environment, frequency offset estimation based on FFT algorithm is mentioned, as its bad performance for low SNR case, rapid DS-SS signal acquisition scheme based on Doppler frequency compensating and frequency domain matching is proposed to improve the performance of acquisition under the circumstances with low SNR. The early-late delay locked loop is designed for code tracking and the impacts of the loop’s parameters ζ and BL are discussed in detail. In the end, Costas loop structure with detailed phase-detector characteristic derivation is designed for carrier phase tracking, at the same time, it is proved to be effective in carrier phase tracking via Matlab simulation.