Study On Pseudo-code Synchronization Of Direct Sequency Spread Spectrum Communication System

Spread spectrum (SS) technology is a frontier technology that develops quickly in thefiled of communication, which initially is used in military communication field. At thesame time, the SS technology is also widely applied to civil communication since it hassome advantages including strong anti-interference, good anti-fading and security andmultiple access communication can be realized. Direct sequence spread spectrum (DSSS)technology is used most widely, upon which the essence of SS technology can be mastered.The pseudo-code used as SS sequence in the SS system is a random sequence which issimilar to the statistic characteristics of white noise, the synchronization of which is criticalto realize SS communication system and the performance of the whole SS system isdirectly influenced by the merits of synchronous performance. The synchronization ofpseudo-code includes two parts which are capturing and tracking, rough synchronization isachieved in the capturing stage and fine synchronization is implemented in the trackingstage on the basis of capturing. The capturing and tracking algorithms forpseudo-codes are studied in this dissertation, and certain capturing algorithms areimproved.The main work of this dissertation is as follows: First, the common pseudo-code, forexample, m sequence, Gold code and M sequence, are studied, especially m sequence, thegeneration methods, frequency spectrum, self-correlation and cross-correlation of msequence are studied and simulated. Second, the common capturing algorithms forpseudo-code is researched, which includes serial capturing algorithm, parallel capturingalgorithm, series-parallel mixed algorithm and matching filter method. At last, the commontracking algorithm for pseudo-code is discussed including non-coherent delay lock trackingloop and non-coherent tracking loop with Ï„ swing.Capturing is still the key part in the research of pseudo-code research. The commoncapturing algorithm is time-consuming, the circuit structure of which is very complex andis very difficult to realize. Two improvements are made in order to achieve the goal ofcapturing pseudo-code. First, an auxiliary circuit is designed on the basis of serial capturingalgorithm and false alarm probability is decreased and capturing time is shortened. Second,a fast capturing method based on FFT using non-coherent algorithm is proposed, which isappropriate to work in the environment with a low signal to noise ratio (SNR) and the conclusion has been validated by simulation using Matlab. The capturing algorithm has animportant application value since the SS system often works in the environment with a lowSNR.The tracking is carried out on the basis of capturing, in the capturing stage, althoughthere is a difference of one code chip in the pseudo-code waveform of the transmitter andthe receiver, the correctness of demodulation and spread spectrum in the receiver are notguaranteed, the pseudo-codes in the receiving signal is locked by the local pseudo-codes inthe tracking stage, as a result, the phase difference is shortened between the transmitter andthe receiver and strict synchronization is realized. Two common incoherent tracking loopsfor pseudo-codes are introduced in detail, the accurate synchronization for pseudo-codes isachieved on the basis of successful capturing.