The Research Of Dispreading And Synchronization Technology In DS System With High Spreading Gain

With the development of communication technology, the future information warfare isbecoming increasingly fierce. Under the pressure of high-tech war, the spread spectrumcommunication technology which is depended on its strong anti-interference ability, lowinterception rate, information hiding and so on, has been the research hot spot in military field inrecent years. Because of the short and uncertain transmitting time, the burst communication canavoid the interference effectively. With the burst communication and direct spread spectrumtechnology combined, the anti-jamming performance can get better improved. Synchrony whichis a very important issue in communication system, is the precondition and foundation ofinformation transmission, the key to determine the performance of receiver.This paper which is under the background of wireless communication system, mainlyresearches and analyzes the theoretical and practical issues encountered during theresearch of DS system receiver with higher gain, and the emphasis is pseudo-random codesynchronization and carrier synchronization. The article firstly introduces the overallframework of system and signal of transmitter, then according to specific indicators of systemand through analyzing different options, chooses matched filter method to realizepseudo-random code synchronization which can shorten the time of code capture. Meanwhiledue to the effect of Doppler, there will be frequency difference between the local carrier ofreceiver and the signal received. The paper uses the method of pilot sequence to sweep, andfrequency difference can be quickly reduced to less than 5kHz. In searching for relatedpeak,due to traditional method based on fixed threshold can not get good performance andadapt to the practical environment, we select the adaptive threshold algorithm. Whencapture is completed and the phase difference is in the 1/2 chip, it will start delay-locked loopwhich automatically adjusts the local PN code according to error, make the local PN code alwayskeeping accurate synchronization with the PN code of received signal. The signals which havebeen synchronal go into dispreading unit to dispread the sign bits.At carrier synchronization, unlike frequency estimation algorithms which are based onclosed-loop such as AFC whose frequency range is±1kHz, this paper researches a fast carriersynchronization algorithm based on open-loop plus a combination of closed-loop. Firstlywe use a FFT open-loop system to estimate frequency difference quickly. Meanwhile whetherthe frequency is positive or not can be determined by the feature that the spectrum ofcomplex signal is unilateral. The frequency difference can be reduced to the band of phase-locked loop through FFT only one time and the frequency range is more than AFC.Then the remaining deviation can be adjusted by the Costas-loop. The method withsweeping, FFT and combined with Costas-loop saves more time than the direct use ofphase-locked loop, AFC and so on, also the range of frequency offset can be up to±30kHz. In addition, the pilot sequence required is not too much, and implement is simple.This is a major improvement on the algorithm. At last, we built a complete systemsimulation based on MATLAB platform, from the generation of signal model to output oflast synchronization, simulate the algorithm and analyze the results. And it is verified onthe hardware platform. Simulation results show that the selection and implementation ofsynchronization algorithm in the paper reach the target. The minimum SNR supported isabout -19dB. Greater than this value it can achieve fast capture and carrier synchronization.So it has a strong practical value.