Design And Realization Of DSSS-GMSK Modulated System

With great capabilities in its anti-interference performances,low probability of interception and high measurement accuracy,the spread-spectrum communication system(SSCS)is widely studied and applied.Meanwhile,Guassian-filtered Minimum Shift Keying(GMSK)modulation is a typical form of Continuous Phase Modulation(CPM),with characteristics of constant envelope,narrow bandwidth and rapid stop-band attenuation.Direct Sequence Spread Spectrum(DSSS)system based on GMSK modulation is equipped with the combined strength of SSCS and GMSK,resulting in its great research significances for both military and civil uses.This paper aims to research on the DSSS transmitting and receiving end design,including following works:Firstly,based on basic theories of DSSS system,the main performance indexes are given and several common spreading sequences are analyzed.Then we explain the GMSK modulation and demodulation principles in detail,and carry out the performances comparison of GMSK and MSK signals.After analyzing several GMSK modulation and demodulation methods,an advanced waveform storage is proposed,which is an effective solution to minimize the amplitude error of the modulated signals and to balance the quadrature carrier phase.Secondly,the design schemes of DSSS system transmitting and receiving end are studied thoroughly.In the conventional receiving end design,dispreading is conducted after demodulation,which is easy to achieve.Nevertheless,underutilized spreading gain results in poor system performances under a low signal noise ratio(SNR)environment.To counter this problem,a method that the signals at the receiving end are simultaneously demodulated and dispreaded.An advanced matched filter is adopted to accomplish pseudo-code acquisition,and the related operations between the local spread-spectrum signals and each spread-spectrum receiving unit are used to restore the original data.Stimulation experiments are also carried out to certify its great performance in a low SNR environment.Finally,On the basis of DSSS theory,this paper presents the DSSS hardware system,including platform design,module setting and system testing.