Research On Key Technologies For Reliable Acceptance Of Direct-sequence Spread Spectrum System

As one of the most important information transmission technologies in the information age,direct-sequence spread spectrum(DSSS)is widely used in military anti-interference communication and mobile communication systems due to its low interception rate,information hiding,multiple access communication and strong anti-interference ability.In recent years,with the rapid development of direct sequence spread spectrum technology in the civil and commercial fields,higher requirements have been placed on the reliable signal receiving technology of DSSS system.In DSSS communication,a carrier offset may happen due to the relative movement between the receiver and the transmitter,the unstable crystal oscillator and the transmission delay.Taking a high dynamic environment as an example,the high-speed movement of the aircraft often causes a Doppler frequency shift of hundreds of hertz.At this time,the reliability of DSSS system is mainly affected by the carrier synchronization module.At the same time,the wireless channel is a complex time-varying channel,so the spread spectrum signal will usually be transmitted from the transmitting end to the receiving end through different paths.This situation will eventually cause multipath fading and even deep fading.In severe cases,it may even interrupt communication.Therefore,how to overcome multipath fading has become an important problem in DSSS communication.Diversity and merging technologies are two of the main ways to get over multipath fading and deep fading.Based on the typical problems mentioned above,this paper mainly studies the carrier synchronization technology and diversity and combining technologies in the DSSS communication system.First,from the development background and research status of spread spectrum communication,the basic principle and key technologies of DSSS communication are briefly introduced.Secondly,combined with the synchronization requirements of DSSS,the carrier synchronization technology in the reliable receiving technology is studied and analysed.This paper makes theoretical research and simulation analysis on many traditional carrier synchronization algorithms in time domain and frequency domain.In view of the shortcomings of the traditional carrier synchronization algorithm,This paper proposes an improved interpolation synchronization algorithm.This improved algorithm makes full use of the information of the first and last spectral lines and solves the problem that the traditional frequency domain synchronization algorithm cannot work in the case of extreme frequency deviation.It also uses the natural logarithm of the spectral line value to replace the spectral line,which makes the estimation of frequency deviation more accurate.Finally,a new carrier synchronization scheme is obtained by combining piecewise PMF-FFT algorithm with the improved interpolation synchronization algorithm,which can optimize the structure and performance of the traditional algorithm simultaneously.In the part of diversity and merging technologies research,this paper first discusses the traditional combining methods such as maximal ratio combining,equal gain combining,as well as the typical hybrid combining.The analysis shows that the performance of the maximal ratio combining is the best but the complexity is very high,while the technology of equal gain combining is easy to realize,but the performance is poor under the condition of incoherent merging.In view of the contradiction between the complexity of the conventional diversity merging technology and the merging performance,a MMSE combining scheme with the forward threshold detection is proposed.In this scheme,the forward threshold is used to detect and filter the branch signals,which not only simplifies the complexity of the conventional MMSE merging algorithm,but also improves the merging performance under the interference condition.At the end of this paper,a high-performance DSSS system scheme design is given.The system uses DSSS technology,while combining multiple low-density parity-check coding(LDPC),cyclic code shift keying(CCSK)spread spectrum technology and time diversity to improve the anti-interference performance of the system and achieve broadband reliable information transmission in complex electromagnetic environments.The improved carrier synchronization scheme and diversity merging scheme are applied to DSSS system to form an improved signal receiving scheme.It is compared with the traditional receiving scheme by simulation and the feasibility of this study is verified from the perspective of the system.