Robust Mobile Spread Spectrum Underwater Acoustic Communication

Underwater acoustic(UWA)channel in shallow water is an extremely complex channel which is referred to as a doubly spread channel with limited spectrum width.The mobile UWA communication channel is even more complex.Besides the factors of multipath spread and frequency selective fading,the influences of Doppler interference and time-varying channel should be considered in mobile UWA communication channel.Spread spectrum(SS)communication is a communication system with strong anti-interference ability,good concealment and multiple access capability.Spread spectrum communication has been applied widely in military communication and secure communication for a long time.However,the spread spectrum communication has a long code period and poor antiDoppler capability,and its performance degrades greatly in time-varying channels.Therefore,it is valuable to study the algorithms of spread spectrum system receiver in the Doppler-delay double spread underwater acoustic channel,which makes the spread spectrum system playing a good role in the mobile communication.In this paper,we will focus on the estimation of the Doppler-delay dual spread underwater acoustic channel and the receiver algorithm of SS communication.Firstly,we analyze the physical characteristics of shallow water UWA and the characteristics of mobile UWA communication channel in this paper.we estimate and compensate the delay-Doppler function of the underwater acoustic channel at the communication system receiver for Doppler interference and time-varying multi-path spread interference,firstly.Three methods of mobile UWA communication channel delay-Doppler function estimation is studied,the first method based on fractional Fourier transform,the second method based on narrowband signal interblur function and the third method based on DSSS signal structure are studied.Secondly,a more robust mobile spread spectrum UWA communication algorithm is studied in this paper,including direct-sequence spread spectrum(DSSS)communication,cyclic shift keying(CSK)spread spectrum communication,traditional M-ary spread spectrum communication,combination spread spectrum communication and M-ary cyclic shift keying spread spectrum communication(M-CSKSS)for the residual carrier phase fluctuation interference after Doppler compensation.The correlation of spread spectrum sequences used commonly is discussed,and the appropriate spread spectrum sequences are selected under different application backgrounds.Improved differential energy detector(DED)and double differential correlation detector(DDCD)are proposed for fast carrier phase fluctuation interference.For the generalized M-ary spread spectrum system,based on the M-ary energy detector(MED),the inconvenience caused by converting the residual carrier phase fluctuation interference into a real number is reduced.MED was improved then the frequency domain energy detector(FDED)algorithm for M-CSKSS is proposed;By using TRM to assist these receiver algorithms to achieve better results,the improved vector TRM algorithm is proposed combined with the vector signal processing algorithm.Finally,the above algorithms proposed are verified by experiment data in multiple waters in this paper.The main purpose of this paper is to provide reliable delay-Doppler estimation and compensation for spread spectrum communication and spread spectrum receiver algorithm.The stability and reliability of the proposed algorithms are verified by experiments on fixed point and mobile underwater acoustic communications.