Research On Underwater Conmmunications Techniques Based On Direct-Sequence Spread Sprectrum Techniques

With the sustaining development of Underwater Acoustic (UWA) communications' application domain, the research of UWA communications techniques has become today's hotspot. The signal's fading which is caused by underwater acoustic channel multipath propagation is the main factors influencing the performance of underwater acoustic communications systems. Because of spread spectrum techniques' ability of resisting multipath interference and multi-access,it is the key techniques which is used to transmit datas in underwater wireless acoustic communications networks.This thesis investigates the feasibility and performance of using Direct-Sequence Spread Spectrum modulation for datas transmission in underwater acoustic communications system.Firstly, the characteristics of underwater acoustic channel was introduced,meanwhile,two simulated shallow sea underwater acoustic channel were given.Then,spread spectrum techniques and π/4 quarto phase-shift keying(π/4QPSK) were discussed and according to the properties of underwater acoustic channels, the direct-sequence π/4 quarto phase-shift keying (DS-π/4QPSK) with quadrature spreading modulation was present.After that the techniques of channel coding, synchronization and RAKE receiver were investigated,the method using a Willard sequence for synchronization of system and acoustic channel multipath time-delay estimate was found.At last,the underwater acoustic spread spectrum communications system was designed based on the DS-π/4 QPSK modulation and RAKE receiver,and the communications system was simulated in two short distance underwater acoustic channels. The simulation results predicted that the communications system could satisfy the requirements of the information transmission in the short distance underwater acoustic channel.