Research On Mobile Underwater Acoustic Spread Spectrum Communication Technology

Underwater acoustic communication technology is mainly used for underwater information interaction.In mobile underwater acoustic communication,multi-path interference and Doppler effect are the main factors affecting communication.In order to overcome the above problems and achieve reliable underwater acoustic communication,direct sequence spread spectrum technology is adopted in this paper and studied on the basis of it.The direct sequence spread spectrum system has the ability to resist multipath interference and noise,and can reliably transmit underwater information under negative signal-to-noise ratio.In order to solve the problem of Doppler frequency offset in mobile underwater acoustic communication,the double-differentially coded technology is studied and the upper limit of the Doppler performance of the technology is analyzed.The reliability of the communication system is improved by coding and block interweave technology based on prime algorithm.In mobile underwater acoustic communication system,if the relative speed is larger,the double-differentially coded technology can't offset the compression or expansion effect on the signal caused by the Doppler frequency shift in the time domain,which will cause inaccurate synchronization and demodulated signal error.Considering the excellent correlation performance and anti-Doppler ability of Chirp signal,Chirp signal is used as the synchronization head to study Doppler factor estimation and frame synchronization algorithm,mainly studies the algorithm based on positive and negative frequency sweep HFM signal and compared through simulation the algorithm performance is optimal.Using Doppler factor estimation and frame synchronization algorithm can accurately estimate the larger relative speed and the Doppler compensation.However,the absolute constant speed cannot be maintained in the actual movement,the residual Doppler frequency offset after the former compensation can be offset by the double-differentially coded technology,without using other compensation methods,which reduces the complexity of the system.Aiming at the multipath problem of underwater acoustic channel,the Virtual Time Reversal Mirror technology is studied.It is used to adaptively match the channel to achieve channel focusing,increase the peak energy of the main path and reduce the sidelobe energy to suppress inter-symbol interference.The Virtual Time Reversal Mirror technology needs accurate channel estimation to achieve better equalization effect,so the channel estimation algorithm based on compressed sensing theory of Matching Pursuit and Orthogonal Matching Pursuit is studied.The simulation results show that the spread spectrum communication system which uses the Orthogonal Matching Pursuit algorithm to estimate the channel for Virtual Time Reversal Mirror can improve the reliability of information transmission effectively.Finally,the performance of the double-differentially coded spread spectrum communication system has carried on the experimental verification,and the mobile underwater acoustic communication experiment has been carried out in the channel pool.The results show that when there is a relative motion between the sender and the receiver,Doppler estimation,coarse compensation and frame synchronization are performed by using Chirp signal as the synchronization head,and the performance of the communication system with fine compensation by the double-differentially coded technology performs well.At the same time,the performance of the communication system is improved by the Virtual Time Reversal Mirror technology,which further verifies the simulation conclusion.