Research On The Technology Of Underwater Acoustic Communication Based On OFDM-MFSK

Underwater acoustic communication(UWAC)which is the only way to complete the long distance information interaction is a key technology to ensure the information transmission for underwater platform application.Underwater acoustic channel is a temporal-spatial-frequency variation wireless channel.Due to the severe multipath propagation,Doppler spread and complex environmental noise,the high quality acoustic communication becomes extramly difficulties.Moreover,underwater platform work scene changing and random motion are become new challenge for UWAC.Therefore,it has a great practical significance and broad application prospect to study robust high speed UWAC in complex environment.This paper focus on the key technology research of UWAC.Aiming at the engineering problems in the practical application.Using multi-carrier OFDM-MFSK as the communication system,high precision of channel estimation techniques,time-frequency combined processing technology,broadband Doppler estimation and compensation technology and broadband efficiency low-distortion transmission system design method are researched in this paper.As a kind of multi-carrier technology,OFDM-MFSK has the advantages of good anti multipath,high communication rate and simple implementation.Using non-coherent method to detecte the MFSKsignal,the sensitivity of Doppler and phase noise is better than OFDM system.From theory and simulation aspect,the effects of timing synchronization error,cyclic prefix length and carrier frequency offset on the performance of the system are studied.Spectral efficiency of the system is analyzed,and a new scheme of OFDM-MFSK-DPSK with high spectral efficiency is proposed for deep sea vertical channel.The application effect of the probabilistic PAPR technique in OFDM-MFSK system is studied.The OFDM-MFSK modulation mode is flexible,which can balance the communication rate and robust performance.It is suitable for the complex environment of underwater acoustic communication.Shallow sea horizontal channel with severe multipath propagation and frequency selective fading which caused the submarine communication system with Inter-SymbolInterference(ISI)and the signal to noise ratio of Partial subcarrier loss.In order to address these problem,a joint time-frequency domain processing technology is proposed.firstly,The compressed sensing(CS)technology is adopted to estimate the high precision parameter of sparse underwater acoustic channel.OMP algorithm has poor estimation results in the noiseenvironment and the numerical instability of the matrix inversion when solving the least square(LS)algorithm.To address these issues,A MMP-DCD algorithm is proposed.this algorithm uses tree search strategy and minimum residuals principle to get the best support,and DCD algorithm is used to avoid the numerical instability.Then,the channel shortening technology is used to achieve channel equalization,which can effectively compress the channel length,restrain the ISI and improve the signal to noise ratio(SNR)of the system.Finally,the frequency diversity technique is used to avoid the energy loss caused by the frequency zero.In this paper,the principle of passive time reversal mirror,virtual time reversal mirror and the frequency domain MMSE channel shortening technique based on single array elements are compared and analyzed.Diversity signal combining method is also studied,and a new method based on group SNR confidence combining method is proposed.The simulation and lake experiment results show that the proposed joint processing technology can effectively suppress the multipath interference and improve the system performance.The random motion of the submarine brings the Doppler effect to the OFDM-MFSK communication system,which destroys the orthogonality between the subcarrier,causes InterCarrier-Interference(ICI),and seriously affects the performance of the system.Due to the ultra wideband characteristics of underwater acoustic channel,the Doppler frequency offset of different subcarriers are inconsistent which makes the Doppler estimation difficult.In this paper,the influence of sampling frequency offset and Doppler frequency spread on the performance of the system is analys.The covariance spectrum technique and the conventional Doppler compensation technique are discussed.The conventional Doppler estimation and compensation algorithm has low accuracy and poor stability in the time-varying multipath underwater acoustic channel environment.To address these problem,A modified two stage Doppler estimation and compensation algorithm based on subcarrier energy is proposed.Firstly,the pulse pair is adopted to estimate the coarse Doppler and complete preliminary compensation.Then,the subcarrier energy of each symbol is used to high accuracy estimate the residual Doppler and complete the final compensation.Based on the sparse characteristics of OFDM-MFSK symbol spectrum,a known frequency subcarrier with frequency-domain oversampling block-by-block Doppler estimation and compensation algorithm is proposed.The Doppler estimation and compensation is completed through frequency-domain oversampling processing once of each symbol with block by block mode,which can effectively reduce the computational complexity of traditional method.The simulation,pool and lake experiment results show that the two proposed algorithm has good Dopplerestimation accuracy and stability performance in the time-varying multipath UWACA environment.In order to address the worse performance of broadband matching and larger distortion of emission waveform in communication transmitter system with multi-resonant broadband wide-directivity transducer.A new transmission system design method which combined with pre-equalizer and real frequency technology is proposed.This proposed method could expand the system bandwidth,impove the work efficiency and reduce the signal distortion.In this paper,the design problem of communication transmmition system is analyzed,and real frequency direct technique(RFDT)and simplified real frequency technique(SRFT)are researched.The core idea of pre-equalization design is given in detail,and the realization of equalizer is also studied.The simulation and pool experiment results show that the proposed method can effectively ensure the work efficiency and improve the waveform distortion.As one of the computer aided technology,this technology avoid the establishment and analysis of the equivalent model parameters of the transducer,and needless to select the topology of the matching network.therefore,the proposed menthed has a good application prospects.