Improvement Of Channel Estimation And Equalization Module Based On OFDM Underwater Acoustic Communication System

Today's social science and techno logy is developing rapidly,and the vast ocean space has gradually attracted people's attent ion.The exploration o f the ocean requires the transmission of underwater informat ion,with underwater acoust ic co mmunications as the main representative techno logy.However,due to the co mplex marine environment and high acoust ic noise o f the underwater acoust ic channel,strong mult ipath effect,large delay spread and significant Doppler influence,the system implementation o f underwater acoust ic co mmunications techno logy has beco me an engineering difficult y.In the early stage of the laboratory,an underwater acoust ic communicat ions system based on OFDM was developed.However,when the relat ive mot ion o f the transceiver changes too fast,the sea surface fluctuates great ly,and the seawater medium is uneven,the co mmunications performance is great ly reduced.Based o n experimental data analysis,it is found that the main cause o f the above problem is that the Doppler effect in the underwater acoustic channel is aggravated,which destro ys the orthogonally between subcarriers in OFDM,resulting in system performance degradation.The channel est imat ion module and channel equalizat ion module in the underwater acoustic co mmunication s system are the main modules for processing the Doppler effect.Therefore,this study focuses on the improvement and optimization o f the above two modules.Based on the experimental data analysis,the main reason for the above problems is that the Doppler effect in the underwate r acoust ic channel is intensified,result ing in the sub-carriers in OFDM no longer orthogonal to each other,which leads to a significant increase in the system error rate.The channel est imat ion module and channel equalizat ion module in the underwater aco ust ic co mmunications system are the main modules for processing the Doppler effect.Therefore,this study focuses on the improvement and opt imizat ion of the above two modules.By analyzing the specific influence o f the Doppler effect on the channel est imat ion and equalizat ion module,the improvement o f the channel estimat io n module is: introducing the Doppler depth factor;opt imizing the channel est imat ion algorithm based on co mpressed sensing,adding in the construct ion o f the sensing matrix The t ypical set of Doppler factors is given and the construct ion rules are given.A new regularization parameter select ion method is proposed.The improvement of the channel equalizat ion module is as fo llows: the minimum mean square error equalizat ion based on the diago nal loading factor is proposed;the index cyclic assignment of the regularizat ion parameter and the diagonal loading factor is proposed.The improved system receiver through the above two modules is changed fro m an ICI-ignorant receiver to an ICI-aware receiver.Finally,the above improvements are simulated in the simulated underwater acoustic mult ipath t ime-varying channel,which proves that the improved ICI-aware receiver can achieve high-speed reliable communicat ions with severe Doppler effect.Although the ICI-aware receiver has a better ant i-Doppler effect,the receiver complexit y is relat ively high when the Doppler effect is small.Therefore,based on the turbo standard,the receiver is improved to an adaptive iterative receiver,which automat ically ba lances the relationship between system performance and co mplexit y.The specific improvements include: introducing a priori informat ion into the channel estimat ion module to obtain addit ional pilots;changing the MMSE equalization based on a priori informat ion in the channel equalizat ion module;and obtaining a priori information and a probabilit y conversion manner fro m the decoding module.Simulation o f the underwater acoust ic mult ipath t ime-varying channel proves that the adapt ive iterat ive receiver can reduce the system co mplexit y as much as possible while ensuring the performance of the communicat ions system.Based on the above improvements,this paper uses the experimental data o f the Qiandao Lake '17 Lake test in the laboratory to verify the ICI-aware receiver and adaptive iterative receiver.The results show that the improved two receivers meet the expected requirements.The experimental data demodulat ion error rate decreases,and the system performance is improved.