Research On Channel Estimation And Iterative Equalization In High Velocity OFDM Underwater Acousitc Communication System

The underwater acoustic communication system to support high-speed mobile terminal research,is one of the hot research.The response of the underwater acoustic communication channel in the mobile state changes with time and frequency,and it is necessary to overcome the influence of the channel by using the appropriate communication system and the processing algorithm.Orthogonal frequency division multiplexing(OFDM)can solve the inter-symbol interference(ISI)caused by underwater multipath,but the estimation and equalization of the channel is a key part of the receiver signal processing.In this paper,we mainly study the iterative channel estimation and equalization techniques based on pilot data.The channel estimation module uses the pilot and the soft information generated in the iterative process to track the channel response.The equalizer adopts the iterative equalization method to overcome the influence of channel time variation.The main contents of this paper are as follows:Firstly,the transmission characteristics of underwater acoustic waves are studied,and the simulation analysis of underwater acoustic channels is realized based on BELLHOP.Based on the complex characteristics of the actual underwater acoustic channel,this paper establishes the time-varying simulation model of underwater acoustic channel based on the statistical model of time-dependent Rayleigh fading on the basis of BELLHOP's physical channel model,which provides a new method for channel estimation and equalization Simulation background.Secondly,according to the time-varying characteristics of the underwater acoustic channel,the MMSE iterative channel equalization scheme based on soft information is studied.The generation of symbolic soft information in the iterative process provides a measure of reliability for the detection of symbols,which can be used as a priori information of the next iteration,More conducive to the elimination of channel-induced ISI and sub-carrier interference ICI.In order to further reduce the complexity of the equalization,this paper makes full use of the band-like characteristics of the time-varying channel frequency domain matrix.Simulation results and algorithm analysis show that the preprocessing of the channel at the receiving end not only guarantees the system performance,but also greatly reduces the equalization complexity.Thirdly,in order to obtain the channel state information of the time-varying underwater acoustic channel,the article fits the parameters of the residual time-varying channel by the base expansion model(BEM)after the Doppler spread compensation.The BEM channel estimation algorithm based on least squares(LS)and minimum mean square error(MMSE)criterion is realized respectively.In order to further improve the system performance,combined with the previous iterative equilibrium thought,the iterative equalization of the soft symbol information as auxiliary pilot into the estimation module to achieve the iterative estimation of the channel.The simulation results show that the channel estimation algorithm based on BEM can realize the estimation of time-varying underwater acoustic channel,and the system performance is further improved after the estimation module is embedded in the iterative equilibrium.Finally,according to the background of the simulation of underwater acoustic communication,a experimental platform of underwater acoustic communication is built with LabView and Matlab.Some underwater experiments are carried out based on this platform.The platform equipped with a complete set of OFDM communication system,you can achieve the basic communication functions.The actual performance of the communication system and the related algorithm in underwater communication can be verified by placing the transceiver in the real underwater environment directly.