Research On The Least-Symbol-Error-Rate Adaptive Turbo Equalization Algorithm In Underwater Acoustic FTN Communications

With the continuous advancement of the construction of a maritime power,the application range of underwater acoustic communication has also been expanded.Traditional low-rate underwater acoustic communication technology has been difficult to meet the needs of realtime processing of information,and realization of high-speed underwater acoustic communication transmission technology has become very important.Faster Than Nyquist(FTN)technology can break the Nyquist criterion and enable the system to obtain a higher transmission rate,but it will introduce additional inter-symbol interference(ISI).In addition,due to the strong time-varying multipath effect of the underwater acoustic channel,there will be more serious ISI at the receiving end.Equalization technology can effectively eliminate ISI,among them,the most commonly uesd equalization technology in underwater acoustic communication is Turbo equalization,which can usually be divided into Turbo equalization based on channel estimation and Turbo equalization based on direct adaptation.The latter does not require channel estimation,so the complexity is relatively low and it is not sensitive to timevarying channels.Most of the equalization algorithms of traditional receivers adopt the Minimum-MeanSquare-Error(MMSE)criterion,and use the mean square error as the performance measurement index.However,the symbol-error-rate(SER)is the direct measurement index of the communication system performance.A lot of simulation and theoretical analysis show that even if the communication system reaches the MMSE,it may not reach the Least-SymbolError-rate(LSER).However,the equalization algorithm designed based on the LSER criterion can better improve the performance of the receiver.This paper studies and analyzes the adaptive Turbo equalization algorithm based on the LSER criterion,hoping to effectively eliminate the serious ISI in the underwater acoustic FTN system.The main work is summarized as follows: First,the underwater acoustic communication technology,FTN technology and underwater acoustic channel equalization technology are reviewed,and the characteristics of the underwater acoustic channel and the FTN transmission theory are analyzed.Secondly,the traditional equalization algorithm is introduced,and the derivation of the adaptive LSER equalization algorithm under the linear structure and the decision feedback structure is introduced.On this basis,the LSER equalization algorithm is combined with Turbo equalization,and the a priori soft symbol and the posterior soft symbol are respectively applied to the LSER adaptive Turbo equalization algorithm.Through simulation analysis under Rayleigh fading multipath channel,the results show that these two algorithms have good bit error rate performance.Taking into account the higher randomness and complexity of the actual underwater acoustic communication,this paper adopts the space diversity receiving technology in Chapter 4,and applies the inner layer iteration of the posterior soft symbol to the LSER adaptive Turbo equalization algorithm.Through the simulation analysis under the Rayleigh fading multipath channel and the processing of the underwater acoustic experimental data of Qiandao Lake,the results show that the algorithm can carry out reliable communication in the underwater acoustic FTN system with fast time-varying and strong multipath interference,and it has a fixed practical value.