Bidirectional Soft Information Iterative Reception Algorithm In Underwater Acoustic Faster Than Nyquist System

In recent years,military and civilian marine information engineering has been rapidly developed,and the widespread application requirements of underwater communication urgently require the support of high-rate and reliable communication technologies,especially in the field where information needs to be processed in real time.At present,underwater acoustic communication technology is the main technical means for underwater wireless communication,and the underwater acoustic channel causes aliasing of received signals due to its own severe time-varying,multipath,Doppler and other effects,resulting in serious inter symbol interference(ISI);faster than Nyquist(FTN)technology can break the limits of the Nyquist criterion,allowing the system to achieve higher transmission rates,but at the expense of introducing its own ISI.How to eliminate the severe ISI with the lower complexity algorithm at the receiving end of the underwater acoustic FTN system becomes the main problem that restricts the application of FTN transmission technology to underwater acoustic communication.Therefore,this paper analyzes and studies the receiver-side equalization technology of the underwater acoustic FTN system.The main work is summarized as follows:1.In this paper,the characteristics of underwater acoustic channel are analyzed in detail,and the FTN transmission theory and Mazo boundary are deduced and simulated to verify the feasibility of FTN transmission.The reason for the generation of ISI in underwater acoustic FTN transmission system is grasped in essence.2.The equivalent model of single-carrier underwater acoustic FTN transmission system is derived.Based on this model,the applicability of traditional equalization algorithm in underwater acoustic FTN transmission system is studied and analyzed,including linear equalization algorithm such as minimum mean square error(MMSE)equalization algorithm,decision feedback equalization algorithm and Turbo equalization algorithm.3.Aiming at the characteristics of fast change of underwater acoustic channel,the bidirectional frame structure is designed.Based on the channel estimation algorithm and the soft information interaction principle of MMSE-Turbo structure,a bidirectional time domain equalization suitable for FTN transmission system with low complexity is proposed.The Turbo soft information iterative receiving algorithm is simulated in Rayleigh fading multipath channel and analog time-varying underwater acoustic channel,which verifies that the algorithm has good bit error rate performance and low complexity.4.The above algorithm is applied to the underwater acoustic FTN receiver.The spatial diversity technique is introduced and its time-reversed merging algorithm is derived.A bidirectional time domain equalization Turbo soft information iterative diversity receiving algorithm is proposed.In order to speed up the convergence,the time domain equalization is extended to time-frequency two-dimensional equalization,and a bidirectional time timefrequency joint equalization Turbo soft information iterative diversity receiving algorithm is proposed.5.We conducted experiments on different acceleration factors,different distances,different seasons,and different movement rates in Qiandao Lake,Zhejiang.The experimental results show that the proposed algorithm has good BER performance in different application scenarios of underwater acoustic FTN transmission system,and has certain practical application value.