Joint Multi-branch Combining And Turbo Equlization For Underwater Acoustic Cooperative Communication

Since the beginning of the new century,with the development of society and the increase of people’s demand,the marine economy has been developed unprecedentedly,and the underwater acoustic communication has also attracted the attention of all walks of life.However,the underwater acoustic channel has its inherent disadvantages: large propagation loss,complex environment,strong multipath effect,etc.,which seriously restrict the development of underwater acoustic communication.In order to improve the reliability of the underwater acoustic communication link,this thesis makes an in-depth study on the underwater acoustic cooperative communication.On this basis,Turbo Equalization Technology and multiple access interference(MAI)elimination technology are combined.The main research results of this thesis are as follows:Firstly,the characteristics of underwater acoustic channel are investigated deeply,which is the premise of underwater acoustic communication research.BELLHOP model is used to generate impulse response of underwater acoustic channel in simulation.The multipath effect in underwater acoustic channel is serious,which leads to serious inter-symbol interference(ISI),in order to solve this problem,we introduce adaptive soft decision feedback Turbo Equalization(SDF-TE),which solves error propagation problem in traditional decision feedback equalization(DFE)caused by hard decision,making full use of the useful information after decoding and being able to adapt to different channels.The principle of adaptive SDF-TE is introduced in detail in this thesis,and the law of exit information change in the iterative process is explained.The simulation results show its good performance on eliminating(ISI).Then,the signal to noise ratio(SNR)of the receiver is far lower than the expected value because of the large propagation loss of the underwater acoustic channel.In order to solve this problem,the cooperative diversity technology is applied to the underwater acoustic communication to form the underwater acoustic cooperative communication system.The traditional time-sharing cooperation mode needs two time slots,and the end-to-end transmission delay is large.In order to overcome this problem,the decoding distributed carrier frequency forwarding mode is considered,that is,the source node sends a single frequency signal,the relay node directly performs frequency conversion forwarding,and the destination node uses band-pass filters to distinguish the signals of different branches.Since cooperative diversity is usually only suitable for flat fading channels,that is to say,multi-branch combination can only effectively improve the SNR of the receiver under the mild ISI channel,and it is not suitable for the frequency selective fading channel such as underwater acoustic channel.Therefore,the equalization technology is introduced into underwater acoustic cooperative communication to overcome ISI.Because the separation structure of combining and equalization cannot make the processed signal performance reach the optimal level,we combine the multi-branch combining and equalization,that is,the combination of multi-branch combining and adaptive SDF-TE,and propose a joint multi-branch combining and Turbo Equalization(J-MBC-TE)algorithm.Multi-branch combining and equalization can be carried out synchronously,and soft information can be exchanged iteratively between ASDFE and decoder.ASDFE reconstructs ISI by using soft information fed back by decoder,and then subtracts ISI from received signal,so as to eliminate residual ISI and improve the correctness of final decoding.Finally,in view of the MAI problem in the multi-user underwater acoustic cooperative communication,on the basis of the adaptive J-MBC-TE,a feedback filter is added to feed-back other user information,and the idea of interleaver-division multiple access(IDMA)uses different interleavers to distinguish user information is used for reference,and proposes an adaptive joint multi-branch combining and iterative multi-user(J-MBC-IMUD)algorithm.In an ASDFE,multi-branch combining,equalization and MAI elimination can be carried out at the same time,and soft information can be exchanged iteratively between the ASDFE and the decoder.ASDFE uses the soft feedback information of the current user corresponding decoder and the soft feedback information of other user corresponding decoder to reconstruct the ISI and MAI respectively.