Research On Time Domain Equalization Of Single Carrier Underwater Acoustic Communication In Shallow Water Environment

In shallow water environment,underwater acoustic(UWA)communication is faced with severe challenges from the UWA acoustic channel such as severe multipath spread and Doppler,rapidly time-varying both in time and frequency domain,frequency-dependent transmission loss,strong interference from noise,and limited communication bandwidth.This paper firstly introduces the basic time-domain equalization theory of single carrier,then deduces the passive time reversal(PTR)mirror,the decision feedback equalizer(DFE),the time-domain channel estimation and followed by computer simulations.On this basis,this paper carries out several relevant researches on single carrier UWA communication in time-varying channel,short-range high-rate UWA communication,multiuser UWA communication and under-ice acoustic communication.All of these algorithms have been tested using multiple lake,sea or uder-ice experimental data.Firstly,it begins with the research on robust single carrier UWA communication in the time-varying channel.To time-varying Doppler effect and multipaths,this paper proposes such a solution as follows: At first,according to the estimated result of the delay-doppler spread function,the received signal will be converted by interpolation to a new discrete signal;Then,the new siganl is divided into shorter isometric fragments each of whose time domain length is less than the channel coherence time.So the characteristic of channel(including the multipath and the Doppler)is approximately time-invariant within so short period.The current fragment's Doppler shift could be compensated by that of the previous one.And the current estimated channel characteristic could be updated on the basis of the previous one.At last,a single-channel DFE with embedded phase lock loop(PLL)is utilitised to eliminate the residual intersymbol interference(ISI)and Doppler.The proposed method does not use so many training symbols and it can greatly improve the efficiency of the system.It has been demonstrated by the two sea experiments in Xiaochangshan island and Bayuquan port that the method is robust.Secondly,research on short-range high-rate UWA communication based on vector hydrophone array.This paper propose a communication scheme that multiband transmission combined with bidirectional PTR-DFE on the basis of vector signal processing:(1)In high-rate UWA communication the energy of each sympol is so small that it is seriously disturbed by the channel noise,the linear combination of signal from velocity channel and pressure channel using hydrophones could improve the signal-noise ratio(SNR)of received signal.Combined utilization of multiple hydrophones could acheive spatial processing gain by PTR.(2)Multiple bands transmission divides the relatively wide band into sevaral sub-bands and within each subband the informatin is transmitted in parallel.Problems that the sampling rate is high and the weak ability of anti multipath spread will be solved effectively.(3)The bidirectional PTR-DFE is proposed to solve the problem of error propagation problem.It combines the equalized result of forward equalizer and backward equalizer to decrease the probability of false decision.So,the proposed short-range high-rate UWA communication scheme is composed of vector hydrophone array,multipleband transmission and bidirectional DFE.The scheme was tested using sea experimental data,achieving a data rate of 48kbit/s and bandwidth efficiency of 4.29 bits/s/Hz,with very low bit error rate(BER).Moreover,research on multiple users UWA communication.This paper studies space-division multiple access(SDMA)based on single carrier,computing signal to interference and noise ratio(SINR)of each user when using PTR.To asynchronous multiple users UWA communication,this paper proposes the method of adaptive passive time reversal(APTR).Under the constraint that interference between multiusers is zero,APTR calculates the optimal matching weight vectors in the frequency domain and then transformed to time domain by IFFT based on the idea of adaptive beamformer.On the basis of asynchronous multiuser UWA communications,this paper further studies MIMO single-carrier UWA communication,proposing a solution that PTR combined with successive interference cancellation algorithm i.e.PTR-SIC-DFE.In the latter part of this paper,the effectiveness and robustness of the adopted methods above are verified by computer simulation and sea experiments.At last,under-ice acoustic communication is concerned.The research is carried out mainly on the two aspects that under-ice channel characteristics and under-ice single carrier acoustic communication according to real under-ice data.The analyzed result of under-ice channel data shows that the under-ice acoustic channels are very stable with long channel coherence time.The spatial coherence of under-ice channel is dependent on the environment.Using only one receiver,this paper realizes data transmission at rate of 32kbits/s in under-ice environment.The under-ice MIMO data processing result shows that the almost invariant sound speed gradient causes the distribution of signal energy at the receving array is strongly relevant to the depth of transducer.The basic PTR-DFE could succeed in decoding the MIMO data when using two hydrophones.