Study On Time Domain Equalization Technology Of Single Carrier Underwater Acoustic Communication System In Harsh Ocean Environment

Underwater acoustic channel under harsh ocean environment is one of the most complex channel environments.Single-carrier underwater acoustic communicatio n based on acoustic waves faces huge challenges to achieve effective and reliab le communication.Under severe sea conditions,the characteristics of the underwate r acoustic channel are mainly reflected in the following aspects: when sound waves propagate in the ocean,the multipath effect of the signal will be caused by the scattering of the sea surface,seabed and other media;when there is relativ e movement between the transmitting and receiving ships,and the movement of wate r currents,Causing the Doppler effect.Compared with the underwater acoustic velocity,there is a serious Doppler expansion phenomenon;the characteristics of the acoustic channel are constantly changing with factors such as tides,noise,temperature,depth,and geographic location,resulting in a time-varying nonstationary channel characteristic.Compared with the multi-carrier communication system,the advantages of single-carrier communication system are mainly reflected in: better anti-Dopple r effect;lower signal peak-to-average power ratio;high spectrum utilization.This is the most needed performance for underwater acoustic communication.However,single-carrier technology has worse anti-multipath effect than multi-carrie r technology.Channel equalization technology is a key technology to resist channel time-varying,Doppler,multipath and other issues.The traditional single-carrie r time-domain equalization technology has limited effects on complex underwate r acoustic channels.In view of the above problems,this paper conducts the followin g analysis and research:The performance of different equalization techniques is compared under different underwater acoustic channel environments.The principle and simulatio n experiment analysis proves that the fractionally spaced two-way decision feedback equalizer based on the recursive least squares algorithm is more suitable for the single channel of harsh ocean underwater acoustic channels.This equalizer can curb multipath,Doppler,time-varying and other problems in underwater acoustic channels to a certain extent.In order to further suppress the delay spread and Doppler effect in the harsh ocean underwater acoustic channel,this paper proposes a method of closed-loo p Doppler decision feedback equalizer with variable tracking step.Through the method of variable step size,on the one hand,the traditional embedded phase-locked loop decision feedback equalizer can only handle the narrow-band Doppler effect and the smaller phase jitter phenomenon,which can be expanded to stably handle the equalization with the bad doppler effect structure.On the other hand,the traditio n a l closed-loop Doppler decision feedback equalizer tracking phase is improved.The effectiveness of the scheme proposed in this paper is verified in simulation and Qiandao Lake experiment.On the basis of the above research,the method of variable step-length channel equalization based on passive time mirror for dual-spreading channels is furthe r studied.This equalization scheme combines sub-path passive time reversal mirro r technology to realize sub-path Doppler compensation.And according to the actua l underwater acoustic channel characteristics,the sub-path Doppler compensation is improved to block Doppler compensation.The experimental data verifies tha t applying the time reversal mirror technology of sub-path/block Doppler compensation to channel equalization can effectively resist the double spreading effect and time-varying characteristics.Through the above research,the robustness of the single-carrier system for dualexpanded underwater acoustic channels under severe sea conditions has been greatly enhanced,providing a basis for a new underwater acoustic multi-mod e communication system(including single-carrier mode).