Study On Underwater Acoustic Communication Technique Between Mother Ship And Underwater Platform

Underwater operation platform coupled with mother ship is an important part of the marine developments.The information exchange between surface ship and underwater platform plays an important role.The communication methods can be divided into two categories.One is uplink underwater acoustic communication(UAC)applied to transmit data,which needs higher communication rate.The other is downlink UAC used to transmit command,which needs strong robustness.Mother ship and underwater operation platform have unbalanced resources.Mother ship's resources are unlimited in a sense while underwater operation platform's resources are constrained.It is the main topic of the paper to achieve the effective and robust communication link in the asymmetric resources situation through reasonable design corresponding transmitter and receiver structure.UAC between surface ship and underwater platform often occurs in the mobile environment characterized by Doppler effect and time varying channel.The Doppler estimation and compensation are the primary problems in mobile UAC.Usually we can obtain Doppler coefficient by estimating the signal's frequency.However,CW signal is susceptible to UAC channel.The Doppler estimation method based on measuring pseudo random sequence carrier frequency offset by Chip-Z transform is proposed.The channel's stop and pass band effects will be overcome through the spread spectrum processing.At the same time,due to the spreading gain and pseudo random sequence excellent auto-correlation,the method can be used in low SNR environment to estimate the most powerful path's Doppler coefficient.Doppler compensate filter based on the Farrow filter structure is proposed.It can take into account the amount of computation and compensation accuracy thus will be applied in the broadband Doppler compensation.Due to the existence of the estimation and compensation error,the residual Doppler effect is still existed after the compensation.So it is necessary to deal with the residual Doppler effect.Differential pattern time delay shift coding and differential coding spread spectrum UAC method are proposed.The two methods have a strong tolerance for Doppler effect.Above two methods can achieve robust decoding after wideband Doppler compensation,dispense with residual Doppler processing,and are used in variable-space dependent underwater acoustic channel.The system performance is verified by the sea trial,and the both two methods are suitable for complex mobile UAC environment.Differential OFDM UAC system can effectively suppress the channel effects in high SNR environmentby means of differential demodulation in the receiver.The higher communication rate can be obtained with a relatively simple transmitter receiver structure.It has a good application prospect in the uplink UAC.When differential OFDM UAC is used in the uplink high speed UAC system,higher requirement is needed for the receiving SNR.In view of the low SNR problem and the complex environment impact in the uplink UAC,the vector time reversal mirror method is proposed which can be applied in the surface ship's receiving signal.The vector sensor can obtain both sound pressure and vibration velocity at the same times,and we can obtain the processing gain by combining the sound pressure and the vibration velocity signals.The proposed single vector time reversal mirror can effectively improve the input SNR of the receiver,and complete the channel equalization through the vector channel matching.Vector array time reversal mirror can improve the output SNR of the receiver and suppress the side lobe of the Q function.The above method combined with other high-speed communication system(such as DMPSK-OFDM)can meet the need of uplink UAC.Downlink UAC is used to transmit command,which puts forward higher requirements for the robustness and security of the link.In view of the demand,the paper proposes the UAC scheme based on parametric array.In combination with pattern delay differential coding,spread spectrum,SC-MPSK and other communication systems,the specific parameters of the transmission scheme are given.The position estimation from the surface ship to the underwater platform is realized by using the uplink UAC signal,which provides the information of target position and pitch angle for downlink directional communication.Parametric array using the nonlinear effects can obtain differential frequency wave,which has wide band and narrow directivity.The multi-path channel will be restrained by the narrow directivity which will simplify downlink communication receiver structure,so as to reduce the underwater platform resource overhead.Moreover,we verify the validity of the parametric array used in downlink UAC by the sea trial.