Research On Long-Range Underwater Acoustic Communication In Shallow Water

With the advancement of the national maritime power strategy,the development of the marine economy is moving from the offshore to the far sea,and the naval strategy is also extending from the offshore defense to the far sea,which puts forward higher requirements for the underwater acoustic(UWA)communication technology,hoping that it can achieve a longer communication distance and has robust communication performance.This paper focuses on the passive time reversal(PTR)Pattern time delay shift coding(PDS)underwater acoustic(UWA)communication,combined with the idea of diversity combining,and focuses on solving the problems of severe multipath spread and low signal-to-noise ratio(SNR)faced by long-range UWA communication in shallow water.Aiming at the influence of Doppler and time-varying characteristics of channels in mobile UWA communication,the corresponding solutions are proposed and verified by simulations and field experimental data.Firstly,the characteristics of UWA channels in shallow water are analyzed in detail,and the problems to be solved to realize the long-range robust UWA communication in the shallow water are pointed out.The generation mechanism of multipath,Doppler and time-varying properties of UWA channels and the influence on the acoustic signals are expounded in detail.Based on the sonar equation,the limited available bandwidth of UWA communication is analyzed from the perspective of energy.Some characteristic functions of UWA channels are given in order to analyze and explain the physical characteristics of underwater acoustic channel mathematically.In addition,the characteristics of UWA channels are studied and analyzed by using the measured data,focusing on the Doppler effect on the channel characteristics,the causes of the multipath,the temporal coherence of the channel and the spatial correlation in the vertical direction.The analysis of the characteristics of UWA channels show that channel equalization and Doppler compensation are indispensable means to achieve reliable and robust UWA communication in shallow water.And for long-range UWA communication,it is necessary to make full use of the energy of multipath to improve SNR.Secondly,a maximum ratio passive time reversal(MRPTR)method is proposed to solve the problem that the PTR focusing gain is weakened in the actual marine environment.The performance of PTR is analyzed through theoretical derivation and measured data.The analysis show that the temporal compression property of PTR can effectively suppress the multipath spread,while the spatial focusing property can improve the main-side lobe ratio of the q-function.Theoretical derivation and simulation analysis of selection combining(SC),equal gain combining(EGC)and maximum ratio combining(MRC)are carried out.The results show that SC has the lowest output gain,and the output performance of EGC is easily affected by the difference of SNR among branches,while MRC has the best output gain.And regardless of the differences in the channel conditions of each branch,the performance of MRC can always remain stable.Aiming at the problem that the PTR focusing gain is weakened in the actual marine environment,the idea of MRC is introduced into the PTR method and the MRPTR method is proposed.The MRPTR weight coefficients are theoretical deduced,and the deduction result indicate that the weight coefficients are only related to the input noise power of each array element,and are not affected by the channel multipath structure,which avoids the influence of the errors caused by the signal amplitude estimation in conventional MRC.A pre-weighting based passive time reversal method is subsequently proposed,which has equivalent performance to MRPTR.The correctness of the combining weight coefficients of MRPTR is verified by simulation,and it is proved that MRPTR has better output performance than conventional PTR.The data processing results of the remote sea trial in shallow water also show that,compared with the conventional PTR UWA communication,the PDS UWA communication system based on MRPTR has a higher output SNR and a lower bit error rate(BER).Subsequently,in view of the degradation of the PTR output performance of the vertical receiving array under low SNR,a virtual-channel-based passive time reversal(VCPTR)technology is proposed to improve the performance of UWA communication.The concept of a virtual channel is proposed,its mathematical expression is given,its construction process is discussed and deduced in detail,and a principle simulation is carried out.The simulation results show that the virtual channel has less signal fading,but it still has complex multipath structure.Therefore,VCPTR technology is proposed to realize the equalization of the virtual channel,and its focusing gain is deduced and simulated.The experimental results of the shallow sea remote UWA communication show that when the SNR is too low to accurately estimate the subchannels,the VCPTR can obtain the same output performance as the MRPTR with fewer array elements,which verifies the practicality and effectiveness of PDS UWA communication system based on VCPTR in low SNR environments.Finally,in view of the Doppler effect and the time-varying characteristics of the channel in mobile UWA communication,a PDS UWA communication scheme based on dynamic adaptive decoding(DAD)is proposed.According to the decoding characteristics of PDS,a symbol-based Doppler compensation(SB-DC)method is proposed,which performs real time Doppler delay compensation on the next symbol by calculating the delay error caused by Doppler to the current symbol.The PDS UWA communication system based on SB-DC is simulated and analyzed using the measured channel data.The simulation results show that SB-DC can compensate the time-varying Doppler of the communication signal in real time,thereby reducing decoding errors.Due to the time-varying characteristics of the UWA channel,the equalization processing of the PTR on the channel cannot achieve the expected effect.Therefore,an adaptive copy-correlation decoding(ACD)method is proposed,which can overcome the effects of the time-varying channel by updating the reference signal of the copy-correlator in real time.Finally,a DAD scheme is proposed by combining SB-DC and ACD,and its decoding performance is simulated and analyzed.The simulation results show that DAD can further reduce the communication BER on the basis of SB-DC.The decoding performance of DAD-based PDS underwater acoustic communication system is further analyzed by using the experimental data of the Songhua River UWA communication test.The experimental results verify the feasibility and robustness of the DAD-based PDS UWA communication system in mobile UWA communication.