Research On Acoustic Channel And Directional Underwater Acoustic Communication In Shallow Ice-Covered Regions

The acoustic channel in shallow sea ice areas includes underwater and cross-ice acoustic channels.Compared with the characteristics of severe multipath expansion,serious Doppler frequency shift,and limited communication bandwidth in the open shallow water acoustic channel,the shallow sea ice area acoustic channel also has more serious transmission loss and noise interference due to the existence of ice,which brings a more severe challenge to the research of underwater acoustic communication and crosses ice acoustic communication.Facing the application background of underwater acoustic communication and cross-ice acoustic communication in shallow sea ice areas,this paper carries out relevant research on the theoretical derivation and numerical analysis of acoustic channels from two aspects of shallow sea ice area and ice waveguide,on this basis,it provides support for parametric array acoustic communication system and verifies the numerical research results through Songhua River ice area test.First,a model based on the proportional boundary finite element method is proposed to analyze waveguide propagation characteristics in shallow sea ice areas.The robust and efficient numerical calculation is achieved by deriving and solving the waveguide eigenequation,and then the underwater acoustic channel characteristics are studied.Sound wave propagation in shallow water waveguides with ice layers involves complex physical processes such as reflection and transmission.This includes the scattering of sound waves at the fluid-solid interface and the propagation in solids.To deeply understand the acoustic coupling process of the ice water waveguide,the normal mode method is used to analyze the waveguide’s propagation characteristics and frequency response characteristics in the shallow sea ice area.Considering that the accuracy and efficiency of the first stage mode solution of the normal mode method will directly affect the numerical performance,this paper derives the nonlinear eigenequation used to describe the waveguide in the shallow sea ice area from the control equation and the principle of virtual work and gives the numerical solution method of the nonlinear eigenequation on the premise of considering the half-space fluid seabed.Compared with the traditional numerical method,the derived scaled boundary finite element formula can accurately calculate the real and complex domain eigenvalues of the ice-water coupled waveguide; In terms of numerical value,the algebraic linearization algorithm is introduced to solve the problem that Scholte Stoneley waves at the ice water interface cannot be considered based on inverse iteration.In addition,considering the difference of variable dimensions used by the ice water coupling system,to further improve the stability of the numerical solution,the error convergence characteristics of waveguide processing in shallow sea ice areas based on the proportional boundary finite element method are analyzed.Finally,the underwater acoustic channel’s propagation and frequency response characteristics are analyzed based on the calculated waveguide modes.Secondly,the ice waveguide acoustic channel is studied.In this paper,the simulation of the simplified model is carried out.Based on the multi-mode numerical method,the propagation characteristics of the ice waveguide under non-uniform,one-side,and two-side fluid loads are discussed.For calculating modal eigenvalues of free uniform elastic plates,the complex plane root searching algorithm is studied based on the spoke angle principle,and the complex plane root searching strategy is given.For the modal eigenvalues of the non-uniform ice layer,a spectral method framework is constructed based on the evolution equation,and the configuration scheme of the framework for solving the problems of the non-uniform ice layer and fluid-loaded ice layer is analyzed,which proves that the numerical method is widely applicable,robust and efficient.The propagation characteristics of the non-uniform ice layer under symmetrical and linear sound velocity gradients are further analyzed.Under an elastic ice layer,ice surface displacement reception response is analyzed from ice thickness,signal frequency,receiving location,and ice ridge interference.Finally,the ice acoustic communication technology based on the parametric array is studied.In this paper,based on the parametric array,combined with the aforementioned numerical model and qualitative analysis,the method of effective acoustic information transmission based on parametric array emission is discussed.The sound propagation experiment in the ice area was carried out to verify the correctness of the numerical model.A recursive filtering algorithm is proposed to pre-modulate to overcome the loss of communication performance caused by differential frequency wave distortion.The multi-channel comparison between the parametric array and the ordinary transducer in the Songhua River ice communication experiment is given,and the decoding performance of the recursive filter pre-modulation under different phase shift keying modulation modes is analyzed.Using parametric array sonar and piezoelectric accelerometer as experimental means,the experimental research on cross-ice acoustic communication is carried out: firstly,the displacement vibration of the ice surface is received in the direction of the acoustic axis of the parametric array,and the cross-ice communication is realized by using Pattern time delay difference communication system; The accelerometer has further deviated from the acoustic axis direction of the parametric array,which verifies the communication robustness of the time hopping spread spectrum system against low SNR cross ice acoustic signals.