| Extremely low frequency(ELF)/ very low frequency(VLF)electromagnetic waves have strong permeability to conductive media such as earth and seawater,and are widely used in geophysical exploration,underground communications,underwater communications and other fields.However,ELF/VLF communication systems are not only limited in bandwidth,but also have typical non-Gaussian characteristics of channel noise,which seriously limits the communication performance of ELF/VLF systems.In recent years,the heated ionospheric modulation ELF/VLF electromagnetic wave technology has promoted the development of broadband ELF/VLF communications.Therefore,it is of great significance to study the noise interference characteristics and suppression methods of broadband ELF/VLF communications.This dissertation analyzes the characteristics of atmospheric noise in ELF/VLF communication and studies the model of atmospheric noise.On this basis,it focuses on the suppression of atmospheric noise in ELF/VLF communications,the performance of wideband VLF communication systems under Symmetric alpha-stable(SαS)noise and the motion noise suppression of superconducting quantum interference device(SQUID)receiving system.The main work and contributions of this dissertation are summarized as follows:1)Analyzing and modeling of atmospheric noise characteristics in ELF/VLF communications.Aiming at the problem that the atmospheric noise characteristics are uncertain and the model is not uniform in ELF/VLF communications,which leads to the serious degradation of the receiver performance,this dissertation proposes a method based on SαS distributed atmospheric noise model with different channel bandwidth.In this dissertation,a large number of measured atmospheric noise data are used to analyze the statistical characteristics of atmospheric noise with different channel bandwidth at different times and different scenes.The results show that the distribution of atmospheric noise at different times has certain differences,but also has certain correlation.Subsequently,different distribution models are used to fit the measured data.The results show that the atmospheric noise of different channel bandwidth can be modeled as SαS distribution models.ELF noise is characterized by weak pulse,while VLF noise is characterized by strong pulse.The larger the channel bandwidth is,the larger the characteristic parameter α in the SαS distribution is,that is,the less impulsive the noise is.2)Research on adaptive suppression of atmospheric noise in ELF/VLF communications.Aiming at the problem that ELF/VLF communications are interfered by atmospheric noise and the signal cannot be accurately extracted,this dissertation proposes an adaptive atmospheric noise suppression method based on continuous variational mode decomposition combined with Hausdorff distance(SVMD-HD).This method can continuously extract the mode in the signal when the modal quantity information is missing.The simulation results show that this method has good suppression effect on atmospheric noise,especially under the condition of low signal-to-noise ratio,and the algorithm has low computational complexity,fast convergence speed,clear physical significance and good power spectrum separation effect,and can solve the problems of mode mixing.3)Research on broadband VLF communications system and its performance under SαS noise.Aiming at the problems of low channel transmission efficiency and weak received signal caused by pulse and power frequency harmonic interference in VLF channel,based on the orthogonal frequency division multiplexing(OFDM)system,this dissertation proposes a MSK demodulation method based on power function approximate SαS filtering.In order to further improve the system performance,a communication scheme based on Low density parity check code-Orthogonal frequency division multiplexing(LDPC-OFDM)is proposed,and the decoding method based on Huber measure is used to decode LDPC.This decoding method can reduce the impact of impulse noise and enhance the reliability and effectiveness of the system.The simulation results show that LDPC-OFDM system scheme,Minimum shift keying(MSK)demodulation method based on power function approximation SαS filter and Huber measure decoding method improve the performance of VLF communication system.4)Research on motion noise suppression method based on SQUID receiving system.Aiming at the problem that the SQUID receiving system will be disturbed by motion noise,which will lead to the instability of the received data,this dissertation adopts a variational mode extraction(VME)method to suppress the motion noise.This method follows the principle of minimum energy of the residual signal at the center frequency of the mode after extracting the specific mode,and converts the problem of suppressing the motion noise in the SQUID receiving system into the problem of acquiring the specific mode of the signal.In order to verify the effectiveness of the VME method,this dissertation carries out experiments on the simulation and measured data.The results show that the VME method can effectively and accurately suppress the motion noise in the SQUID receiving system,and the method has lower computational cost and higher convergence speed. |
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