| 5G communication network has been set all over the world in the past 5 years,providing a communication basis for new technologies such as the Internet of Things,autonomous vehicles,telemedicine,etc.In order to achieve the goal of Internet of everything and smart city,5G network cannot satisfy such information transmission scenario.Therefore,6G network has become the focus of communication studies.However,the 6G network works at terahertz band which leads to a small coverage,a large number of communication satellites and ground relay stations are needed to ensure the coverage.Traditional communication technology cannot transmit such amount of information at several Ghz level between satellite-ground,inter-satellite and inter-relay stations.Free space laser communication technology is an ideal solution.Single-mode fiber is applied in FSO system to achieve high data rate.The spatial signal light in the atmospheric channel will be affected by atmospheric turbulence.Due to the randomness of turbulence,the performance of the receiver system changes randomly,which affects the stability of the communication link.When we evaluate the influence of atmospheric turbulence on the performance of communication system,the coupling efficiency of spatial light to a single-mode fiber is the basic parameter,which turns into a random variable due to randomness of atmospheric turbulence.The average coupling efficiency is often used to analyze the system parameters,such as signal-tonoise ratio and bit error rate.However,the relationship between these system parameters and the coupling efficiency is nonlinear.We cannot accurately evaluate the impact of atmospheric turbulence on the system performance only using the average coupling efficiency.It is necessary to analyze the statistical distribution of the coupling efficiency,based on which the signal-to-noise ratio and the bit error rate can be calculated from a statistical perspective.Kolmogorov turbulence model is widely used in the analysis of atmospheric turbulence on space laser communication systems because of its concise mathematical form.However,its conditions are relatively strict,requiring the boundary conditions of atmospheric turbulence with infinitely large outer scale and infinitely small inner scale,as well as the conditions of isotropic and isotropic turbulence.Moreover,theoretical and experimental studies show that there is a large amount of non-Kolmogorov turbulence in the atmosphere,especially in cities with plenty of human activities.The temperature and airflow distribution are complex and variable,and it is difficult to form a stable Kolmogorov turbulence.Therefore,it is inaccurate to use Kolmogorov turbulence model to analyze the influence of atmospheric turbulence on the performance of free space laser communication system.In order to fully study the influence of atmospheric turbulence on the performance of free space laser communication system,this thesis establishes a statistical model of the coupling efficiency of spatial light to single-mode optical fiber based on the nonKolmogorov turbulence model,ant the average SNR and BER of the system affected by atmospheric turbulence are calculated from a statistical perspective.Experiments are designed to verify the accuracy of the statistical model and the impact of atmospheric turbulence on the system performance is studied.The main works of the thesis are as follows:1.The optical system of FSO is fully studied.The difference between the power spectral density function of Kolmogorov,Tatarskii and other classical turbulence models and the non-Kolmogorov power spectrum is analyzed.Based on the non-Kolmogorov power spectrum,the residual variance of spatial light wavefront is calculated and the distribution characteristics of the variance of different Zernike terms are analyzed.2.The physical process of spatial light coupling to a single-mode optical fiber is studied.The modeling process of coupling efficiency is simplified by proper transformation.Based on the non-Kolmogorov turbulence,the statistical model of coupling efficiency is established,and its theoretical analysis is done.The influence of statistical distribution of coupling efficiency on turbulence intensity under different turbulence models is compared,the results show that,in the link design phase,it will reduce link redundancy by 4.9d B in extreme cases using a non-Kolmogorov model to analyze the average coupling efficiency.3.The relationship between the SNR/BER and the coupling efficiency under the direct and coherent detection systems is analyzed.Based on the statistical model of the coupling efficiency,the calculation method of the average SNR and BER of the system is obtained,and the influence of the average SNR and BER with different turbulence intensity using different turbulence models is analyzed,the average SNR error can be reduced by 2.3 d B,and the average BER error can be reduced by 1 order of magnitude in some cases.4.The experiment is designed to obtain the distribution of coupling efficiency under the influence of atmospheric turbulence.Compared with the experimental distribution,the accuracy of the model is verified;At the same time,the communication link is set up,and the BER affected by atmospheric turbulence is obtained.Compared with the theoretical value,the accuracy of measuring the performance impact of atmospheric turbulence free space laser communication system by this method is proved.The research of this thesis provides a more accurate method to analyze the impact of atmospheric turbulence on the free space laser communication system,especially in the design stage of the system.The system loss caused by atmospheric turbulence can be evaluated accurately and ensure the system performance at the same time.Power,space and other resources of the system can be optimized to the best to provide a reference for designers. |
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