| With the progresses and developments of science and technology,especially the exploitation and utilization of space technology,the amount of data that people get from space increases geometrically,which puts forward higher requirements for the transmission rate and bandwidth in the process of transmission.As a new type of communication technology,Optical Wireless Communication(OWC)arises at the historic moment.It can easily realize long-distance,high-speed communication.Therefore,wireless optical communication can be used in inter-satellite,satellite-ground,water-air,air-ground and other different scenarios of communication,among its application prospects.However,the optical signal will be affected by random media.The turbulence effect will cause the wavefront distortion of the laser beam as carrier in the transmission channel.It causes the beam to spread and drift,leading to transverse dispersion,phase and intensity fluctuation.It will seriously reduce the link performance of the OWC system.The transmission characteristics of laser beam in random media are studied from two different angles of numerical simulation and experimental research,and the link performance of OWC system is analyzed.Firstly,we used wave optics computation to simulate the scintillation index in the atmospheric channel based on the extended Rytov theory.We use this model to study the scintillation characteristics of the horizontal link,compare the influence of the receiving aperture,wind speed and transmission distance on the wireless optical transmission.In the underwater channel,the scintillation characteristics of horizontal link are studied by using the power spectral density model of ocean refractive index fluctuation,and the influence of the environmental parameters such as receiving aperture,water velocity and transmission distance on the wireless optical transmission is analyzed.Based on the link characteristics of atmospheric and underwater channels,the water-air vertical link model is established,and the influence of receiving aperture and transmission distance on the wireless optical transmission is compared.For uplink and downlink,Gaussian beam and circular beam are used to investigate.The results show that the annular beam can resist the turbulence induced intensity fluctuation better,and its link performance is better than that of the Gaussian beam.In addition,the reasons for the link performance difference between uplink and downlink are analyzed.Secondly,the transmission characteristics and communication performance of light in atmospheric channels under the influence of turbulence are studied experimentally.The experimental platform of adaptive optics system without wavefront sensing is established.The experimental scheme is designed for different turbulence.For the weak turbulence simulated by phase screen,stochastic parallel gradient descent algorithm is used to correct the wavefront distortion,and the correction effect and convergence rate of the algorithm are analyzed.For the complex turbulence generated by atmospheric turbulence generator,the Gerchberg-Saxton(GS)algorithm for correction,and the algorithm is improved.The correction effect and convergence speed of the original GS algorithm and the improved GS algorithm are compared.The average signal-to-noise ratio and bit error rate of the system are calculated by using the given system parameters and measurement results.The influence of turbulence on the communication performance of the OWC system is analyzed.Finally,the transmission characteristics and communication performance of light in underwater channels under the influence of turbulence are studied experimentally.The underwater optical wireless communication(UOWC)system is built in the underwater channel by simulating the scene of sea surface wave disturbance.The transmission characteristics of laser in the presence of water surface wave disturbance are studied.The results show that the laser scattering is related to turbulence and disturbance depth,and the distribution of laser spots at the receiving end has a certain statistical law.On this basis,non-return-to-zero on-off Keying modulation is used to study the communication performance of 500 Mbps signal in 10 m tap water channel.The research shows that the communication quality of the UOWC system is closely related to the input light intensity,the disturbance range and depth of water surface waves.The innovation of this paper can be summarized as follows: Aiming at the atmospheric random media disturbance in the optical transmission link,a novel wavefront correction technology scheme of transmission optical field distortion is proposed and realized.This scheme according to the principle of transport of intensity equation,an intensity-phase inversion device is designed,adopt this device can quickly obtain the wavefront information of transmission optical field distortion caused by turbulent atmosphere.Combining with the stochastic parallel gradient descent algorithm,realizes the real-time correction and compensation of wavefront distortion,and enhances the elimination real-time and reliability of signal scintillation caused by turbulence,which provides a practical and reliable technical scheme for the application of adaptive optics technology without wavefront sensing in optical wireless communication.In the study of the optical transmission characteristics of water surface waves,the statistical characteristics of airflow disturbance on the surface of water media have been studied and obtained,and the physical characteristics and communication performance of the optical signal of the underwater transmission link have been given an effective characterization of the characteristics of the water surface waves.The related original research result is a supplement to the existing theory of turbulent optical communication,which provides a valuable reference for the effective transmission and communication of optical wireless systems in random media and the improving the anti-interference performance of transmission links. |
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