Research On Adaptive Compensation Of Atmospheric Channel And Receiving Technology In Free Space Optical Communication

Free space optical communication is a wireless communication with free space as the transmission medium and light as the information carrier.Combining the advantages of microwave communication and optical fiber communication,free space optical communication realizes large communication capacity and high-speed transmission,which can be applied to satellite-to-ground,intersatellite and ground communications.It has great strategic needs and application value in military and civil fields.And it is also widely considered as an important technology development field of future communications.The most important factor restricting free space optical communication technology is the random interference of atmospheric channel.The transmission of laser beam in the atmosphere will be affected by atmospheric turbulence,which causes a decrease of beam quality.Then the stability and reliability of communication systems are reduced.In order to reduce the influence of atmospheric channel on communication systems and improve the communication quality,we analyze the atmospheric channel characteristics and study the key technologies such as large-aperture RF/optical hybrid receiving antenna,diversity reception and adaptive compensation in free space optical communication.Firstly,the characteristics of atmospheric channel are studied for atmospheric turbulence.We analyze the beam propagation characteristics in atmospheric turbulence.The simulation interface design of atmospheric channel characteristics is completed by using the graphical user interface of MATLAB.The channel parameters and simulation results affecting communication are analyzed.It is found that the better quality of beam transmission can be obtained by using the laser with the aperture of 10cm and the wavelength of 1550nm.Then the simulation platform of OPSK coherent optical communication is completed with the software of Optisystem.We carry out the 20Gbps QPSK coherent optical communication simulation.When the laser wavelength,line width and the signal-to-noise ratio is 1550nm,100kHz and 22dB,respectively,the error rate of system can be achieved to 10-6.Secondly,for the problem of signal fading caused by atmospheric turbulence,we study the large-aperture radio frequency and optical(RF/optical)hybrid receiving antenna.The structure design of the RF/optical hybrid receiving antenna is proposed.The design of optical signal reception is realized on the radio frequency antenna,which improves the reliability of the system and is suitable for space communication,especially in the field of deep space communication.Then the diversity receiving of antenna array is also studied to solve the problem of signal fading.Based on the extended Rytov theory of ABCD ray matrices and spherical wave model,the channel correlation coefficient of spherical wave in Kolmogorov turbulence is derived.The approximate estimation of channel independent correlation length and the layout of array receiving antenna are obtained.Furthermore,it is found that the method of diversity reception can further reduce the bit error rate of communication systems.Finally,aiming at the non-linear distortion of wavefront distortion caused by atmospheric turbulence,the neural network and deep learning algorithm are combined with the optical orthogonal frequency divisionmultiplexing technology to complete the end-to-end free space optical communication systems based on the autoencoder.We propose the scheme of hybrid autoencoder combining the extended selection mapping.The simulation results show that the proposed scheme of hybrid autoencoder can further reduce the peak-to-average power ratio and bit error rate of communication systems and achieve the adaptive compensation of the channel much better.