| Free space optical communication(FSO)is a kind of communication mode which uses laser beam as carrier and free space environment as channel between two or more terminals.It has gradually obtained the extensive attention of scholars since it supports unlicensed spectrum,strong anti-electromagnetic interference ability,excellent security and so on.The traditional free space optical communication systems mainly adopt the binary on-off keying(OOK)modulation.The advantages of the system are simple structure,easy to build and low cost.However,its disadvantages are relatively low detection sensitivity and spectral efficiency.Therefore,the OOK systems are not suitable for ultra-high speed communication.Compared with the traditional OOK systems,the optical communication systems based on high-order signal modulation and coherent detection have the advantages of good channel selectivity,high spectral efficiency and high detection sensitivity.However,the signal light of high-order signal modulation coherent optical communication system is easily affected by atmospheric turbulence in the transmission process,resulting in the phenomenon of light intensity fluctuation and phase jitter,which leads to the sharp decline of system performance.Therefore,it is of great significance to study and overcome the influence of atmospheric turbulence for the coherent optical communication systems with high-order signal modulation.Firstly,this paper reviews and discusses the background,significance and development of free space optical communication at home and abroad.Then,the atmospheric channel is introduced.After that,in order to suppress the influence of turbulence on high-order free space coherent optical communication,two system design methods are proposed,and the influence of atmospheric turbulence on them is studied and analyzed.Finally,an equivalent experiment is built to verify the effectiveness of the above two system methods.The main contents of this paper are as follows:1.The attenuation,scattering,turbulence and refractive index structure constant model,refractive index fluctuation power spectral density model of atmospheric channel are introduced in this paper.The theoretical model of free-space coherent optical communication system under the influence of atmospheric turbulence is analyzed.The influence of signal intensity fluctuation and phase jitter caused by atmospheric channel on the system is also studied by using plane wave theory.2.According to the characteristics of the atmospheric channel,the coherent free space optical communication system based on double adaptive thresholds with amplitude modulation is proposed.Besides,the expressions of BER and probability density functions of detection thresholds are analyzed and derived.The BER performance of the system is also verified by simulations.3.In order to obtain higher system performance and spectral efficiency,the coherent free-space optical communication system with 8-QAM modulation using joint algorithm of amplitude compensation and digital phase recovery is proposed.The effectiveness of the system is verified by simulation experiments,and the experimental results are compared and analyzed with other systems.4.The experimental platform of free-space coherent optical communication with high-order signal modulation and the atmosphere scintillation playback system are designed and built.Then,the coherent free space optical communication system based on double adaptive thresholds with amplitude modulation and the coherent free-space optical communication system with 8-QAM modulation using joint algorithm of amplitude compensation and digital phase recovery are tested at the rate of 2.5Gbps to verify the performances of the two systems under different turbulence conditions.The research in this paper can provide theoretical support for the design and implementation of free-space optical communication system with high-order signal modulation,and it has certain reference significance and value. |
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