Research On Jointing Probability Shaping And Geometric Shaping In Coherent Optical Communication

As a communication method,Free Space Optical Communication(FSO)utilizes laser as information carrier and the atmosphere as transmission medium,which has attracted extensive attention for its advantages,such as large capacity,high confidentiality,strong anti-interference ability and rich spectrum resources.In order to meet the demand of increasing data capacity,FSO is also developing towards the direction of large capacity and high speed.Therefore,how to achieve larger channel capacity,higher information rate,and better system performance in free space optical communication systems has become an important research topic.There are mainly two systems: in FSO systems,which are Intensity Modulation/Direct Detection(IM/DD)system and coherent detection system,respectively.Compared with IM/DD system,the coherent detection system has higher detection sensitivity by mixing local oscillator light with signal light at the receiving end.More importantly,since the coherent detection system introduces the degrees of freedom of amplitude and phase,the channel capacity can be further improved by constellation shaping technology,which is close to the Shannon limit,thereby achieving higher-speed transmission.On the whole,constellation shaping technology includes probability shaping technology and geometric shaping technology.Therefore,the joint research of probability shaping technology and geometric shaping technology in coherent laser communication system has scientific theoretical significance and practical significance of application.In this paper,the research significance and progress of laser communication technology are described firstly and the research progress of constellation shaping technology are introduced.Then we analyze the influence of atmospheric on laser communication systems.Finally,the application of constellation shaping technology in coherent optical communication system is deeply studied.The main research work of this paper is as follows.1.The effects of the atmospheric on the laser communication are analyzed,mainly including attenuation effect caused by atmospheric absorption and atmospheric scattering and the light intensity scintillation and phase fluctuation caused by atmospheric turbulence.The intensity fluctuation models under different turbulence intensity are analyzed.The system structure of coherent optical communication is studied.The basic principle of coherent detection technology,including homodyne detection,heterodyne detection and phase diversity detection technology,is emphatically analyzed.2.The basic principle and decoding algorithm of LPDC code are studied.According to the characteristics of laser communication channel,starting from the belief propagation decoding algorithm,the closed expression of decoding initial value suitable for laser communication channel is deduced,and the corresponding decoding algorithm is modified.3.The application of probability shaping technology in coherent optical communication system is studied.According to the characteristics of laser communication channel,the optimal probability distribution of input signal is designed,and the closed form expression of the symbol error rate of nonuniform signal under Gamma-Gamma channel is derived.Finally,the effectiveness of the probabilistic shaping scheme is verified by simulation.4.The pairwise optimization algorithm based on the information rate of bit metric decoding is studied.Considering the complexity of this algorithm,the cut-off rate is taken as the objective function to optimize the constellation positions to achieve geometric shaping.The closed form expressions of the cut-off rate in Additive White Gaussian Noise channel and atmospheric turbulence channel are derived,and the multiple interior point method is designed to optimize the constellation point coordinates.The effectiveness of the geometric shaping scheme is verified by simulation.In order to further improve the channel capacity of the system,a joint probability shaping and geometric shaping scheme is proposed to be applied to the coherent optical communication system.5.The system construction process of Optisystem optical communication simulation software is studied.Through the Optisystem optical communication simulation software,the 16 QAM coherent optical communication link is built.The performance of probabilistic shaping,geometric shaping and probabilistic shaping combined geometric shaping schemes under different turbulence conditions are verified by system level simulation.