Digital Signal Processing Algorithm For Mode Diversity Space Optical Communication System

With the expansion of free space optical communication system use scenarios and the substantial growth of the number of terminals,the demand for free space optical communication system traffic is increasing in the whole society.The compensation method for free space optical communication system has become one of the research hotspots in this field.Spatial diversity technology is mostly based on multi antenna reception,and the system complexity is high.The mode diversity technology uses a few mode fiber to receive optical signals,which can support several orthogonal modes to transmit at the same time.The higher-order mode signals excited in the spatial optical channel can be received by mode diversity using the orthogonality of modes.As a result,the mode diversity oriented space optical communication system based on the few mode optical fiber came into being.In this paper,the key basic technologies of the mode diversity oriented spatial optical communication system are explored,and the digital signal processing algorithm of the mode diversity oriented spatial optical communication system is mainly studied.The simulation system and experimental system of the mode diversity combined compensation are built,and its performance is tested.The work of this paper mainly includes the following aspects:Firstly,the basic structure of the mode diversity oriented spatial optical communication system is introduced,and the key technologies of the system are discussed.Starting from the Maxwell equations,the mode transmission characteristics in the SMF are analyzed,the theoretical basis of mode transmission is discussed,and the coherent reception technology is analyzed and introduced.Secondly,starting from all kinds of possible damages in the mode diversity oriented spatial optical communication system,this paper studies the digital signal processing algorithm in the system,analyzes the structure,advantages,disadvantages and theoretical basis of the orthogonal imbalance compensation algorithm,bit synchronization algorithm,polarization demultiplexing algorithm,carrier synchronization algorithm and mode diversity merging algorithm,and combines the preliminary experimental data and simulation means to The availability of the algorithm is verified.An improved carrier synchronization algorithm based on the sum of the two adjacent phase differences is proposed.The theoretical basis of the algorithm is discussed,and the performance of the algorithm is analyzed by simulation.The performance of the algorithm is compared with that of the fourth power frequency offset estimation algorithm and the sum of the one adjacent phase differences algorithm.Finally,the simulation system of spatial optical communication based on mode diversity is established by using vpitransmissionmaker and MATLAB.In the simulation transmission,each mode is regarded as an independent channel,and the digital coherent reception and the digital signal offline processing are adopted at the receiving end.The maximum ratio combining algorithm is applied to the offline digital signal processing.The communication performance of the three modes of single mode fiber receiving with three modes and six mode few mode optical fiber diversity is simulated and analyzed.A mode diversity oriented spatial optical communication experimental system is built,and the experimental data of QPSK modulation formats under three turbulence intensities are collected,and the performance of six mode combining and single mode receiving is compared..