| In the satellite-to-ground laser communication,the laser beam carrying information must pass through the earth's atmosphere in the transmission link.Under the influence of atmospheric turbulence,there will be some adverse phenomena for the beam at the receiving plane,such as fluctuation of laser intensity,fluctuation of arrival angle and light spot drift,which seriously affects the quality of satellite-ground laser communication system.Therefore,it is imperative to study the effect of atmospheric turbulence on satellite-to-ground laser communication and the corresponding antiturbulence technology.Among the many anti-jamming technologies,the channel coding technology has been widely used in the field of wired optical communication,due to its advantages of fast transmission rate,high energy utilization efficiency and low installation cost.Among them,LDPC is gradually becoming more and more popular in the coding technology of satellite and earth communication system because of its advantages of close to Shannon's theoretical limit,low coding and decoding complexity and suitable for hardware implementation.With this as the research background,based on the satellite-earth three-level power spectrum model,combined with the difference of the statistical characteristics of turbulence in the earth's atmosphere as well as the aperture average effect,this paper improved the expression of the light intensity scintillation of oblique link under the condition of moderate to strong turbulence,and expended the application scope of original satellite-earth light communication channel model.On this basis,the LDPC coding technology is further applied to the satellite-ground optical communication system,the simulation model of FSO system is established,and the influence of LDPC code usage on the bit error rate of the communication system is analyzed.The main research work of this paper includes:(1)Based on the three-layer power spectrum turbulence model of satellite-toground channel,the stratospheric atmosphere with a power law of 5 is treated differently from the boundary layer and the free troposphere,and the turbulence outer scale effect which was often neglected in the analysis of the fluctuation characteristics of the light intensity in the past is taken into consideration.Further combining with the aperture average effect at the receiving end,the theoretical model of atmospheric turbulence light intensity fluctuation in the oblique link in the satellite-ground laser communication system is perfected,and the applicable range of zenith Angle in the satellite-ground channel scintillation index model is extended from the weak turbulence condition(0?60°)to the full link.(2)Based on the EW optical channel fading model,the LDPC code in FSO system is simulated.The noise characteristics of FSO system and the influence of LDPC code parameters on system performance are analyzed,including decoding algorithm,code length,code rate and maximum iteration times.The simulation results show that the LDPC coding technology can significantly improve the bit error rate of the communication system.Among the decoding algorithm,PD BP decoding has better decoding performance.When the SNR is 8d B,the decoding algorithm reduces the bit error rate of the uncoded system by about 6 orders of magnitude when the turbulence is weak.And LDPC code with longer code length,lower code rate and larger maximum iteration times has stronger error correction ability.(3)Based on the modified satellite-to-ground link channel model,the influence of LDPC code on the performance of satellite-to-ground light communication is discussed.The influence of downlink channel parameters on bit error rate of satelliteground communication system is analyzed,including the size of receiving aperture and zenith Angle.At the same time,the selection of LDPC coding scheme for satelliteground optical communication system under different SNR conditions is discussed in order to achieve the balance between hardware computing efficiency and bit error rate performance. |
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