| Communication can be divided into two categories: wired communication and wireless communication.As the mainstream means of communication,wired communication has the advantages of fast speed and high capacity,but it is limited in areas where cable and optical fiber are not easy to lay.Free space optical communication technology is widely used as supplement and backup of wired and microwave communications in military and commercial communications due to its advantages of flexible installation,strong confidentiality and no need to apply for high frequency resources.This paper briefly introduces the background and research status of FSO technology,and analyses the basic working principle of ground-to-ground FSO technology and the advantages and disadvantages of the technology itself.Due to the randomness and complexity of the atmospheric channel,it is impossible to select appropriate modulation mode at transmitter modulation and reception mode at receiver for suppressing the influence of atmospheric channel and improving the performance of FSO system.The main work and achievements of this paper are as follows:(1)Firstly,in this paper,for the ground-to-ground FSO communication system,the absorption and scattering effects of light by solid particles and aerosol particles in the near-Earth atmosphere,the scintillation effect of light intensity caused by atmospheric turbulent motion,and the phase drift effect caused by non-uniform scale turbulence are studied.In addition to the influence factors in the atmospheric channel,the free space optical communication system as point-to-point communication inevitably has the aiming problem at the transmitter and receiver after long-distance transmission.In this paper,Gamma-Gamma probability distribution model of atmospheric turbulence intensity fluctuation,which is consistent with the measured data,is used to establish a joint atmospheric transmission channel model combined with the effects of atmospheric particle absorption and scattering,aiming error and phase drift caused by inhomogeneous turbulence.(2)Modulation technology is one of the key technologies of free space optical communication technology.At present,most of FSO systems use intensity modulation technology and phase modulation technology.The effect of phase drift caused by turbulence of non-uniform scale on the performance of phase modulation system is simulated and analyzed.From the point of view of improving the transmission capacity of instant information,orthogonal subcarrier modulation(OFDM),a multi-carrier modulation technology,greatly improves the system capacity.From the point of view of reducing the influence of atmosphere on modulation mode,the polarization of light in FSO link is less disturbed by atmosphere.In this paper,the principle and performance of FSO system based on OOK,BPSK,PSK-OFDM and Polsk modulation are analyzed in detail,and the average BER and average communication capacity of FSO system with several modulation modes under ideal channel and atmospheric joint channel are simulated and analyzed.(3)In order to provide theoretical basis for the receiving mode of polarization modulation free space optical communication system,a FSO system based on polarization modulation/coherent reception is built by using Optisystem simulation software.The system is built on the platform of Optisystem software.The effects of atmospheric attenuation,communication distance and phase error between local oscillator source and signal light on coherent receiving system are simulated and analyzed.The direct reception,zero-difference coherence and heterodyne coherence reception are compared.The effects of turbulence intensity,signal-to-noise ratio,normalized threshold,normalized jitter coefficient and normalized beam width on BER and outage probability of coherent FSO system are quantitatively analyzed by simulation.The research results of this paper can be used in the simulation research and system construction of FSO communication system,and provide a theoretical basis for the performance of free space optical communication system and related modulation technology,reception technology research. |
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