| Free-space optical(FSO)communication refers to optical communication that uses lasers as the source of transmission and the atmosphere as the transmission medium.FSO has the advantages of wide bandwidth,no spectrum license required,and high transmission rate which has attracted widespread attention in recent years.However,due to the particularity of the communication environment and communication channel,signal transmission in the atmospheric attenuation and the pointing error problem have become the bottleneck of FSO communication distance and communication performance.Especially in remote communication,atmospheric turbulence will cause fading,which will lead to the interruption of communication,which seriously affects the widespread use of FSO.In order to solve this problem,the development of relay assisted transmission technology has become increasingly hot in recent years,and the setting of the relay node position affects the performance of the communication system.Orthogonal Frequency Division Multiplexing(OFDM)has the advantages of anti-frequency selective fading,effective countermeasures against interference between signal waveforms,and high frequency band utilization.It is applied to high-speed data transmission in fading channels,and has gradually begun to be used in the field of FSO communications.The OFDM coherent detection method has become a better detection method because of its advantages,such as high reception sensitivity and strong polarization film dispersion ability.Therefore,relay technology and coherent OFDM detection technology have become the focus of research to improve the performance of FSO communication systems.This paper mainly studies the interrupt performance of coherent OFDM serial relay system and parallel relay system.On this basis,the influence of relay node location on the optimization of FSO system interrupt performance is analyzed.The structure design of the problem of the frequency stability of the local vibration signal and the signal light during the coherent detection of the receiver is performed.The main research content is as follows;1.For the medium-strong turbulent flow environment and long-distance transmission scenarios,a theoretical model of a coherent OFDM serial relay system under the Gamma-Gamma distributed atmospheric channel model is proposed,which is applied to the FSO communication serial decoding and forwarding(DF)relay system.The location of the relay was studied.Considering the effects of atmospheric attenuation,atmospheric light flicker and pointing error on the transmission performance of FSO communication relay system,a joint channel model of free-space optical communication system is established.Based on this channel model,an end-to-end closure probability expression for a free-space optical coherent OFDM serial relay system is deduced using the MeijerG function.Simulation analysis of the serial decoding and relaying system with different relay positions under atmospheric and atmospheric conditions is carried out.The optimization of the relay location can effectively improve the performance of the FSO communication serial relay system.This part of the study provides a feasible method for the application of long-distance communication in a medium-strong air turbulence environment,and optimizes the system's interruption performance.2.In the case of high communication performance and moderate transmission distance,a theoretical model of a coherent OFDM parallel relay system under the Gamma-Gamma distributed atmospheric channel model is proposed.Based on this channel model,the closed form expression of end to end interruption probability of coherent OFDM serial relay system in free space optical communication is derived by using MeijerG function.The parallel DF relay system is modeled and simulated under medium and strong turbulent flow environment,and the influence of the number of parallel links,turbulence intensity and relay node position on the outage probability is obtained.This system model can effectively improve the interrupt performance of the system,providing a theoretical basis for the design and parameter selection of coherent OFDM-FSO systems.3.A free-space optical communication tunable optical coherent detection OFDM device is proposed and its implementation scheme is given.This device solves the problem that the frequency stability of the local vibration signal and the.signal light in the coherent detection at the receiving end of the system is at a high order of magnitude. |
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