Clear Air Turbulence Effects And Correction Technology In Free-space Optical Communication

With the rapid development of the information technology, traditional microwave communication system cannot meet the increasing demand for the performance of transmission rate and security. Free-space optical (FSO) communication has many potential advantages, such as large communication capacity, excellent security performance, resistant to electromagnetic interference, less power consumption, less volume, less mass, and being license-free. Recently it has turned to be a hot topic in the field of wireless communication research. When laser beam propagates through the atmosphere as the information carrier, it will be influenced by power attenuation and turbulence effects, and the performance of FSO systems will be degraded. The atmospheric effects have become an obstacle that limits the wide application of FSO technology.Focusing on the FSO application, this dissertation investigated the mechanism of atmospheric effects on the laser propagation and its correction technology. The dissertation consists of five parts as follows:Firstly, the mechanism of atmospheric effects on the laser propagation in FSO was theoretically studied. Some physical properties of atmosphere and atmospheric phenomena such as clouds and fogs were introduced. The problem of power attenuation caused by atmospheric absorption and scattering was analyzed. The physical model and the statistical characteristics of atmospheric turbulence were illustrated. Based on basic theories of the laser propagation through atmosphere, the turbulence-induced effects on the laser signal of FSO systems, such as intensity fluctuation, beam wander, beam spreading, angle of-arrival fluctuation and spot dispersion, were systematically discussed.Secondly, the mathematical model of bit error rate(BER) for IM/DD FSO system operating in the atmospheric channel was modified. According to the basic composition and fundamental principle of FSO system, the BER model was divided into three modules including atmospheric propagation, photoelectric detection and threshold decision. Considering the influence of atmospheric attenuation, beam spreading and intensity fluctuation, the atmospheric propagation module was applicable to weak turbulence conditions. The photoelectric detection module was based on the output characteristic of APD in the IM/DD FSO system. The influence of multiplicative noise was added to the threshold decision module. On the basis of the former three modules, the modified mathematical model of BER was proposed.Thirdly, a set of simulation experiment FSO system was designed and implemented, and experimental results such as scintillation index, BER and intensity probability distribution were obtained under various atmospheric conditions at a distance of1kilometer. By means of data processing, the influence of shot noise on the measurement of scintillation index was analyzed, and an improved method which is effective in low signal-to-noise ratio was proposed. By substitution of the experimental parameters into classical and modified BER mathematical models, the accuracy of the two models were compared. Furthermore, BER values calculated with maximum likelihood fitting distribution and lognormal distribution were both compared with measured values.Fourthly, based on the modified BER model, a kind of optimum threshold obtained by slow-varying statistics such as signal intensity and scintillation index was proposed, and the influence induced by turbulence was corrected. Characteristics of the optimum threshold were researched by numerical simulation and experimental measurements. For convenience, an approximate analytic expression of the optimum threshold was also obtained by means of nonlinear fitting. The results showed that the system performance can be improved by this kind of optimum threshold. The optimum threshold can lead to performance loss relative to ideal level; however it is acceptable for typical FSO communication systems operating under weak fluctuation conditions. If the signal intensity is kept constant, the fluctuation of the optimum threshold level can be neglected.Finally, the overflow problem of gamma-gamma distribution model in numerical simulation was analyzed, and two modified model by which overflow can be effectively avoided were deduced. A scheme of frequency-shift-keying (FSK) FSO system was designed, and the system performance was simulated by the Optisystem software. The simulation results show that the scheme is feasible.The results of this dissertation are not only applicable to the simulation research of BER performance for FSO systems, but also can provide reference for engineering design, site selection and correlative theoretical researches.