| Free-space optical (FSO) communication is a new wireless broadband communication technology. Comparing with fiber communication and RF communication, FSO communication has many advantages for ourdoor optical wireless communication, such as no laying optical cables, low cost, short construction period, high bandwidth, small volume, good communication security, and so on. However, FSO links are always vulnerable to some effects from free-sapce. Scintillation caused by atmospheric turbulence is the most important effect, which leads to random fluctuation in the amplitude and phase. Another main effect is pointing errors which are the misalignment between transmitter and receiver. The errors are usually due to the random jitter caused by the platform. The random jitter of the beam will impact the normal communications due to the narrow laser beamwidth. It increases link error rate and affects the quality of communication link. In summary, the effects from scintillation and pointing errors have significantly limited the application of FSO communication technology. In order to improve the performance of FSO communication links, some works must be carried out to study the mechanism of these effects in FSO communication links. Some methods must be explored to suppress them. Therefore, this dissertation studies the performance of FSO links under the influence of scintillation, and then it investigates the performance of FSO links with the combined effects from scintillation and pointing errors. Furthermore, spatial diversity technology is investigated for suppressing this combined effect.Firstly, the basic principle and the research status of the FSO communication technology are described, meanwhile, the development of the beam atmospheric propagation theory, the researches on the performance of the FSO links with the effects from the scintillation caused by the atmospheric turbulence and the pointing errors caused by the platform jitter are summarized. The analysis of the relevant theories and calculations on the atmospheric optical propagation is given. The channel structure is introduced. The theory of the atmospheric turbulence, optical wave models in free-space, and basic theory atmospheric optical propagation are carefully described and analyzed.Secondly, the performance of FSO links under the effect of the scintillation is deeply studied. Base on the Gamma-Gamma channel model, the closed-form bit-error rate (BER) expressions for on-off keying (OOK), pulse position modulation (PPM), digital pulse interval modulation (DPIM), and circular polarization shift keying (CPolSK) have been given by considering direct detection. The performances of FSO links with these modulation schemes have been analyzed. Comparing with CPolSK direct detection systems, CPolSK coherent systems have been discussed by considering the average number of received signal photons per bit.Thirdly, quantitative differences in the size of large-aperture between CPolSK and OOK are further discussed. The theoretical model is proposed to analyze the quantitative differences with the impact from communication wavelength, link length, and the received average signal-to-noise ratio (SNR). These results are useful for the design of FSO systems.Fourthly, the performance of FSO links is systematically studied with considering the combined effects from scintillation and pointing errors,. The channel model with the combined effect is analyzed. The theoretical model is established for the analysis of the channel capacity with some important system parameters. The closed-form channel capacity expressions for direct detection and coherent detection systems have been derived, and the performances of two kinds of systems have been analyzed with different system parameters. The results indicate that the channel capacity is affected by the combined effect. Increasing the detector size, transmitter power, and decreasing the beamwidth, the channel capacity can be improved. For a given transmitter power, the design method for channel capacity optimization has been studied and the optimum system parameters have been given based on the theoretical model.Fivethly, spatial diversity, a method to suppress the combined effects, has been further studied. Based on K distribution channel model, the performances of the transmitter and receiver diversity have been analyzed, respectively. With the analysis on the performance of the spatial diversity under the effect of scintillation, the theoretical model is established for analyzing the performance of the spatial diversity systems by considering the combined effects from scintillation and pointing errors. The BER mathematical expressions have been given for transmitter diversity and receiver diversity with the three combining method (optimal combining, equal gain combining, and selection combining). The simulation results indicate the advantages of the spatial diversity.This dissertation has certain significance and reference value for the design of FSO communication systems. It provides basic theory for analyzing the performance FSO links under the combined effects from scintillation and pointing errors. It also provides experimental foundation for the realization of future FSO communication systems.
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