The performances of fiber-based free-space optical (FSO) communications over gamma-gamma distributed turbulence are studied for multiple aperture receiver systems. The equal gain combining (EGC) technique is considered as a practical scheme to mitigate the atmospheric turbulence. Bit error rate (BER) performances for binary-phase-shift-keying-modulated coherent detection fiber-based free-space optical communications are derived and analyzed for EGC diversity receptions through an approximation method. To show the net diversity gain of a multiple aperture receiver system, BER performances of EGC are compared with a single monolithic aperture receiver system with the same total aperture area (same average total incident optical power on the aperture surface) for fiber-based free-space optical communications. The analytical results are verified by Monte Carlo simulations. System performances are also compared for EGC diversity coherent FSO communications with or without considering fiber-coupling efficiencies.

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Quadrature amplitude modulation (QAM) is an exciting technology to accomplish a high communication data rate without increasing the system bandwidth, which can undoubtedly greatly improve the performance of free-space optical (FSO) communication systems. With the increase of experimental and theoretical results about atmospheric turbulence, scientists have confirmed that the anisotropy and non-Kolmogorov property cannot be ignored for atmospheric turbulence. In this paper, utilizing the new anisotropic non-Kolmogorov (ANK) turbulence spectrum models for the satellite links that are applicable to both vertical and slant links, we derive the average bit error rate (BER) expression of rectangular QAM for a Gaussian-beam transmission through the weak ANK ground-to-satellite uplink under the influences of both scintillation and beam wander. Numerical results display that, when the zenith angle is less than a certain angle, the average BER of the ground-to-satellite uplink decays with the enhancement of the anisotropic factor, but while the zenith angle is greater than the certain angle, the contrary trend will occur. Our work will benefit the design optimization of QAM/FSO communication systems in ANK satellite links.

With the deepening of atmospheric turbulence research, scientists have discovered that the anisotropy of turbulence cells cannot be ignored. The anisotropic non-Kolmogorov turbulence model is more in line with the actual situation of atmospheric turbulence. However, the recent experimental results of Korotkova et al. and Beason et al. display that the turbulence cell has an anisotropic tilt angle, that is, the long axis of the turbulence cell may not be horizontal to the ground but has a certain angle with the ground. Therefore, it is urgent to analyze whether the anisotropic tilt angle is necessary in the turbulence spectrum. In this study, we develop the anisotropic non-Kolmogorov turbulence spectra for the horizontal and vertical links in the presence of the anisotropic tilt angle. Then, based on these spectra, the analytical expressions of the scintillation index for the horizontal and vertical links are derived under weak fluctuation condition. The results indicate that the anisotropic tilt angle is necessary in the vertical link but not in the horizontal link.