Beam shaping effects on MIMO free-space optical communication systems

Multiple Input Multiple Output (MIMO) systems are employed in Free Space Optical (FSO) communication links to improve the link reliability in the presence of atmospheric turbulence. In this thesis, we consider a MIMO FSO system with practical transmitter and receiver configurations that consists of a radial laser array with Gaussian beams and a detector array with Gaussian apertures. Using the extended Huygens-Fresnel principle in weak atmospheric turbulence, we have derived formulations to find the average power and the power correlations on the finite sized detectors. This lets us to quantify the performance metrics such as the power scintillation index, the aperture averaging factor and the average bit error rate () as a function of system parameters, i.e., transmitter and receiver ring radius, number of Gaussian laser beams, number of detectors, laser source size, detector aperture radius, degree of source coherence, link distance and the structure constant of atmosphere. At first, by the help of the derivations the performance of multiple-input single-output (MISO) FSO system is investigated using both for coherent and for partially coherent Gaussian sources. Then, we improve our derivations and investigate the performance of MIMO FSO systems. In this way, the performance of MIMO FSO system is compared to that of MISO FSO, single-input multiple-output (SIMO) FSO and single-input single-output (SISO) FSO systems. MISO systems are then employed in underwater wireless optical communication (UWOC) links to mitigate the degrading effects of oceanic turbulence. To quantify the scintillation index of the MISO UWOC system, the Huygens - Fresnel principle is used with the novel equivalent structure constant of atmosphere. The oceanic turbulence parameters such as rate of dissipation of mean-squared temperature, rate of dissipation of kinetic energy per unit mass of fluid, Kolmogorov microscale, the ratio of temperature to salinity contributions to the refractive index spectrum, link distance and the wavelength which are expressed by the novel equivalent structure constant of atmosphere. Using the Matlab program, we present graphs and investigate the effect of system parameters on the performance metrics.