Free space optical communication has received a great deal of attention in some applications like military, civilian and educational research on recent decades. The advantages of this technology including high power efficiency, quick installation, portability, high security in data transmission against eavesdropping, impervious to electromagnetic interference and high speed data transmission are some prominences of it against other communication technologies, and also are reasons of replacing other technologies with this technology in some communication applications like last mile connection, inter-satellite communications, underwater communications and terrestrial optical communications. Despite of its good features, there are a number of problems that FSO links confront. They are incding line-of-sight requirement, perpetual transmitter/receiver alignment, signal fading due to atmospheric turbulence and signal attenuation due to light absorption and scattering. Absorption of optical signals is due to the presence of water droplets and carbon dioxide within the atmosphere, where as scattering is due to fog and haze, as well as rain and snow. The coefficient of scattering depends on the optical wavelength and the size of scattering particles. Absorption of optical signals is due to the presence of water droplets and carbon dioxide within the atmosphere, where as scattering is due to fog and haze, as well as rain and snow. The coefficient of scattering depends on the optical wavelength and the size of scattering particles. Dense fog is the most dominant weather effect, resulting in over 300 dB/km attenuation coefficient so the presence of dense fog is confining the achievable link range (distance) to about 100 meters and the common way to increase FSO range in this situation is using RF back up for link. Another channel effect that mentioned prior is signal fading due to atmospheric turbulence. Atmospheric turbulence causes random fluctuation of the intensity of the optical radiation. The main reason of atmospheric turbulence is temperature variation of air that consequently changes atmospheric refractive index. This phenomenon is called scintillation. The typical fades due to atmospheric turbulence is about 100 miliseconds and such this fade could deteriorate link outage and increase its BER and high power penalty. These effects make detecting process difficult or even impossible. In other words system performance extremely depends on the weather status. The effect of these shortcomings can be diminished by some ways such as increasing the transmitter power or receiver sensitivity, increasing the number of transmitters or receivers, information coding, etc In this thesis, a complete procedure of design and simulation of a free space optical communication system is considered. Simulation results are compared to laboratory tests results (for 2m distance) to evaluate the accuracy of simulations. In presented simulator all parameters that affect the transmitted light beam including noise of transmitter, receiver and background, effect turbulence, channel attenuation and offsets of transmitter and receiver, are studied. The results show that the simulation results are in good compatibility with laboratory tests results, and also show that atmosphere conditions and the offsets of transmitter and receiver that ruin the straight trajectory of system, exteremly affect systems function. Key words: FSO, turbulence, attenuation, simulation.
