**3. Formulations**

Free space optical communication link requires a good understanding of the atmosphere as the laser beam has to propagate through it. The atmosphere not only attenuates the light wave but also distorts and bends it. Attenuation is primarily the result of absorption and scattering by molecules and particles (aerosols) suspended in the atmosphere. Distortion, on the other hand, is caused by atmospheric turbulence due to index of refraction fluctuations. Attenuation affects the mean value of the received signal in an optical link whereas distortion results in variation of the signal around the mean. Often, atmospheric attenuation can be the limiting factor in an optical communication link through the atmosphere [16].
