3 dB/km for multimode fiber cables and 0.2 to 0.5 dB/km in the case of single-mode fiber cables. In contrast, the atmospheric attenuation of laser power in optical wire- less transmission can vary dramatically and is difficult to predict. Atmos- pheric attenuation loss spans from 0.2 dB/km in exceptionally clear weather to 350 dB/km in a very dense fog. The main reason behind this phenomenon is that the moisture (fog) particles are SO tiny and dense that they act as millions of tiny prisms, and when a band of light shines through them, the signal is distorted and dissipated. The effect of fog on FSO atmospheric attenuation is comparatively more severe than rain. This is because the fog droplet radius (5 to 20 um) is on the order of the laser wavelengths that results in noticeable attenuation (up to 350 dB/km) compared to the attenuation (20 dB/km) introduced by the rain droplet with sizes between 200 and 2,000 um. Therefore, the effect of weather attenuation on FSO laser power is considered one of the major obstacles for deployment because it reduces the uptime or availability of FSO communication systems. In order to explain the underlying reason for atmospheric attenuation in further detail, it is essential to state the two types of atmospheric impair- ment in FSO systems that cannot be neglected. The first type of impair- ment is described as scintillation, which by definition is the intensity fluctuation in the received optical signal. Scintillation causes error bursts during periods of atmospheric turbulence that is originated by temperature gradients near the ground, 4 resulting in localized random pockets of vary- ing indices of refraction. When the size of these scintillation pockets is smaller than the diameter of the laser beam in an optical transceiver, the Chapter 9 optical beam experiences distortion, which is observable as a nonuniform optical intensity across the wave front (see Figure 9-9).4 On the other hand, when the size of the pockets is larger than the diam- eter of the laser beam, the beam w