**3.1. Aerosol**

1° ≈17

172 Contemporary Issues in Wireless Communications

*θ* is a divergence angle between transmitter and receiver FSO units.

**Figure 6.** Small angles – divergence and spot size between transmitter and receiver.

**Divergence Range Spot Diameter** 0.5 mrad 1.0 km ~ 0.5 m (~ 20 in) 2.0 mrad 1.0 km ~ 2.0 m (~ 6.5 ft) 4.0 mrad (~ ¼ deg) 1.0 km ~ 4.0 m (~ 13.0 ft)

**3. Mathematical model of atmospheric turbulence**

**Table 1.** The divergence, range, and spot diameter.

of the signal around the mean.

mrad→1 mrad

The geometric path loss for an FSO link depends on the beam-width of the optical transmitter, the path length (*L*), and the divergence angle (*θ*). Transmitter and receiver aperture diameters are quantifiable parameters, and are usually specified by manufacturer. Table (1) illustrates the relation of divergence in (mrad), range in (km), and spot diameter in (inches or feet).

The atmospheric attenuation is one of the challenges of the FSO channel, which may lead to signal loss and link failure. The atmosphere not only attenuates the light wave but also distorts and bends it. Transmitted power of the emitted signal is highly affected by scattering and turbulence phenomena. 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

≈ 0.0573°

Aerosols are particles suspended in the atmosphere with different concentrations. They have diverse nature, shape, and size. Aerosols can vary in distribution, constituents, and concen‐ tration. As a result, the interaction between aerosols and light can have a large dynamic, in terms of wavelength range of interest and magnitude of the atmospheric scattering itself. Because most of the aerosols are created at the earth's surface (e.g., desert dust particles, human-made industrial particulates, maritime droplets, etc.), the larger concentration of aerosols is in the boundary layer (a layer up to 2 km above the earth's surface). Above the boundary layer, aerosol concentration rapidly decreases. At higher elevations, due to atmos‐ pheric activities and the mixing action of winds, aerosol concentration becomes spatially uniform and more independent of the geographical location. Scattering is the main interaction between aerosols and a propagating beam. Because the sizes of the aerosol particles are comparable to the wavelength of interest in optical communications, Mie scattering theory is used to describe aerosol scattering [8].


**Table 2.** Radius ranges for various types of particles.

Such a theory specifies that the scattering coefficient of aerosols is a function of the aerosols, their size distribution, cross section, density, and wavelength of operation. The different types of atmospheric constituents' sizes and concentrations of the different types of atmospheric constituents are listed in Table (2) [7,9].
