**3.3. Atmospheric attenuation**

Atmospheric attenuation is defined as the process whereby some or all of the electromagnetic wave energy is lost when traversing the atmosphere. Thus, atmosphere causes signal degra‐ dation and attenuation in a FSO system link in several ways, including absorption, scattering, and scintillation. All these effects are varying with time and depend on the current local conditions and weather. In general, the atmospheric attenuation is given by the following Beer's law equation [14]:

$$
\tau = \exp(-\beta L \ ),
\tag{7}
$$

where,

*τ* is the atmospheric attenuation;

*β* is the total attenuation coefficient and given as

$$
\beta = \beta\_{\text{abs}} \beta\_{\text{scat}};
\tag{8}
$$

*L* is the distance between transmitter and receiver (unit: km);

*β*abs is the molecular and aerosol absorption, this parameter value is considered as too small so, we can neglected;

*β*scat is the molecular and aerosol scattering.
