4. Turbulence models

One of the most crucial steps in the attempts to mitigate the degradation in the performance of optical communication systems is accurate modeling of the atmospheric turbulence under different conditions. Below are some of the irradiance functions presented in terms of probability distribution functions.

#### 4.1 Lognormal distribution

The lognormal distribution is one of the most widely employed for weak atmospheric turbulence distribution. Here, the irradiance value received at the receiver follows the distribution [9]

$$f\_A(a) = \frac{1}{2a\left(2\pi\sigma\_x^2\right)^{\frac{1}{2}}} \exp\left(-\frac{\left(\ln a + 2\sigma\_x\right)^2}{8\sigma\_x^2}\right) \tag{1}$$

where μ<sup>x</sup> is the mean value of fading and σ<sup>2</sup> <sup>x</sup> is the fading covariance.

Figure 1. Some turbulence mitigation techniques in FSO systems.
