## 3 Challenges in FSO Communication
### FSO System and Channel Models
#### 3.2.2 FSO Channel Models  
This section provides an overview of various channel models that can be utilized to predict the performance of FSO systems. These models are essential for understanding how different environmental factors and conditions affect the effectiveness and reliability of FSO communications. The models incorporate PDFs derived from turbulence intensity to characterize the impact of turbulence on FSO signals. Analyzing these models allows us to gain insights into how factors like AT, weather conditions, and signal attenuation impact system performance, enabling us to optimize and better anticipate the behavior of FSO systems in real-world scenarios. As an optical beam travels through the atmosphere, it experiences atmospheric loss. The signal power attenuation follows the exponential Beer-Lambert law and is expressed as [3]  
\[h_{\ell}(\lambda,z)=\frac{P(\lambda,z)}{P(\lambda,0)}=\exp(-\sigma(\lambda)z) \tag{8}\]  
where \(h_{\ell}(\lambda,z)\) represents the loss as a function of the propagation path length \(z\) at wavelength \(\lambda\). The signal power at a distance \(z\) and the emitted power at \(z=0\) are expressed as \(P(\lambda,z)\) and \(P(\lambda,0)\), respectively. The total extinction coefficient or the attenuation coefficient \(\sigma(\lambda)\) per unit length is defined by [3]  
\[\sigma(\lambda)=\alpha_{m}(\lambda)+\alpha_{a}(\lambda)+\beta_{m}(\lambda)+ \beta_{a}(\lambda), \tag{9}\]  
where \(\alpha_{m,a}\) are the absorption coefficients for molecules and aerosols, respectively, and \(\beta_{m,a}\) represent the scattering coefficients for molecules and aerosols, respectively. The attenuation \(h_{\ell}\) is considered a constant scaling factor over an extended period, ensuring consistent behavior. Additionally, it depends on the size and distribution of scattering particles, along with the wavelength used. This attenuation can be described regarding visibility, which is directly measurable from the atmosphere. Empirically, attenuation is characterized in relation to visibility as [3]  
\[\sigma(\lambda)=\frac{3.912}{V}\bigg{(}\frac{\lambda}{550}\bigg{)}^{-q}, \tag{10}\]  
where \(V\) represents visibility (in kilometers), and \(q\) is a parameter influenced by the particle size distribution in the atmosphere as described by the Kruse model [90]:  
\[q= \begin{cases}1.6&V>50\text{ km}\\ 1.3&6\text{ km}<V<50\text{ km}\\ 0.585V^{1/3}&V<6\text{ km}\end{cases}  \tag{11}\]  
Additionally, Kim introduces an extended model designed to improve accuracy in low-visibility situations. The Kim model is described as [90]  
\[q= \begin{cases}1.6&V>50\text{ km}\\ 1.3&6\text{ km}<V<50\text{ km}\\ 0.16V+0.34&1\text{ km}<V<6\text{ km}\\ V-0.5&0.5\text{ km}<V<1\text{ km}\\ 0&V<0.5\text{ km}\end{cases}  \tag{12}\]  
Pointing Error or Misalignment Fading  
An FSO link is a LoS communication system that uses a narrow optical beam, which requires precise pointing accuracy to ensure the effective performance and reliability of optical systems [16]. PE and signal degradation at the Rx are typically caused by wind loads and thermal expansions, leading to random movements of the building.