## 3 Challenges in FSO Communication
### Jamming  
The wireless medium's inherent openness renders it vulnerable to both intentional and unintentional interference. A significant source of unintentional interference is interference from neighboring cells. Conversely, intentional interference, like jamming attacks, represents adversarial actions targeting a victim Rx [74]. Therefore, as wireless links, the reliability and security of the entire FSO system are susceptible to intentional jamming. This threat is particularly significant in military platforms or other security-sensitive communication protocols controlled by FSO networks, which can be targeted and disrupted by adversaries [75]. The impact of jamming on RF systems, including wireless multihop networks, has been extensively studied from various perspectives [76; 77; 78; 79; 74]. FSO systems are also vulnerable to jamming because the spectral width of the laser light in FSO communications is typically confined to specific wavelengths to ensure compatibility with optical fiber communication. Usually, FSO communication operates within very narrow wavelength ranges, such as 1300 nm or 1550 nm. Consequently, the operating frequency of a legitimate user link is readily accessible in FSO systems, making them highly susceptible to jamming attacks [75]. Additionally, in an FSO system, it is crucial to optimize the field-of-view (FoV) based on the specific application. The FoV is defined as the solid angle through which a detector can receive an incoming optical signal. As illustrated in Figure 7a, the optical signal emitted by a jammer is captured when it falls within the FoV; otherwise, it passes through unobstructed. Typically, the FoV of the receiving aperture is kept wide to maximize signal reception, mitigate the effects of fading due to AT and PE, address aperture misalignment issues, and prevent communication link failures caused by obstacles in the FSO system [75]. In tracking-based FSO communication systems, a wide FoV of up to 60\({}^{\circ}\) is employed to increase the tracking range and reduce optical aberrations [80]. Additionally, coarse PAT (CPAT) systems consider FoVs up to 70\({}^{\circ}\)[7; 81]. Due to the inherent fluctuations of UAVs, the optimal FoVs for UAV-based FSO links are significantly larger than those for ground-based FSO links [82]. Moreover, aperture averaging is an essential technique in FSO systems, as the unpredictable nature of the received signal caused by fading imposes severe constraints on system performance. Aperture averaging is a technique used to control and define the size of the detector or receiving aperture needed to capture the traversing optical field as effectively as possible. This process is particularly crucial for FSO systems operating in strong and saturated regimes. Aperture averaging helps to mitigate signal scintillation or irradiance flux variations (reducing fade probability), minimize the impacts of angle of arrival (AoA) fluctuations and boresight angle, and optimize the average signal power at the detector plane. Additionally, it serves as a primary method to ensure spatial diversity [46; 83]. A comprehensive discussion on aperture averaging is provided in Section 4.9. Therefore, to maintain effective FSO communication through the continuous establishment of transmission links, it is crucial to have a larger aperture diameter, which consequently results in a wide FoV [46; 82]. A narrow FoV restricts the operational coverage area, making it less ideal for FSO systems. Conversely, a wider FoV increases ambient light background noise, which can be mitigated through various methods to capture a larger portion of the incoming optical beam at the FSO Rx [82; 84]. Also, a higher FoV offers a broader coverage area and improved signal reception. For instance, a larger aperture diameter (8 inches) significantly reduces error probability in comparison with a smaller aperture diameter (2 inches) [46].