## 3 Challenges in FSO Communication
### Atmospheric Channel Effects Phenomena
#### 3.1.4 Reliability and Security Concerns  
FSO communication inherently benefits from enhanced security due to its highly directional and narrow optical beams, making interception challenging without direct physical access to the LoS path. However, security concerns in FSO communication, such as eavesdropping, jamming, and physical obstruction, can significantly impact the propagation and reliability of optical signals [2]. These challenges must be addressed through a combination of advanced encryption, AO, dynamic beam management, and robust system design. By mitigating these security risks, FSO communication can offer a secure and efficient alternative to traditional RF-based systems, particularly in applications where high data rates and secure transmission are paramount. This section provides a detailed discussion of these FSO security concerns. Eavesdropping  
Similar to wired links, wireless optical transmission is inherently more secure than RF transmission due to the high directionality of optical beams as opposed to the almost broadcast nature of RF signals, making them significantly harder to intercept, though not impossible [48]. However, there are scenarios where the threat of eavesdropping from one or more malicious actors may arise. Eavesdropping in FSO communication involves intercepting the optical beam to gain unauthorized access to the transmitted data. This can happen because of AT, optical beam divergence, channel scattering, and PE. Consequently, adversaries might access information through scattering over a non-LoS (NLoS) channel and scattering over a LoS channel caused by aerosol particles. Eavesdropping may be feasible if both the legitimate and the adversary Rxs are positioned close to each other within the FSO links [48; 49; 50]. Additionally, eavesdropping is possible when part of the beam radiation is reflected by small particles and detected by an external observer not in the LoS of either communication peer. However, the power received by the eavesdropper in such cases will be significantly less than in an equivalent RF scenario. An alternative scenario involves the eavesdropper blocking the laser beam to collect more power. In this case, the legitimate Rx would notice a significant decrease in the average received power, prompting the communication to be stopped for security reasons [48]. Traditionally, upper-layer cryptographic algorithms, which assume a limited computational capability of eavesdroppers, have been sufficient to protect confidential messages. However, due to the complexity of key generation and allocation, along with the current advanced hardware available to eavesdroppers, these cryptographic algorithms may not meet future wireless network security requirements. In addition to these upper-layer algorithms, physical (PHY) layer key generation and encryption methods, which exploit channel reciprocity to generate key packets [51], and PHY layer wiretap coding, which leverages the difference in channel quality between the Tx-to-Rx and Tx-to-eavesdropper channels to protect confidential messages [52; 53], can be employed to secure confidential messages in the FSO-RF relay network [50]. The challenge of ensuring secure communications in the presence of external eavesdroppers is a well-known issue in digital communications. There is increasing interest in the PHY layer secrecy performance over fading channels, leading the research community to focus on PHY layer security (PLS) as a means to avoid eavesdropping without relying on complex encryption techniques in the upper layers [54]. Consequently, the PLS of FSO links has been studied over \(\mathcal{M}\)[54; 55; 56] and \(\Gamma\Gamma\) turbulence channels [48]. Additionally, ref. [57] explores the PLS of a LoS FSO link employing Orbital Angular Momentum (OAM) multiplexing. The information security risks of an FSO communication system with an eavesdropper outside the laser beam, who could obtain information through a non-LoS scattering channel, are examined in [58].