Hybrid Radio Frequency (RF) and Free Space Optics (FSO) communication systems represent a promising solution for modern communication challenges, offering the robustness of RF and the high data rates of FSO. As these systems become increasingly important in applications ranging from last-mile connectivity to satellite communication, ensuring their security, particularly at the physical layer, has emerged as a critical concern. This survey explores the state of the art in physical layer security (PLS) for mixed RF-FSO systems, highlighting key techniques, challenges, and solutions. We also outline future research directions to enhance the security and performance of these hybrid communication systems. By integrating insights from the latest research, this paper aims to provide a comprehensive understanding of the current landscape and future potential of PLS in mixed RF-FSO systems.

As wireless communication networks evolve to support more devices and higher data rates, ensuring secure communication has become increasingly critical. Physical Layer Security (PLS) offers a promising approach to safeguarding data transmission by exploiting the physical properties of the communication channel. However, implementing PLS techniques often requires significant energy resources, which can be a limiting factor, particularly in resource-constrained environments like IoT networks. Energy harvesting (EH) technology presents a viable solution by enabling devices to harness ambient energy, thereby supporting the continuous operation of PLS mechanisms. This paper explores the role of energy harvesting in enhancing physical layer security, discussing various EH techniques, their integration with PLS strategies, and the challenges and future directions in this emerging area of research.