Free Space Optical Communications Security and Reliability Trade-off: A Survey

The security and reliability trade-off (SRT) of free space optical (FSO) communications are the most critical features that should receive significant attention, especially with the continued development of optical wireless communications. FSO communication systems covering an ultra-broad range of unlicensed spectrum have emerged as a promising solution to mitigate conventional radio frequency (RF) spectrum scarcity ranging from a few meters to several kilometers in communication distance. Despite the potential advantages of FSO technology, its link reliability degrades due to atmospheric turbulence and other environmental factors including fog, sands, heavy rain, pointing errors, and scintillation. Moreover, although FSO communications are considered more secure than RF communications in general, various threats compromise the security of FSO communications. Therefore, this paper is motivated to examine the security and reliability vulnerabilities imposed by the external threats and channel effects for improving the FSO systems. This survey provides an overview of several crucial aspects of FSO technology, reliability, and security, illustrating the trade-off between security and reliability, and the implications of using the opportunistic schedule schemes to enhance the diversity gain, security, and reliability of FSO communications. Additionally, this paper includes a table outlining previous works regarding these subjects.