Information Security Risks Outside the Laser Beam in Terrestrial Free-Space Optical Communication

Consider a terrestrial free-space optical (FSO) communication system with a potential eavesdropper outside the laser beam who can capture the information through a non-line-of-sight scattering channel. We study the information security risks in this system under different atmospheric conditions and show that the risks can be neglected for the high-visibility condition while they become serious for the low-visibility situation. We further show the secrecy capacity when the eavesdropper changes locations and present a 2-D distribution. Finally, the secrecy capacity outage probability of a turbulence-induced FSO system is derived. It is found that the secure transmission system performance is very sensitive to the background radiation and required transmission rate under strong turbulence."?>.