Unified Capacity Results for Free-Space Optical Communication Systems Over Gamma-Gamma Atmospheric Turbulence Channels

Transmit power control, as in the mobile wireless channels, can enable a robust and spectrally efficient communication through atmospheric turbulence in terrestrial free-space optical (FSO) channels. With optical bandwidths in excess of several GHz and eye safety regulations limiting the transmit optical power, the per hertz signal-to-noise ratio (SNR) in terrestrial FSO systems can possibly become limited, especially true for future high-bandwidth and long-haul applications. Hence, power control becomes significant in terrestrial FSO systems. However, a comprehensive study of dynamic power adaptation in the existing FSO systems is lacking in the literature. In this paper, we investigate FSO communication systems capable of beam power control with heterodyne detection and direct detection based receivers operating under shot noise-limited conditions. Under these considerations, we derive unified exact and asymptotic capacity formulas for the Gamma-Gamma turbulence channels with and without pointing errors; these novel closed-form capacity expressions provide new insights into the impact of varying turbulence conditions and pointing errors. Further, the numerical results highlight the intricate relations of atmospheric turbulence and pointing error parameters in typical terrestrial FSO channel setting. A concrete assessment of the impact of the key channel parameters on the capacity performances of the aforementioned FSO systems is performed revealing several novel and interesting insights.