In terrestrial free-space optical (FSO) communication systems, adaptive power control at the optical laser transmitters is crucial not only to prolong the life span of the laser sources, but more importantly to maintain robust and spectrally efficient communication through atmospheric turbulence. However, a comprehensive study of dynamic power adaptation in existing FSO systems is lacking in the literature. In this paper, we investigate FSO communication systems capable of adaptive laser power control with heterodyne detection (HD) and direct detection (DD) based receivers operating under shot-noise-limited conditions. Under these FSO systems considerations, we derive unified exact and asymptotic formulas for the capacities of Gamma-Gamma atmospheric turbulence channels with and without pointing errors; these novel closed-form capacity expressions are much simpler and provide new insights into the impact of varying turbulence conditions and pointing errors. Finally, the numerical results highlight the intricate relations of atmospheric fading, pointing error, and large-scale channel parameters in a typical terrestrial FSO channel setting, followed up by an accurate assessment of the key parameters determining the capacity performances of the aforementioned FSO systems revealing several interesting characteristics.

翻译：在地基自由空间光通信系统中，自适应光激光发射器功率控制不仅对延长激光源寿命至关重要，更重要的是能通过大气湍流维持鲁棒且高频谱效率的通信。然而，现有文献尚未对现有FSO系统中的动态功率适配进行系统性研究。本文研究具备自适应激光功率控制能力的FSO通信系统，该系统采用外差检测和直接检测接收机，并在散粒噪声受限条件下工作。针对上述FSO系统特性，我们推导出有/无指向误差的伽马-伽马大气湍流信道容量的统一精确与渐近公式；这些新颖的闭式容量表达式形式更简洁，为不同湍流条件与指向误差的影响提供了新见解。最终，数值结果揭示了典型地基FSO信道场景中大气衰落、指向误差与大尺度信道参数之间错综复杂的关系，并准确评估了决定上述FSO系统容量性能的关键参数，展现出若干有趣的特性。