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.

翻译：与移动无线信道类似，发射功率控制能够在地面自由空间光（FSO）信道中实现稳健且频谱高效的大气湍流通信。由于光学带宽超过数GHz，且人眼安全法规限制了发射光功率，地面FSO系统的每赫兹信噪比（SNR）可能受限，这对于未来高带宽、长距离应用尤为明显。因此，功率控制在地面FSO系统中变得至关重要。然而，现有文献缺乏对FSO系统中动态功率适配的全面研究。本文研究了具备光束功率控制能力的FSO通信系统，其接收机采用外差检测与直接检测方式，并在散粒噪声受限条件下工作。基于这些考量，我们推导了存在与不存在指向误差时伽马-伽马湍流信道的统一精确容量公式及渐近容量公式；这些新颖的闭式容量表达式为理解不同湍流条件与指向误差的影响提供了新视角。此外，数值结果揭示了典型地面FSO信道中大气湍流与指向误差参数间的复杂关联。通过对关键信道参数对上述FSO系统容量性能影响的具体评估，研究揭示了若干新颖且具有启发性的现象。