Terrestrial free-space optical (FSO) communication systems, while designed to operate on large unlicensed optical bandwidths, are power-constrained due to strict eye safety regulations. The channel fluctuation inherent in terrestrial FSO links also limits the received optical power. Consequently, the available signal-to-noise ratio (SNR) per Hz could become limited; this holds for future terrestrial systems based on coherent optical communications. An efficient and adaptive transmission mechanism is thus crucial at the optical transmitter. However, a critical assessment of the impact of adaptive transmission in terrestrial FSO systems has received less attention in the literature. This work studies terrestrial coherent FSO communication systems employing adaptive beam transmission while detection receiver operate under shot noise-limited conditions. Specifically, we propose a novel exact closed-form expression for the average spectral efficiency of a coherent FSO system with optimal adaptive transmissions over the gamma-gamma turbulence channel with pointing errors. More importantly, we provide a detailed assessment of the impact of turbulence and pointing error impairments on the coherent FSO system performance, revealing several novel and counterintuitive insights. In particular, the extensive numerical results help elucidate the intricacies of analyzing these terrestrial FSO systems and clarify a few misconceptions alluded to in recent related literature.

翻译：地面自由空间光（FSO）通信系统虽然设计用于在无许可的大带宽光频段上工作，但由于严格的眼安全规定而受到功率限制。地面FSO链路固有的信道波动也限制了接收光功率。因此，每赫兹可用信噪比（SNR）可能变得有限；这对于未来基于相干光通信的地面系统同样适用。因此，在光发射端采用高效的自适应传输机制至关重要。然而，文献中对自适应传输在地面FSO系统中影响的严格评估关注较少。本研究探讨了采用自适应光束传输的地面相干FSO通信系统，其检测接收机在散粒噪声受限条件下工作。具体而言，我们针对具有指向误差的伽马-伽马湍流信道上的最优自适应传输相干FSO系统，提出了一种新颖的精确闭式平均频谱效率表达式。更重要的是，我们详细评估了湍流和指向误差损伤对相干FSO系统性能的影响，揭示了若干新颖且反直觉的见解。特别是，广泛的数值结果有助于阐明分析这些地面FSO系统的复杂性，并澄清了近期相关文献中提及的一些误解。