Terrestrial free-space optical (FSO) communication systems, while designed to operate on large unlicensed optical bandwidths, are fundamentally power-constrained due to stringent eye-safety regulations. Moreover, channel fluctuations inherent to terrestrial FSO links further reduce the received optical power. Consequently, the achievable signal-to-noiseratio (SNR) per hertz could become limited -- especially for future terrestrial FSO systems based on coherent communications. This necessitates the development of efficient and adaptive communication strategies at both the optical transmitter and receiver. However, a comprehensive assessment of adaptive coherent terrestrial FSO systems remains largely unexplored in the existing literature. This paper investigates terrestrial FSO communication links employing adaptive coherent transmission and synchronous heterodyne detection-based reception operating in the shot-noise-limited regime. Specifically, we propose a novel exact closed-form expression for the average spectral efficiency limit of coherent FSO communication systems with optimal adaptive signaling over gamma-gamma turbulence channels 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. 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：地面自由空间光通信系统虽然设计用于在大型无许可光学带宽上运行，但由于严格的人眼安全规定，本质上受到功率限制。此外，地面自由空间光链路固有的信道波动进一步降低了接收光功率。因此，每赫兹可实现的信噪比可能变得有限——特别是对于未来基于相干通信的地面自由空间光系统而言。这需要在光发射机和接收机两端开发高效的自适应通信策略。然而，现有文献中对自适应相干地面自由空间光系统的全面评估在很大程度上仍未得到探索。本文研究了在散粒噪声限制区域内运行、采用自适应相干发射和基于同步外差检测的接收的地面自由空间光通信链路。具体而言，我们针对具有指向误差的伽马-伽马湍流信道上采用最优自适应调制的相干自由空间光通信系统，提出了一种新颖的精确闭式平均频谱效率极限表达式。更重要的是，我们详细评估了湍流和指向误差损伤对相干自由空间光系统性能的影响，揭示了一些新颖且反直觉的见解。大量的数值结果有助于阐明分析这些地面自由空间光系统的复杂性，并澄清了近期相关文献中提及的一些误解。