Coherent free-space optical (FSO) communication is recognized as a key enabler for ultra-high-capacity fronthaul and backhaul links in next-generation wireless networks. Spectrally efficient $M$-ary quadrature amplitude modulation (MQAM) formats are well-suited for these links. However, theoretical analyses of adaptive MQAM transmissions over terrestrial FSO channels remain limited. In this letter, we first derive the spectral efficiency limit of adaptive unconstrained MQAM over gamma-gamma turbulence with pointing error. We then show that adaptive transmissions using only six square MQAM constellations performs close to the theoretical limit (within $0.10$-$0.12$ bits/s/Hz) across a wide range of signal-to-noise ratios and channel conditions.

翻译：相干自由空间光通信被认为是下一代无线网络中实现超高容量前传与回传链路的关键技术。频谱效率高的$M$进制正交幅度调制格式非常适合此类链路。然而，针对陆地自由空间光信道的自适应MQAM传输的理论分析仍较为有限。本文首先推导了在存在指向误差的伽马-伽马湍流信道中，无约束自适应MQAM的频谱效率极限。随后证明，仅使用六种方形MQAM星座的自适应传输方案，在广泛的信噪比与信道条件下，其性能可接近理论极限（差距在$0.10$-$0.12$ bits/s/Hz以内）。