📝 SummarXiv

Abstract

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 long-haul terrestrial systems based on coherent optical communications. An efficient and adaptive transmission mechanism is thus crucial at the optical source. 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 FSO communication systems employing adaptive beam transmission while detection receivers operate under shot noise-limited conditions. Specifically, we propose a novel exact closed-form expression for the average spectral efficiency of these FSO systems with transmit beam power adaptation over the gamma-gamma turbulence channel with pointing error. More importantly, we provide a detailed assessment of the impact of turbulence and pointing error impairments on the performance of the aforementioned FSO systems, 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.