Contents I. Introduction
Optical wireless (OW) technology has drawn considerable attention as a means of implementing reliable high-capacity outdoor systems that may not be implemented by conventional fiber optics [1]. Hero experiments involve intersatellite or satellites-to-earth links that have demonstrated the potential of OW system utilization in extreme application scenarios [2], culminating in the recent moon-to-earth OW communication [3]. Outdoor OW are equally important in down-to-earth systems that are used to interconnect buildings or business networks in urban environments at very high capacities that are comparable with fiber-based solutions [4]– [6]. The widespread application of outdoor OW, however, is hindered by atmospheric effects that induce a severe penalty on the link budget. Fog, snow, rain and air pollution account for a relatively static loss in the OW channel that needs to be compensated by amplification, coding and/or diversity [7]– [9], while the existence of variable temperature air pockets in the transmission path imparts a time-varying loss that is generally described as fading. Fading is also typically tackled by providing an adequate link margin, while a number of techniques have been proposed with a goal of reducing the impact of fading and minimizing the margin that is required. Beam focusing [10], aperture averaging [11]– [13], spatial and temporal diversity [12], [14]– [21], coding [21]– [25], relaying [26]– [28] and amplification [28]– [31] are candidate techniques that have been proposed to effectively minimize the impact of fading and contribute to more reliable outdoor OW links.
