Coding for Free-space Optical Channels

Abstract

High bandwidth demands in metropolitan area networks (MANs) and requirements for flexible and cost-effective service cause the imbalance known as the “last mile bottleneck.” Fiber-optics, RF, and copper/coaxial lines are the main state-of-the-art technologies to address the high bandwidth requirements. The incompatibility of RF/microwave and optical communication technologies due to large bandwidth mismatch between RF and optical channels is now widely believed to be the limiting factor in efforts to further increase the transport capabilities in the future. For this reason, RF/microwave–optical interface solutions that will enable aggregating multiple RF/microwave channels into an optical channel are becoming increasingly important. Free-space optical (FSO) communication is the technology that can address any connectivity needed in optical networks, be in a core, edge, or access [1, 2]. In MANs, the FSO can be used to extend the existing MAN rings; in enterprise, the FSO can be used to enable local area network (LAN)-to-LAN connectivity and intercampus connectivity; and the FSO is an excellent candidate for the last-mile connectivity. However, an optical wave propagating through the air experiences fluctuations in amplitude and phase due to atmospheric turbulence. The intensity fluctuation, also known as the scintillation, is one of the most important factors that degrade the performance of an FSO communication link, even under the clear sky condition. FSO links are considered as a viable solution for the last mile bottleneck problem because of the following properties: (1) high directivity of the optical beam provides high power efficiency and spatial isolation from other potential interferers, a property not inherent in RF/microwave communications, (2) the FSO transmission is unlicensed, (3) large fractional-bandwidth coupled with high optical gain using moderate powers permits very high-data rate transmission, (4) the state-of-the art fiber-optics-communications employ intensity modulation with direct detection (IM/DD), and the components for IM/DD are widely available, and (5) the FSO links are relatively easy to install and easily accessible for repositioning when necessary. The FSO communication may also be considered as an enabling technology to bring different technologies together and easy to integrate with variety of interfaces and network elements.