Topology Control and Pointing in Free Space Optical Networks

Title of Document: TOPOLOGY CONTROL AND POINTING IN FREE SPACE OPTICAL NETWORKS Yohan Shim, Doctor of Philosophy, 2007 Directed By: Associate Professor Steven A. Gabriel Department of Civil and Environmental Engineering Free space optical (FSO) communication provides functionalities that are different from fiber optic networks and omnidirectional radio-frequency (RF) wireless communications in that FSO is optical wireless (no infrastructure installation cost involving fibers) and is highly directional (no frequency interference). Moreover, its high-speed data transmission capability is an attractive solution to the first or last mile problem to bridge to current fiber optic network and is a preferable alternative to the low data rate directional point-to-point RF communications for inter-building wireless local area networks. FSO networking depends critically on pointing, acquisition and tracking techniques for rapidly and precisely establishing and maintaining optical wireless links between network nodes (physical reconfiguration), and uses topology reconfiguration algorithms for optimizing network performance in terms of network cost and congestion (logical reconfiguration). The physical and logical reconfiguration process is called Topology Control and can allow FSO networks to offer quality of service by quickly responding to various traffic demands of network users and by efficiently managing network connectivity. The overall objective of this thesis research is to develop a methodology for self-organized pointing along with the associated autonomous and precise pointing technique as well as heuristic optimization methods for Topology Control in biconnected FSO ring networks, in which each network node has two FSO transceivers. This research provides a unique, autonomous, and precise pointing method using global positioning systems (GPS) and local angular sensors (e.g., tilt sensors or inertial navigation systems), which is applicable to both mobile and static nodes in FSO networking and directional point-to-point RF communications with precise tracking. Through medium (264 meter) and short (40 meter) range pointing experiments using an outdoor testbed on the University of Maryland campus in College Park, sub-milliradian pointing accuracy is presented. The short range experiment involves an automatic pointing system implemented by real-time kinematic GPS, bi-axial tilt sensors, a helium-neon laser ( 633 λ = nanometers) and a two-axis gimbal, which shows that the automatic pointing system is reliable. In addition, this research develops fast and accurate heuristic methods for autonomous logical reconfiguration of bi-connected ring network topologies as well as a formal optimality gap measure tested on an extensive set of problems. The heuristics are polynomial time algorithms for a congestion minimization problem at the network layer and for a multiobjective stochastic optimization of network cost and congestion at both the physical and network layers. TOPOLOGY CONTROL AND POINTING IN FREE SPACE OPTICAL NETWORKS