Hybrid Free Space Optical (FSO) and Radio Frequency (RF) networks promise highly available wireless broadband connectivity and quality of service (QoS), particularly suitable for emerging network applications involving extremely high data rate transmissions such as high quality video-on-demand and real-time surveillance. FSO links are prone to atmospheric obscuration (fog, clouds, snow, etc) and are difficult to align over long distances due the use of narrow laser beams and the effect of atmospheric turbulence. These problems can be mitigated by using adjunct directional RF links, which provide backup connectivity. In this paper, methodologies for modeling and simulation of hybrid FSO/RF networks are described. Individual link propagation models are derived using scattering theory, as well as experimental measurements. MATLAB is used to generate realistic atmospheric obscuration scenarios, including moving cloud layers at different altitudes. These scenarios are then imported into a network simulator (OPNET) to emulate mobile hybrid FSO/RF networks. This framework allows accurate analysis of the effects of node mobility, atmospheric obscuration and traffic demands on network performance, and precise evaluation of topology reconfiguration algorithms as they react to dynamic changes in the network. Results show how topology reconfiguration algorithms, together with enhancements to TCP/IP protocols which reduce the network response time, enable the network to rapidly detect and act upon link state changes in highly dynamic environments, ensuring optimized network performance and availability.
Hybrid free space optical/radio frequency (FSO/RF) networks promise broadband connectivity, high availability and quality of service (QoS), together with the capability of autonomous reconfigurability to deal with changing atmospheric and traffic conditions in dynamic environments. Nodes with n-connectedness (multiple transceivers) offer great flexibility in constructing new network topologies. Moreover, topologies using hybrid links are more effective in changing atmospheric conditions than those, using either communication modality alone. While FSO links can be expected to be available >99% of the time on links up to 1km in length, high performance RF provides backup connectivity in heavily obscured conditions. We have designed and implemented gimbal-mounted, hybrid FSO/RF nodes with combined apertures for joint pointing, acquisition, and tracking (PAT) operation. These nodes incorporate directional RF antennas for PAT network setup and management, and FSO links for very high data rate transmission. We describe these hybrid nodes and their performance, our hybrid network simulations, and our re-configurable network testbed for high data rate video transmission. Our simulations include realistic modeling of obscuration, traffic management, and topology control to deal with link non-availability and optimization of network performance. Hybrid, directional networks are scalable and provide low probability of intercept/detection (LPI/LPD) operation, especially in FSO mode.
Optical wireless networks are emerging as a viable, cost effective technology for rapidly deployable broadband sensor communication infrastructures. The use of directional, narrow beam, optical wireless links provides great promise for secure, extremely high data rate communication between fixed or mobile nodes, very suitable for sensor networks in civil and military contexts. The main challenge is to maintain the quality of such networks, as changing atmospheric
and platform conditions critically affect their performance. Topology control is used as the means to achieve survivable optical wireless networking under adverse conditions, based on dynamic and autonomous topology reconfiguration. The topology control process involves tracking and acquisition of nodes, assessment of link-state information, collection and distribution of topology data, and the algorithmic solution of an optimal topology. This paper focuses on
the analysis, implementation and evaluation of algorithms and heuristics for selecting the best possible topology in order to optimize a given performance objective while satisfying connectivity constraints. The work done at the physical layer is based on link cost information. A cost measure is defined in terms of bit-error-rate and the heuristics developed seek to form a bi-connected topology which minimizes total network cost. At the network layer a key factor is the traffic matrix, and heuristics were developed in order to minimize congestion, flow-rate or end-to-end delay.