Stuart D. Milner

Hybrid free space optical/RF networks for tactical operations

We discuss our research and development in high availability, high bandwidth, tactical communications and networks using hybrid (free-space optical (FSO)) and RF links. To achieve this, the primary high data rate bridges use free space laser links, in parallel with high performance RF links. To manage this dynamic network, in which nodes may lose connectivity in the optical domain, the RF domain, or both, requires an innovative approach to network operation. The software that controls the network, must be aware of the actual and potential physical connectivity of the network, must exploit connectivity to maximize quality of service (QoS), and be able to handle the constant changeover on individual links between optical and RF connectivity. The key technologies under research and development include: topology and diversity control software; hardware for pointing, acquisition and tracking (PAT); and a combined aperture FSO/RF transceiver with joint PAT.

Aperture averaging for optimizing receiver design and system performance onfree-space optical communication links

Feature Issue on Optical Wireless Communications (OWC) We have developed a flexible, empirical approach for optimizing the design of free-space optical communication links by using multiframe image analysis of received intensity scintillation patterns. This is a versatile way to perform aperture-averaging analysis. A high-performance digital camera with a frame-grabbing computer interface is used to capture received intensity distributions of a He-Ne laser beam propagating in weak and intermediate turbulence conditions. The aperture-averaging results demonstrate the expected reduction in intensity fluctuations due to increasing the receiver aperture diameter for various strengths of turbulence. Aperture averaging improves the bit error rate.

Autonomous reconfiguration in free-space optical sensor networks

This research focuses on the physical and logical control and reconfigurability of network topologies through intelligent and dynamic rearrangement of nodes in an optical wireless sensor network. We address high data rate sensor networks (e.g., infrastructure monitoring; surveillance), which consist of gigabit per second, narrow beam, free-space optical links between fixed and/or mobile nodes. In our approach, the seamless operation of such networks requires maintenance of wireless link connectivity and quality and at all times, amidst, for example, changing atmospheric, and traffic and platform conditions. This is achieved by dynamic reconfiguration through topology control. We address the problem of dynamic formulation of topologies, which contain only two transceivers per communications node or switch. The task of reconfiguration requires the formation of a biconnected graph or a ring topology. The problem is similar to the traveling salesman problem and is NP complete. We address the mixed integer programming formulation of this problem, and show that it does not scale even for a small network. We then focus on heuristics for dynamic, autonomous reconfiguration. Using simulations, we investigate tradeoff between solution quality and computational time. We also investigate the effectiveness of these dynamic reconfiguration heuristics compared to fixed, degraded topologies.

Nature-Inspired Self-Organization, Control, and Optimization in Heterogeneous Wireless Networks

In this paper, we present new models and algorithms for control and optimization of a class of next generation communication networks: Hierarchical Heterogeneous Wireless Networks (HHWNs), under real-world physical constraints. Two biology-inspired techniques, a Flocking Algorithm (FA) and a Particle Swarm Optimizer (PSO), are investigated in this context. Our model is based on the control framework at the physical layer presented previously by the authors. We first develop a nonconvex mathematical model for HHWNs. Second, we propose a new FA for self-organization and control of the backbone nodes in an HHWN by collecting local information from end users. Third, we employ PSO, a widely used artificial intelligence algorithm, to directly optimize the HHWN by collecting global information from the entire system. A comprehensive evaluation measurement during the optimization process is developed. In addition, the relationship between HHWN and FA and the comparison of FA and PSO are discussed, respectively. Our novel framework is examined in various dynamic scenarios. Experimental results demonstrate that FA and PSO both outperform current algorithms for the self-organization and optimization of HHWNs while showing different characteristics with respect to convergence speed and quality of solutions.

Obscuration minimization in dynamic free space optical networks through topology control

Free space optical (FSO) networks are emerging as a viable, cost effective technology for rapidly deployable broadband communication infrastructure. The main drawback of this type of networks is their dynamic performance, especially under adverse weather conditions and high nodes mobility. Topology control is used as the means to achieve survivable optical wireless networking under hostile 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. Design, analysis and comparison of algorithms and heuristics for configuring optimized topologies in dynamic environments are presented. Heuristics were developed for ring networks (2 optical transceivers per node) as well as for 3-degree networks (3 optical transceivers per node). This paper focuses on the design of efficient and scalable algorithms for physical layer topology optimization. That is, algorithms to select the topology configuration which optimizes a given physical layer objective. Performance and scalability results are shown for the various heuristics used, in different scenarios and for different network sizes.

Studies of pointing, acquisition, and tracking of agile optical wireless transceivers for free-space optical communication networks

Free space, dynamic, optical wireless communications will require topology control for optimization of network performance. Such networks may need to be configured for bi- or multiple-connectedness, reliability and quality-of-service. Topology control involves the introduction of new links and/or nodes into the network to achieve such performance objectives through autonomous reconfiguration as well as precise pointing, acquisition, tracking, and steering of laser beams. Reconfiguration may be required because of link degradation resulting from obscuration or node loss. As a result, the optical transceivers may need to be re-directed to new or existing nodes within the network and tracked on moving nodes. The redirection of transceivers may require operation over a whole sphere, so that small-angle beam steering techniques cannot be applied. In this context, we are studying the performance of optical wireless links using lightweight, bi-static transceivers mounted on high-performance stepping motor driven stages. These motors provide an angular resolution of 0.00072 degree at up to 80,000 steps per second. This paper focuses on the performance characteristics of these agile transceivers for pointing, acquisition, and tracking (PAT), including the influence of acceleration/deceleration time, motor angular speed, and angular re-adjustment, on latency and packet loss in small free space optical (FSO) wireless test networks.

Reconfigurable optical wireless sensor networks

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.

On How to Circumvent the Manet Scalability Curse

An approach is offered to resolve the fundamental non-scalability of peer-to-peer, MANET networks by introducing ultra-broadband (up to and beyond 1 Gb/s), hybrid (FSO/RF) directional backbone nodes that significantly increase the end-to-end throughput of lower tier nodes. This approach represents a very different thrust from those which attempt to circumvent the limitations of low data rate (up to 54Mb/s) and short distance (hundreds of feet) MANET nodes by software fixes such as power control, MIMO, and clever modulation schemes, which are at best stop-gap measures. Our two-tiered approach is scalable based on its combination of MANET clusters interconnected with very broadband directional connections. The MANET clusters are operated within the scalability limits set by scalability density pattern and the base-station oriented directional connections use very narrow beams to provide massive spatial re-use

Optimizing performance of hybrid FSO/RF networks in realistic dynamic scenarios

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.

Multi-objective optimization techniques in topology control of free space optical networks

To mitigate the effects of obscuration and attenuation, free space optical (FSO) wireless networks employ topology control to dynamically reconfigure node connectivity based on changes in link state. Our previous research in topology control has focused primarily on creating heuristics for computing an optimal topology either in the physical layer (minimize bit error rate) or in the network layer (minimize congestion). This research presents a multi-objective optimization formulation of the topology control problem whereby physical network cost and network layer congestion are jointly minimized. Our formulation uses a weighting method for specifying preference in the physical or network layer, based on existing heuristics for each respective layer. Discrete event simulation was used to perform an evaluation of the weighting method in our formulation. By studying scenarios with different normalization factors, number of nodes (N), and weight sampling, we have categorized the conditions in which the multi-objective optimization topology control formulation is inferior, superior or incomparable to using just the respective physical or network layer only formulations. The simulation results show that the combined topology solution is preferable to the individual heuristics in over 80% of cases. Finally, we present possible strategies for assigning preference, and obtaining a Pareto optimal solution.