Multi-Transceiver Free-Space-Optical Structures for Mobile Ad-Hoc Networks
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Radio frequency-based communication has been the dominant way of wireless networking throughout the last two decades. Although new medium access control (MAC) technologies have been adapted to provide better per-node throughput, we have arrived to a point where the frequency spectrum saturates because of the overwhelmingly high data load caused by ever-increasing usage of multimedia content. To remedy this problem of diminishing end-to-end per-node throughput, we propose a novel &ldquo optical antenna &rdquo model that is tessellated with multiple free-space-optical (FSO) transmitter and receiver (transceiver) pairs that exploit spatial reuse of the shared medium and are capable of handling extremely high data rates using optical modulation techniques. However, because of the highly directional nature of FSO transceivers, nodes face a serious issue to communicate reliably: line-of-sight (LOS) alignment. First, we propose four different types of FSO antennas, each with a number of optical transceivers. Then, we present an auto-alignment protocol to opportunistically probe and detect available links to neighbors in a mobile setting. Later, we present the propagation model of optical communication for a single link and required simulation extensions to realistically simulate networks of multi-transceiver FSO nodes. Next, we present a set of simulation results that demonstrate the characteristics of an optical wireless link. We evaluate the performance of different node designs under a number of system parameters such as: different environment settings (indoor and outdoor), mobility, visibility, and node density. After this base set of simulations, we identify the major problem with networks of mobile FSO nodes with highly directional multiple transceivers: <bold>intermittent connectivity</bold >. To remedy this issue, we propose two different buffering schemes: node-wide buffering and per-flow buffering. We, then, present the results of major simulation settings using the two buffering mechanisms. We conclude that such buffering mechanisms are vital for the realization of free-space-optical mobile ad-hoc networks. We also investigate other possible ways to use this directional nature of FSO transceivers and consider efficient relative localization of nodes on a 3 dimensional terrain. Later, we focus on a prototype implementation of such a multi-element FSO antenna and auto-alignment protocol and demonstrate that the proposed system is implementable using off-the-shelf components. We conclude by providing results of various mobility experiments using our prototype.