03-05-2012, 03:39 PM
ADAPTIVE POWER AND DIVERGENCE TO IMPROVE AIRBORNE NETWORKING AND COMMUNICATIONS
ADAPTIVE POWER AND DIVERGENCE TO IMPROVE AIRBORNE NETWORKING AND COMMUNICATIONS.pdf (Size: 282.77 KB / Downloads: 22)
Abstract
Free-space optical (FSO) links have the potential to provide high-bandwidth channels for data and communications applications. FSO can complement existing radio-frequency (RF) links by allowing the rapid transfer of high-volume data, such as images, that are ill suited to the lower bit rate RF links. A significant challenge for FSO links is the need for the transmitter and receiver to be tightly aligned to establish a viable connection. The inaccuracies inherent in position calculation and pointing equipment are a major part of this challenge. In this paper, we explore the use of adaptive transmitter power and adaptive beam divergence to overcome the inherent inaccuracies by relaxing the alignment criteria.
A theoretical model of an FSO system was constructed using Gaussian beam propagation and system parameters from commercial systems. Assuming initial alignment, the receiver was moved away from the transmitter optical axis until the connection was lost, as determined by the available power budget. The transmitter power and divergence were then varied systematically to determine the dependence of the alignment error on these parameters. The calculations were repeated for several choices of the distance between the transmitter and receiver.
Analysis of the calculations demonstrated several important trends. The maximum allowable error is independent of the distance, except where field of view is a limiting factor. Certain combinations of divergence and power, while suboptimal for one distance, provide a relaxed error limit for many distances. Using the calculation results, some initial design criteria can be determined, along with strategies for improving the performance of FSO-based communication systems involving both stationary and moving transmitters and receivers.
Introduction
Free-space optical (FSO) communication can provide a means for establishing high-bandwidth links for a variety of applications, including inter-satellite communication, airborne internet, and inter-building communication in urban settings [1-3]. A significant problem with FSO links is the need for highly accurate alignment of the transmitter and receiver to establish a viable connection. Inaccuracies and delays in the positioning equipment and instabilities in the physical platform create inevitable alignment errors. The problem is made more difficult when one or both transceivers is in motion.
One solution to the alignment problem is to adaptively change the properties of the transmitted optical beam. By modifying the divergence and the transmitted power, the link can exhibit a greater tolerance for alignment error by providing a sufficient link budget over a larger spatial volume. Solutions for the optimal beam parameters have been obtained over both extremely long (i.e. inter-satellite) [3] and relatively short (inter-building) [2] distances. In these cases, the motion is well described by a combination of statistical models and motion equations. The motions were small enough that the receiver’s field of view (FOV) was not a significant factor and rapid positioning changes were not required. For air-to-air and air-to-ground networking in a rapidly changing environment, these previous solutions are inadequate [4]. A different approach is therefore required for finding the optimum parameters.
In this paper, we perform an initial analytical investigation of how the performance of an FSO link depends on beam divergence, transmitter power, and the distance between the transmitter and receiver. A simple link-budget approach is used to reduce the problem to one of collecting sufficient power at the input lens of the receiver optical system. Based on this approach, we calculate the maximum allowable positioning error in the
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direction transverse to the optical axis of the transmitter, further limited by the FOV of the transceiver. The results of the calculations provide some initial suggestions for selecting the beam divergence and transmitter power to enhance the likelihood of establishing the initial link and of maintaining a viable link for the applications noted above.
Theory
In a standard FSO transmission module, the optical components consist of a single-mode fiber cable and a collimating lens. The divergence of the transmitted optical beam depends on the distance zfl between the end of the fiber and the lens, the focal length f of the lens, and the initial field distribution at the end of the fiber. The power at any point beyond the lens depends on the divergence, the field distribution, and the initial power Pi at the end of the fiber. As the field distribution at the output of a single-mode optical fiber closely approximates a Gaussian profile, the theory of Gaussian beam propagation is used to analyze the output of the module