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In this paper we consider the effectiveness of sharing a small pool of reserved time slots of a Time-Division-Multiple-Access ( T D M A ) frame among a large number of ground stations to overcome rain fading. With this approach, a system dynamically assigns time slots from the reserved pool to ground stations experiencing fade depths above the built-in margin. Powerful error correcting codes can be introduced to occupy the extra time slots, providing 10 dB or more of extra fade margin. Because a large number of ground stations are competing for the limited reserved pool, blockage can occur if the number of simultaneous fades exceeds the maximum number that can be accommodated. Some factors that influence the effectiveness of resource sharing are the mutual fade statistics at the various sites, the traffic distribution within the network, the number of earth stations, the size of the reserved pool, and the rain outage objective. Since the mutual fade and traffic statistics are unavailable, we develop models that can be used to find a conservative bound on the required size of the reserved pool. The rain model accounts for diurnal, seasonal, and geographical correlation among attenuation events. Results for a maximum resource-sharing gain of 10 dB show that reserving six percent of the time slots ensures a realized fade gain in excess of 9 dB for a down-link outage objective of 0.005 percent if there are more than 50 ground stations in the network, each with two percent or less of the traffic. I. I N T R O D U C T I O NIn an earlier paper,' a shared-resource concept was described for increasing the rain fade margin of a digital satellite system by as much as 10 dB above the design fade margin. With this approach, unused time slots of the Time-Division-Multiple-Access ( T D M A ) frame are made available to ground stations experiencing rain fading above the R A I N M A R G I N I M P R O V E M E N T 1 6 7
With the increasing need for efficient exploitation of the 14/11 and 14/12 GHz frequency bands, the satellite link designer is faced with the problem of designing communication links in an environment where propagation impairment can be severe. In order to provide a communication service which will meet the desired quality and availability objectives, the designer may choose to employ any of the numerous propagation impairment restoration techniques which have been implemented or proposed in the literature. To determine the optimum technique for a given purpose requires a detailed technical and economic comparison of the candidate restoration schemes with specific reference to the defined service objectives. This paper details a computer program, IMPRES, which has been designed to provide a consistent and reproducible basis for evaluating impairment restoration techniques. The program draws on a knowledge-base containing a profile of each restoration scheme which includes information such as the cost of implementation, the impact on the network operators and users, and the technological and economic risk associated with each scheme. The software design is such that this knowledge-base may be easily updated and extended to accommodate advances in technology and changing costs, thus ensuring that the unified approach to the restoration technique selection problem provided by IMPRES remains valid in the rapidly changing satellite communication environment. KEY WORDS Propagation Fade countermeasures* This paper is based on work performed under the sponsorship of the International Telecommunications Satellite Organization (INTELSAT). Any views expressed are not necessarily those of INTELSAT. Computer-aided designOf the variety of restoration techniques which have been proposed for reducing or eliminating propagation impairments, no single technique can be identified as optimum over a wide range of communication network conditions. The attractiveness of each technique, and thus the selection of the optimum technique, will depend on the application and will be influenced by many technique specific factors, such as the restoration performance, the cost and difficulty of technique implementation for existing earth-station operators, and the technical maturity of the technique, and by network-dependent factors such as the impact of the technique on network operations, and the number and geographical spread of earth-stations in the network. In order to provide solutions to satisfy a variety of communication requirements, it is therefore necessary to perform individual analyses of candidate restoration techniques for each specific requirement. Clearly, in view of the repetitive nature of such analyses, a computer-based solution to the restoration technique comparison problem is desirable, not only to increase the efficiency of the link design process but also to provide a consistent framework for restoration technique comparison.This paper reports on the development of a computer program for the automated comparison of propa...
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