TDMA and CDMA.doc - Resource Management in CDMA-based Satellite Networks Dorothy Kabagaju Okello Department of Electrical Computer Engineering McGill

TDMA and CDMA.doc - Resource Management in CDMA-based...

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Resource Management in CDMA-based Satellite Networks Dorothy Kabagaju Okello Department of Electrical & Computer Engineering McGill University Montreal, Canada April 2004 A thesis submitted to McGill University in partial ful_llment of the requirements for the degree of Doctor of Philosophy. c 2004 Dorothy Kabagaju Okello i Abstract There is interest, supported by successful _eld trials, in the use of satellite communications at the Ka band (30/20 GHz) and beyond to meet emerging demand for broadband interactive multimedia services. The key advantages of operation at Ka band are availability of bandwidth and favorable implications for terminal size, cost and mobility. We study two problems related to bandwidth management of the uplink in a multibeam, CDMAbased, GEO satellite. Our focus is on the delivery of data services with rigid constraints on bit-error rate and elastic constraints on data rate. The _rst of the two problems concerns the design of the coverage areas of the satellite beams. We were interested speci_cally in the adaptation of beam shape to inhomogeneity in the geographic distribution of the user population, and in the impact of beam shaping on the set of transmission rates that are compatible with prescribed constraints on transmission powers and signal-to-interference ratios. Assuming that the spatial distribution of users is known, we construct an algorithm which computes beam coverage regions to equilibrate
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the per-beam user populations. The impact on the set of feasible bit-rate allocations is quanti_ed through numerical experiments. Comparison with uniform beam shapes suggests that the adaptive approach is superior in terms of the number of concurrent transmissions that can be supported. The second problem concerns the allocation of bit rates in a setting where user bit-rate requirements are assumed de_ned by averages over moving windows of constant length. We use a frame-based channel model characterized by fading coe_cients which, though statistically variable, are assumed known to the controller at the start of each frame. The implied temporal elasticity in quality-of-service provides opportunity to achieve economies in transmitted power. The value of such opportunity is quanti_ed by comparison of two extreme cases. We develop an approximate system model which allows optimization of the rate allocation when the number of users is small, and a heuristic which is useful when the number of users is not small. The associated performance results con_rm the inverse relationship between the per-bit energy required for transmission and the length of the averaging window. ii Sommaire Les essais pratiques ont d_emontr_e que la demande grandissante pour les services multim _edias int_eractifs _a grande largeur de bande peut ^etre soutenue par les communications satellitaires _a partir de la bande Ka (30/20GHz). L'avantage de la bande Ka est la disponibilit _e accrue de largeur de bande et la possibilit_e de construire _a faibles co^uts des terminaux plus petits et plus mobiles. Ce m_emoire traite de deux probl_emes de
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