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

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
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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

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.
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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