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WILEY ENCYCLOPEDIA OF
TELECOMMUNICATIONS
VOLUME 3
WILEY ENCYCLOPEDIA OF TELECOMMUNICATIONS
Editor
John G. Proakis
Editorial Board
Rene Cruz
University of California at San Diego
Gerd Keiser
Consultant
Allen Levesque
Consultant
Larry Milstein
University of California at San Diego
Zoran Zvonar
Analog Devices
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Sponsoring Editor:
George J. Telecki
Assistant Editor:
Cassie Craig
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Illustration Manager:
Dean Gonzalez
WILEY ENCYCLOPEDIA OF
TELECOMMUNICATIONS
VOLUME 3
John G. Proakis
Editor
A John Wiley & Sons Publication
The
Wiley Encyclopedia of Telecommunications
is available online at
http://www.mrw.interscience.wiley.com/eot
Copyright
2003 by John Wiley & Sons, Inc. All rights reserved.
Published by John Wiley & Sons, Inc., Hoboken, New Jersey.
Published simultaneously in Canada.
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Library of Congress Cataloging in Publication Data:
Wiley encyclopedia of telecommunications / John G. Proakis, editor.
p. cm.
includes index.
ISBN 0-471-36972-1
1. Telecommunication — Encyclopedias. I. Title: Encyclopedia of
telecommunications. II. Proakis, John G.
TK5102 .W55 2002
621.382 03 — dc21
2002014432
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
L
LAND-MOBILE SATELLITE COMMUNICATIONS
∗
J
EFFREY
B. S
CHODORF
MIT Lincoln Laboratory
Lexington, Massachusetts
access schemes. Finally, network aspects of LMSC systems
are discussed in Section 6. The primary emphasis of
this section is the issue of internetworking LMSC and
terrestrial data networks.
2.
LAND-MOBILE SATELLITE SYSTEMS
1.
INTRODUCTION
Since the early 1990s there has been significant progress in
the development of land-mobile satellite communications
(LMSC) systems and technology. In general, LMSC service
providers have struggled to compete with their terrestrial
mobile wireless counterparts. However, few doubt that
LMSC systems have a meaningful role to play in the
quest for global wireless access in the twenty-first century.
The key advantage enjoyed by satellite communications
systems is their ability to cover broad geographic areas,
substantially decreasing the terrestrial infrastructure
required and potentially simplifying issues relating to
the coordination of this infrastructure for tasks such as
channel assignment and handover. Moreover, a sufficient
amount of spectrum has been allocated to LMSC systems
such that they represent a good choice for the delivery of
broadband services such as multimedia. Of course, LMSC
systems are not perfect. While fewer satellites may be
required to cover an area, satellites are very expensive
to build and deploy relative to terrestrial base stations.
Moreover, depending on operating frequency, significant
channel impairments must be overcome in LMSC systems.
Nonetheless, the potential of LMSC systems ensures they
will remain an area of intense research and development
for the foreseeable future.
The purpose of this article is to describe in moderate
detail technical issues surrounding LMSC systems. Where
appropriate, references are cited so that the interested
reader can pursue these topics further. In Section 2 a
brief description of several existing and planned LMSC
systems is given. These systems are categorized loosely by
their orbital type and are further subdivided according to
the services they provide. Section 3 discusses propagation
issues, including path loss, signal fading, shadowing, and
the effects of directional antenna mispointing. Strategies
for dealing with channel impairments are discussed in
Section 4. These approaches fall into one of two main
categories: error control techniques such as forward error
correction (FEC) coding and automatic repeat request
(ARQ) protocols, and diversity combining methods. In
LMSC systems, satellite resources are typically a limiting
factor. Hence, efficient use of these resources is critical.
Section 5 addresses this issue with a discussion of multiple
∗
Land-mobile satellite systems come in a variety of orbital
configurations, including geostationary or geosynchronous
earth orbit (GEO), medium earth orbit (MEO), and low
earth orbit (LEO). GEO systems operate at an altitude
of 35,786 km, and have an orbital period of 24 hs. MEO
systems have altitudes ranging from 5000 to 10,000 km
and have orbital periods of 4–6 hs. LEO satellites
orbit at altitudes from 500 to 1500 km with periods
of approximately 2 hs. Orbital mechanics will not be
addressed here, but thorough treatments of this topic may
be found in Refs. 1 and 2. Services provided by land mobile
satellite systems include navigation, fleet management,
broadcast, and duplex voice and data communications, the
primary service of interest in this article.
LMSC systems are characterized by a forward path
and a reverse path between two user terminals. Each
path comprises an uplink between the transmitting
terminal and the satellite, and a downlink between
the satellite and the receiving terminal, as depicted in
Fig. 1. Traditionally, LMSC systems satellites have been
transponders (sometimes referred to as ‘‘bent pipes’’),
where the received uplink signal is simply amplified
and translated to a downlink frequency. More recent
systems have begun to employ processing satellites, where
in addition to amplification and frequency translation,
additional processing, such as demodulation, decoding,
and remodulation is performed.
Table 1 summarizes the nomenclature used to catego-
rize the various operating frequency bands of LMSC and
other wireless communications systems. Original spec-
trum allocations for LMSC systems were in the L and
Uplink
Uplink
Downlink
Downlink
This work was sponsored by the Department of the Army
under A/F Contract F19628-00-C-0002. Opinions, interpretations,
conclusions, and recommendations are those of the authors and
are not necessarily endorsed by the United States government.
1223
Forward path
Reverse path
Figure 1.
LMSC link nomenclature.

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