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6.1992-1953

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CATIO
LOA
MMUNICA~IONS
Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 26, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1992-1953
Ming Louie*, Paul Monte", Randy Tyner*
Denis Rouffet**, Klein S. Gilhousent
Space SystemLoral, Palo Alto, California*
Alcatel, Paris, France**
QUALCOMM, San Diego, California?
Abstract
the GLOBALSTAR payload include six elliptical spot
beam L/S-band antennas, beam hopping and Time
Domain Duplexing (TDD) capability, and efficient reuse of the scarce L-band and S-band spectrum. The spot
beam antenna of the GLOBALSTAR satellite also
include an unique ISOFLUX design, which can
substantially mitigate the "near-far'' problem
experienced by many mobile systems. In this paper,
two candidate payloads are also presented: one uses
Time Domain Duplexing (TDD) - Frequency Division
(FD) - CDMA with beam hopping, and the other one
uses FD-CDMA techniques. Both systems can provide
cost-effective mobile communications services to
GLOBALSTARs users over the world.
The GLOBALSTAR System, is a Low Earth Orbit
(LEO) satellite-based mobile communications system
that is interoperable with the current and future Public
Land Mobile Network (PLMN).
The GLOBALSTAR System concept is based upon
technological advancement in two key areas: (1) the
advancement in LEO satellite technology; (2) the
advancement in cellular telephone technology, including
the commercial application of Code Division Multiple
Access (CDMA) technologies.
This paper describes the general characteristics of
the GLOBALSTAR communications payloads and
analyzes the suitability of their design for global digital
mobile communications.and for Radio Determination
Satellite Service (RDSS). The payload design can
provide efficient communications with small hand-held
or vehicle-mounted user units. The unique features of
1. Introduction
The GLOBALSTAR System consists of three
major segments: the space segment, the ground
segment, and the mobile user segment (Figure 1).
SATELUTES
SPACE
SEGMEM
USER
SEGMEM
I
I
PSTN I PLMN
Copyright 0 1992 American Institute of Aeronautics and
Astronautics, Inc. All rights reserved.
1074
Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 26, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1992-1953
The space segment is comprised of a constellation
of 48 LEO satellites to provide global coverage.
Initially, a constellation of 24 operating satellites will
be launched to provide United States coverage. As
worldwide traffic demand develops, 24 (or more)
additional satellites will be launched to provide
continuous global coverage. The GLOBALSTAR
satellite design can utilize various launch vehicles and
launch services to minimize launch costs.
The ground segment consists of gateway stations,
the Telemetry, Tracking and Command (TT&C)
stations, the Satellite Operation Control Center
(SOCC), and the Network Control Center (NCC).
The gateway stations, and associated equipment handle
the interface between GLOBALSTAR and the Public
Switched Telephone Network (PSTN)/PLMN. The
gateway stations support call processing for
GLOBALSTAR users. The GLOBALSTAR System
has many gateway stations distributed throughout the
United States and the world to interface with various
PSTNs and PLMNs. The TT&C and SOCC will
maintain, monitor, and manage the orbiting
satellites. The TT&C and SOCC also determine the
ephemeris of the orbiting satellites to enhance the
RDSS accuracy and to perform station-keeping duties
for the satellites. The NCC and its data network
handles the user registration, verification, billing, and
other network management functions.
Initially, the mobile user segment will include
three different kinds of user terminals: a vehiclemounted unit, a hand-held unit, and a RDSS-only
unit. Many other options can also be provided to
GLOBALSTAR users for various services. The
GLOBALSTAR system is designed with maximum
system flexibility. The system will:
Provide efficient utilization of spectrum with
minimum coordination among system operators.
Be compatible with other CDMA systems.
Be able to be compatible to many PLMN
systems, such as Group Special Mobile (GSM)
and other digital systems.
Minimize satellite and launch costs for various
launch vehicles.
Allow phased introduction of the GLOBALSTAR
service via different constellations.
0
Allow for system improvement to increase
system capacity and improve service through
technology advancement.
The GLOBALSTAR space segment consists of a
constellation of 48 LEO satellites in circular orbits
with 750 nm (1389 km) altitude. Initially, 24
satellites will be launched into eight orbital planes,
with three equally spaced satellites in each orbital
plane, to provide coverage of United States. If the
global market is opened prior to the launch of the first
GLOBALSTAR satellite, then 48 satellites will be
launched into eight orbital planes, with six satellites
in each orbital plane, to provide global coverage.
Figure 2 shows the global coverage of the 48 satellite
constellation.
For the 24 satellite constellation, each orbital
plane has an inclination angle of 47" and each satellite
has a 45" phase shift relative to the nearest satellite of
the adjacent orbital plane. This 24 GLOBALSTAR
satellite constellation can provide CONUS coverage
100%of the time, with at least two satellites in view
at any time.
The 48 satellite constellation provides global
coverage. In this constellation, there are eight orbital
planes and each plane has six satellites, which are
equally phased within the orbital plane. Each orbital
plane has an inclination angle of 52". Each satellite
has a 7.5" phase shift relative to the nearest satellite
in the adjacent orbital plane. Over the United States,
coverage is such that there are three or more satellites
providing GLOBALSTAR services to the public, for
100%of the time.
The GLOBALSTA
Each satellite has six spot beams which form
"coverage cells" on the surface of the earth for links
between the mobile users and the satellites. Spread
spectrum CDMA techniques, combined with multiple
spot beam antennas, permit the spectrum to be reused
many times over the United States and over the world,
to achieve high spectral utilization efficiency. With
24 satellites in operation, this spectrum reuse is 144
Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 26, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1992-1953
nst
times globally.
With 48 satellites in the
GLOBALSTAR system, the spectrum can be reused
288 times over its global coverage.
n.
satellites payload are discussed in the subsequent
sections.
Initially, the GLOBALSTAR space segment is
sized to exceed the expected total demand for
GLOBALSTAR services in the United States market.
However, the GLOBALSTAR system allows for
capacity growth as the need arises and demand
increases. Additional satellites can be launched to
meet new demand. Technological developments in
multiple beam antenna, low-rate voice encoding
techniques, and spread spectrum techniques will also
allow the GLOBALSTAR system to increase capacity
to meet even more market demand.
There are two versions of the proposed GLOBALSTAR
communications payload: one using L-band (1610 1626.5 MHz) only for the satellite-user links( it is
called System A) and the other using L-band for user-tosatellite link and S-band (2483.5 - 2500.0 MHz) for
satellite-to-user link. (System B). Both System A and
System B use C-band as the feeder links. Both systems
are proposed because of the uncertainty of frequency
allocation for the LEO satellites system.
Pavload of Svstem A
The GLOBALSTAR system has been designed to
meet the technical requirements set forth in the
International Radio Regulations and in Part-25 of the
Commission's Rules and Regulations. With the
spread spectrum CDMA techniques, the system design
of GLOBALSTAR is compatible with RDSS
systems, other RDSShlobile Satellite Service (MSS)
systems, the Radio Astronomy community arid the
GLONASS system. Details of the GLOBALSTAR
Figure 3 illustrates the communications payload of
System A. With System A, the L-band frequency is
used for both uplink and downlink between the satellites
and the mobile users. The uplink signal and the
downlink signal share the same L-band spectrum at
different time slots and at different spot beams. Details
of this TbD-FD-CDMA technique is provided by
another paper of this conference.
1075
C-BAND
RECEIVE
ANTENNA
Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 26, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1992-1953
V
MULTI-PORT
L-BAND AMPLIFIER
L-BAND
MULTI-BEAM ANTENNA
C-BAND
TRANSMT
ANTENNA
I
L-BAND BEAM TIMING DIAGRAM
I
OMJ
LCP
T1.4 x T2.5
2 5x l X3.6
36F o 1.4( F K
2 5 F o3
2.5
36
.6(
T x T x 7 x F o ( F K F o (
RCP
0
10
20
30
40
50
MMSWSTERFRAME
I
60
4
:
I
rdr
m!m
POWERAMPLFIER
LOW W S E AMPLIFIER
NOTE, REDUNDANCY NOT SHOWN
FOR SIMPLICITY
mmunications
The repeater of System A is comprised of two
transponders: an outbound link C-L band transponder,
and an inbound link L-C band transponder.
In any given 10 ms transmission time slot, two Lband beams are used. The signals for each beam are
received on orthogonal polarizations. Separate C-band
receivers for each polarization are provided. The
receivers provide low-noise amplification and downconversion of the outbound signals. Three-for-two
receiver redundancy is provided. Blanking limiters are
included in the receivers to protect the receivers from
harmful interference.
The receiver outputs are routed through separate,
ganged, solid state switches controlled by the payload
timing and control equipment. During the 30 ms
transmit period, these switches toggle at 10 ms
intervals between outputs for beams 1-4, 2-5, and
3-6.
The switch outputs are comiected to two of the six
input ports of the (L-band) Multi-port Amplifier
(MPA). The MPA consists of several L-band
amplifiers operating in parallel which share the
amplificatioii of all input signals. The hybrid
arrangement at the input and output recombines each
amplified signal at its output port. The MPA provides
efficient amplification, even when traffic is imbalanced
between beams, and also provides graceful degradation
in the event of an amplifier failure. Partial automatic
level control is provided within the MPA to provide
additional margin to users when traffic is below
capacity.
After amplification and recombination, the signals
for each beam are filtered and routed through circulators
to the L-band antenna. The output filter assemblies
contain band-reject filters that can be switched into
service when required to protect Radio Astronomy
operations.
L-C Band TransDonder.
The circulators at the L-band antenna input connect
inbound signals through the L-band receive filters to
low-noise preamplifiers. The circulators provide
isolation to protect the L-band preamplifiers during
transmit periods. Additional protection is provided by
limiters at the input of each preamplifier. During the
1077
Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 26, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1992-1953
30 ms receive period the (L-band) MPAs are gated off to
prevent amplifier noise from degrading the return link.
ground command, such that interference into the Radio
Astronomy stations can be minimized when a
GLOBALSTAR satellite is near a Radio Astronomy
station.
Following filtering and preamplification, the Lband signals are routed through return pathway switches
to two L-to-C upconverters. The inbound path
switches, controlled by the timing and control
equipment, connect either beams one to four, two to
five, or three to six, to the upconverters in accordance
with the timing allocations. Signals from beams one,
two, and three are routed to one upconverter, and signals
from beams four, five, and six to the other.
Pavload of Svstem B
Figure 4 shows the payload of System B. In
System B, C-band is used for the feeder links, L-band
for user uplinks, and S-band for user downlinks. The
subsystem is comprised of transmit and receive C-band
antennas, a C-to-S-band forward repeater, a multibeam
antenna that operates at both S and L-band, and an Lband inbound repeater. The subsystem operation is
similar to that for System A, except that satellite time
domain duplex switching and beam hopping are not
used.
The unconverted C-band outputs are then amplified,
filtered and transmitted by the C-band transmit antenna.
Both outputs are transmitted on the same frequency
using orthogonal circular polarization.
Timing and Control Unit.
All timing and control required for beam hopping
and TDD operation are derived from an ultra stable
oscillator. Timing can be adjusted by ground command.
SDecial Attention to Radio Astronomv.
In the payload of System A, a bank of Band
Rejection Filters (BRF) are placed at the output of the
L-band MPA. These filters can be switched in by
In System B, time domain duplexing is not
necessary because the users' uplink and downlink use
different frequencies. Six L-band and S-band beams are
used, with coverage similar to that of System A. Six
times frequency re-use is achieved at S-band and C-Band.
All six L-band beams for the user uplinks are
continuously illuminated. All six S-Band beams for the
user downlinks are continuously illuminated. The
C-BAND
RECEIVE
ANTENNA
CBAND RECEIVERS
MULTI-PORT
SBAND AMWFIER
SA-BAND
MULTI-BEAMANrENNA
R
C-BAND
-lR'lNwrr
I t
LCP
Rcp
NOTE: REDUNDANCY NOT SHOWN
FOR SlMPtlClTY
.
~ommunications
1078
block diagram for this configuration is shown in Figure
5. Frequency reuse is achieved at C-Band through the
use of orthogonal polarization.
C-band Antennas.
Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 26, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1992-1953
Separate C-band antenllas are used for transmission
and reception between the satellite and
gatewayshetwork control. The C-band antennas are
identical to those used in System A.
L-S Band Antenna.
The six L-band and S-band beams share common
apertures, i.e., S-band beam-1 and L-band beam-1, are
generated &om one aperture, S-band beam-2 and L-band
beam-2 from a second aperture, etc. Each beam is
generated by an antenna which contains separate L-band
and S-band arrays. Independent L-band and S-band beam
forming networks provide the required amplitude and
phase excitations to generate essentially the same
coverage contours for each frequency. The S-band
beams achieve a 1.0 dBi greater ISOFLUX gain than the
L-band beams.
Outbound (C-S) Repeater.
The signals for each beam are received on
orthogonal polarizations. Separate C-band receivers for
each polarization are provided. Two C-band receivers
amplify the incoming signals and down-convert them to
the common S-band frequency band. Three different
local oscillator frequencies are used in each receiver to
down-convert the three 16.5 MHz C-band channels to
the same S-band frequency. The six outputs from the
two receivers are then routed to the S-band MPA.
The multi-port amplifier operation is similar to
that of System A. Signal amplification is shared
among all amplifiers and the output hybrid network
recombines the signals to the proper ports. The outputs
are bandpass filtered and routed to their associated Sband beam.
Return signals from the users are received via the
six L-band beams. The output from each beam is
separately filtered, amplified, and upconverted to Cband. Three upconversion frequencies are used for
frequency division multiplexing. Signals from three Lband beams are frequency division multiplexed for
transmission on each of three orthogonally polarized Cband downlinks. The signals are then amplified and
routed to the transmit C-band antenna.
ms illurnin
1079
Unit
S.
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*
The six spot beams of the satellite generate L-band
or S-band elliptical coverage cells on the surface of the
earth. The major axis of these elliptical coverage cells
are aligned with the velocity vector of the satellite
movement, so that the time a user stays within the
same satellite beam "cell" is increased and the number
of call hand-off operations among the satellite beam
"cells" is reduced. Figure 5 illustrates that six elliptical
beans covering the service areas of North America.
e
To provide gain contours which substantially
compensate for differences in the slant range from
the spacecraft to the earth ("ISOFLUX").
To ensure that all points on the earth which have
an elevation angle to the satellite of 10" or more are
illuminated by a beam.
To provide low spillover illumination beyond the
earth's limb in order to minimize potential
interference with other spacecraft.
The GLOBALSTAR satellites' spot beam antennas
are also designed to compensate for the difference in the
satellite-to-user link losses between the "near" and the
"far" users, so that the power flux density of the "far"
users is about the same as the "near" users (Le., an
ISOFLUX design). This GLOBALSTAR antenna
design will reduce the near-far problem experienced by
many cellular type systems. With this antenna design,
harmful interference into the system can be reduced and
the capacity of the system can be increased. The main
objectives used in developing these "ISOFLUX" beams
are:
To cover approximately equal areas of the earth
with each beam.
e
To be aligned approximately parallel to the flight
path, so that a user is illuminated by the same
beam throughout the satellite pass. This criterion
minimizes the number of handoffs required during a
user contact.
Figure 6 illustrates the basic principal of this
ISOFLUX design. The ISOFLUX antenna pattern of
the C-band feeder link is shown in Figure 7. Figure 8
is a typical antenna contour of the L-band or S-band
multibeam antenna.
15
k .;r/I
:I
10 -
-5 -
-900
h
-
1
-50
0
50
100
-40
-20
0
20
40
Isoflux p a t t e r n normalized with respect to nadir
nn
rn
rn
1080
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nn
5. Conclusion
The GLOBALSTAR satellites have been designed
to provide mobile communications services through a
constellation of LEO satellites. To make the satellite
design suitable to the unique characteristics of mobile
communications and of the low orbit satellites, the
GLOBALSTAR payload design included the
ISOFULUX antenna design to minimize the near-far
problem of a mobile system. The payload design also
included features for time domain duplexing such that
the same L-band frequency can be reused for both uplink
and downlink signals.
The GLOBALSTAR satellites are also sized to
provide sufficient capacities (2600 to 2800 full duplex
voice channels with single satellite) for the mobile
users. With this payload design, cost-effective mobile
communications services can be provided to users all
over the world.
The GLOBALSTAR project is developed by an
international team. The authors would,like to thank Y.
Tanguy, F. Berthault, P. Jung, and J. F. Migeon for
their valuable contribution.
1081
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