Loral Aerospace Corporation along with Qualcomm Inc. have developed a satellite system which offers global mobile voice and data services to and from handheld and mobile user terminals with omni-directional antennas. By combining the use of low-earth orbit (LEO) satellites with existing terrestrial communications systems and innovative, highly efficient spread spectrum techniques, the Globalstar system provides users with low-cost, reliable communications throughout the world. The Globalstar space segment consists of a constellation of 48 LEO satellites in circular orbits with 750 NM (1389 km) altitude. Each satellite communicates with the mobile users via the satellite-user links and with gateway stations. The gateway stations handle the interface between the Globalstar network and the OSTN/PLMN systems. Globalstar transceivers are similar to currently proposed digital cellular telephones in size and have a serial number that will allow the end user to make and receive calls from or to that device anywhere in the world. The Globalstar system is designed to operate as a complement to existing local, long-distance, public, private and specialized telecommunications networks. Service is primarily designed to serve the rural and thin route communications needs of consumers, government users, and private networks

Viterbi, Andrew J.

Wiedeman, Robert A.

English

NASA Technical Reports Server

THE GLOBALSTAR MOBILE SATELLITE
SYSTEM FOR WORLDWIDE PERSONAL COMMUNICATIONS
Robert A. Wiedeman
Vice President of Engineering
Loral Qualcomm Satellite Services, Inc.
3875 Fabian Way, Palo Alto CA. 94303
(415)-852-6201
Andrew J. Viterbi
Vice Chairman
Qualcomm Incorporated
10555 Sorrento Valley Rd.
San Diego, CA. 92121-1167
22786
1.0 INTRODUCTION
Loral Aerospace Corporation along with
Qualcomm Inc. have developed a satellite
system which offers global mobile voice
and data services to and from handheld
and mobile user terminals with omni-
directional antennas. By combining the
use of low-earth orbit (LEO) satellites
with existing terrestrial communications
systems and innovative, highly efficient
spread spectrum techniques, the Globalstar
system provides users with low-cost,
reliable communications throughout the
world. The Globalstar space segment
consists of a constellation of 48 LEO
satellites in circular orbits with 750 NM
(1389 km) altitude. Figure 1 shows the
global coverage. Each satellite
communicates with the mobile users via
the satellite-user links and with gateway
stations.
The gateway stations handle the interface
between the Globalstar network and the
PSTN/PLMN systems. Globalstar
transceivers are similar to currently
proposed digital cellular telephones in size
and have a serial number that will allow
the end user to make and receive calls
from or to that device anywhere in the
world.
The Globalstar system is designed to
operate as a complement to existing local,
long-distance, public, private and
specialized telecommunications networks.
Service is primarily designed to serve the
rural and thin route communications needs
of consumers, government users, and
private networks.
Due to the orbit selection and the use of
CDMA spread spectrum technology the
number of circuits over a region is
increased due to multiple satellite
coverage. Each satellite operates as a
repeater in space, eliminating complex call
setup procedures and on-board processing.
Globalstar has been configured to link the
mobile user to a terrestrial gateway
through a single satellite so that the
system requires no satellite crosslinks.
Globalstar offers services which are not
currently available, such as world wide
cellular roaming. Globalstar service
offerings will meet the needs of the
general public both domestically and
globally where it is not cost effective to
provide terrestrial based cellular services.
Signal availability and quality to the user
is the most important factor in choosing a
technology for MSS services. The user
signal availability is determined by a large
291
https://ntrs.nasa.gov/search.jsp?R=19940018313 2020-06-16T15:59:49+00:00Z
number of factors which are statistically
developed to determine what the user
experiences during a MSS call. For
example, a user which is stationary in a
clear area will experience a certain
propagation effect while another userusing
a hand held device while in a vehicle
traveling under a heavily leafed tree
canopy may experience a different value.
The Globalstar user receive signal level
and transmit power is adjusted to account
for impairments. Rural and suburban
propagation models have been developed
by Vogel of the University of Texas II1
which show the probability of needing
margin in certain conditions. These
models when combined with multiple
satellite availability (path diversity) and
elevation angle statistics from the satellite
constellation determine signal availability.
2.0 CDMA IS OPTIMUM FOR THIS
SERVICE
Frequency Division Spread Spectrum
CDMA (FD/SS/CDMA) provides
optimum and flexible service quality to the
end user. The FD/SS/CDMA modulation
choice allows multiple signal paths from
more than one satellite which mitigates
fading and blocking.
The user is provided signals which are
optimized for the subscriber's environment.
Link margin is provided to each user
independently as the gateway senses that
it needs margin. User margin is provided
in three independent ways. First, there is
thermal margin built into each link to
account for the inherent thermal noise
from all sources. Second, a power control
ability is built into the hardware which
when activated boosts the power amplifier
output by up to 10 dB in 0.5 dB steps
both under the direction of sensing circuits
in the user equipment and under control
of the gateway that is handling the call.
This provides a total "link by link margin"
of 11 dB. Third, the Globalstar system
constellation provides additional mitigation
of shadowing and blocking using path
diversity. This path diversity is equivalent
to having more margin on a link by link
basis. When these margins are used with
the suburban/rural model for fading,
shadowing and blocking, the resulting
availability is shown in Figure 2. The
results show that significant improvement
in availability is provided with path
diversity.
For example, the user equipment in a
rural/suburban environment which is
experiencing a fade will first sense a
degradation of the receive frame error
rate. The unit will open loop increase its
power to maintain quality and send a
request to the gateway which increases the
forward link power. On the return link
the gateway measures the frame error rate
and Eb/No and sends up/down power
control commands to the user unit closed
loop. At the same time the uplink signals
are being received by a second or third
satellite, since the Globalstar system
provides multiple satellites in view of the
user's omni-directional antenna. The
signal level of these alternate paths is
measured by the same gateway serving the
user. The gateway software controls both
the power sent to and transmitted by the
user as well as the usage of the alternate
signal paths. At a predetermined point,
depending on the system loading, the
alternate signal paths are combined by the
gateway receivers improving the received
signal from the user. The alternate paths
are monitored for level and stability and
according to preset thresholds the user is
commanded to the best alternate path.
Link performance is assured by power
control which maintains the service quality
constant over a wide range of fading,
shadowing and blocking environments.
For satellite systems fading is Rician in
nature and not typified by the Rayleigh
292
model. Globalstar link budgets provide
adequate margins for Rician fading
conditions while shadowingand blocking is
provided by satellite diversity.
Users in severely degraded situations will
obtain service in excessof that available
from FDMA/TDMA systems. For
example, a user may be in a situation
where the power control and thermal
margin of 11 dB has been exhaustedand
the user is in need of even more margin.
Globalstar's systemwill sensethis situation
and the user transferred to another
satellite which has a less degradedpath.
With CDMA there is no "brick wall" in
capacity or signal quality. The system
operator has control of the user signals
which are requesting services. The
Globalstar system has been designed to
provide maximum capacityunder modeled
conditions which represent average
conditions expectedfor the market served.
When circuit demand begins to peak the
system control facilities adjusts the power
distribution to maintain overall signal
quality, individual signal links continue to
power control as discussedabove. When
the system reaches maximum capacity at
a certain quality, several options are
available to the system operator. As
examples, two of these options are
discussed here. Unlike FDMA or TDMA
systems where the capacity reaches the
"brick wall" and no further users may be
added to the system, the CDMA systems
utilize graceful degradation to maintain
flexibility. The operator may allow more
subscribers to occupy the bandwidth,
slightly degrading the overall signal quality
from a Mean Opinion Score of 3.5 to 3
or less. Alternatively, the system operator
can redistribute a portion or all of the
users over more of the satellites in view,
thus reducing the blocking and shadowing
path diversity for the users which may
need it. During these traffic peaking
conditions some users may experience
some higher levels of noisy transmission
and loss of signal quality for short periods
of time.
The Globalstar system offers the system
operator maximum flexibility in channel
assignment. Since all frequencies are
reused in all of the beams, the interaction
between gateways is minimized. Since the
frequency reuse factor is maximized and
the traffic load is averaged over several
satellites, the utilization of the satellite is
maximized and system loading as discussed
above can exceed 100%. The system, in
addition, has excess circuits available for
handoff and path diversity situations. By
comparison, in FDMA and TDMA the
frequencies must be assigned in blocks to
certain beams and since there cannot be
100% frequency reuse, system efficiency is
therefore reduced. Since the system
efficiency is 100%, the maximum capacity
to a region may be realized and the
capacity to each country can be
maximized.
With CDMA the ability to concentrate
circuits in a small region is important for
emergency and disaster communications.
For these conditions only CDMA has the
flexibility to allow maximum capacity in a
small region. Combinations of demand
assignment, decreased quality, and other
CDMA techniques, can make thousands
of circuits available compared to the "brick
wall" limitation of FDMA/TDMA.
The spectrum utilization with CDMA
systems is maximized. Multiple systems
can share the bandwidth with few
coordination factors. International sharing
with other CDMA operators requires
similar sharing coordinating. Since the
system downlink Power Flux Density
(PFD) around the world is limited and
since operation of uplinks are limited by
EIRP density and health hazard reasons,
these parameters can be coordinated
within reasonably small limits.
! 293
Considering the downlink sharing, the
capacity of the spectrum is the addition of
aIl systems operating with CDMA, less the
inter-system interference. Essentially, this
means that there are many more power
sources (satellites) in view of the region
that is being served. The degradation to
each system is a percentage of its capacity
due to sharing of the interference power
between the systems. However, the
coordinated result is greater than any
single system alone. The usage of the
spectrum by several CDMA systems is
over twice that of a TDMA system.
The advantage of CDMA to various
administrations around the world is the
freedom from spectrum segmentation
required for operation of a FDMA or
TDMA system. Spectrum segmentation,
if implemented, results in warehousing of
spectrum. This means that if country A
and country B are attempting to
coordinate systems and one of these is an
FDMA or TDMA system and the beams
cover both countries, either all or in part,
the portion segmented for the FDMA or
TDMA system is unusable for the second
country even if the first country doesn't
bring its system into use. CDMA on the
other hand prevents this warehousing.
The full bandwidth is available to both
countries. If one doesn't deploy, the other
just experiences less interference in its
system.
3.0 SYSTEM DESIGN FOR
GLOBALSTAR OPTIMIZES THE USE
OF CDMA
The advantage of Globalstar is that it uses
simple and reliable satellites. Forty eight
satellites are launched to provide
maximum path diversity in the temperate
zones and to handle peak traffic. If there
are random failures of several satellites,
the coverage of the system is not affected
most of the time. Therefore, there is no
need to have hot spare satellites on orbit,
nor the need to launch individual
replacement satellites on a crash schedule.
FDMA and TDMA systems, since they
must only have single satellite coverage of
a user, must replace their failed satellites
as they fail.
Globalstar has high elevation angles in the
temperate zones of the world, for example
the average elevation angle in CONUS is
up to 55 degrees. The minimum elevation
angle, occurring only infrequently and for
a very short duration of time is 20 to 32
degrees depending on location. These
high elevation angles and path diversity
mitigate shadowing and blocking. Reliable
path diversity is provided by 100% two
satellite coverage in CONUS, three
satellite coverage up to 90% of the time
and four-satellite coverage up to 35% of
the time.
Globalstar has the lowest path delay of
any of the proposed LEO systems with the
maximum path delay, including vocoder
processing, of less than 100 ms. The
maximum propagation path delay is 18 ms
for the user-satellite-gateway link; the
remainder is processing. Since there is no
onboard processing or inter-satellite links,
the processing is minimized and the
overall path delay is reduced, providing
excellent quality without the need for echo
cancelers, etc.
The implementation method chosen by
LQSS allows an easy extension of
terrestrial cellular developments underway.
Qualcomm Incorporated has developed a
CDMA cellular telephone system which
improves the spectrum efficiency over
analog by a factor of about 15. This
system is being deployed currently in
several wireline cellular systems in the
USA. LQSS plans to modify this system
slightly, to account for increased path
delay and effects of doppler, and utilize it
for satellite delivered cellular telephony.
294
As such, this is the only LEO system
proposed which has a heritage for its
gatewayand user equipment.
A key element in Qualcomm's terrestrial
system implementation is the use of a
Rake receiver which makes use of all
available signal paths to insure that a
quality signal is delivered to the user and
the gateway. A Rake receiver makesuse
of the CDMA modulation to receive many
signal paths simultaneouslyand coherently
combine them to develop the highest
signal input to the decoder. A multiple
finger receiver is operated, with each
receiver assigned to a particular signal
path to decode. A separate receiver
continuously searchesfor signal pathswith
the user's code. Once a path is found it
is assigned to a receiver finger. Logic
within the receiver reviews the signal
levels emerging from these receivers and
performs decisions on combining. It is
unimportant whether theseare direct paths
or multipath signals, all signals received
are available to be combined.
The benefits of the usage of two satellite
coverage and the Qualcomm unique
RAKE receiver design, combined with
mitigation of fading and blocking of the
hand held user equipment, are very
important. Consider the case of a user
which is shadowed by a propagation
impairment. At the onset of shadowing
the user would normally be utilizing a
single satellite path to a gateway. The
second and third satellite paths, although
present would not be in use. As the
shadowing increases the unit senses an
increasedframe error rate, makesan open
loop power increase and requests power
control, the gateway responds and begins
closed loop control. The signal level
remains constantat the shadowedsatellite.
The increased power level will be
transmitted by the second and third
satellites to the gateway and, depending
on system operation be utilized (closed-
loop) to optimize the user's power
requirements. The need for excessivelink
margin is thus avoided.
4.0 SUMMARY
Globalstar will use a constellation of 48
satellites to provide continuous coverage
of the areas of the world requiring mobile
connectivity. The Globalstar system is
designed to operate as a complement to
existing local, long-distance,public, private
and specialized telecommunications
networks. Service is primarily designedto
serve the rural and thin route
communications needs of consumers,
government users, and private networks.
Frequency Division Spread Spectrum
CDMA (FD/SS/CDMA) provides
optimum and flexible servicequality to the
end user allowing multiple signal paths
from more than one satellite which
mitigates fading and blocking. Link
margin is provided to each user
independently as the gateway sensesthat
it needs margin. Path diversity,
guaranteed by multiple satellite coverage
is equivalent to having more margin on a
link by link basis. When these margins
are used for mitigating fading, shadowing
and blocking, the resulting availability is
better than a TDMA system with higher
margin but with no path diversity.
The Globalstar system offers the system
operator maximum flexibility without the
need for difficult synchronization, thereby
allowing many gateways. Since CDMA
maximizesfrequency reuse,the traffic load
is averaged over several satellites, the
utilization of the satellite is maximizedand
system loading can exceed 100%. The
spectrumutilization with CDMA systemsis
maximized. Multiple systems can share
the bandwidth with few coordinating
factors. International sharing with other
CDMA operators requires similar sharing
coordination. The advantageof CDMA to
295
various administrations around the world
is the freedom from spectrum
segmentation required for operation of a
FDMA or TDMA system which reduces
operational efficiency.
Reference [11
NASA Reference Publication 1274, February 1992; Propagation
Effeas for Land Mobile Satellite System; Overview of
Experimental and Modeling Results, Julius Goldhirsh and
Wolfhard J. Vogel
296


The Globalstar mobile satellite system for worldwide personal communications

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server