A new concept for a satellite based public mobile communications system, LOOPUS Mob-D, is introduced, whereby most of the classical problems in mobile satellite systems are approached in a different way. The LOOPUS system will offer a total capacity of 6000 high rate channels in three service areas (Europe, Asia, and North America), covering the entire Northern Hemisphere with a set of group special mobile (GSM) compatible mobile services, eventually providing the 'office in the car'. Special characteristics of the LOOPUS orbit and the communications network architecture are highlighted

Horn, P.

Kuhlen, H.

English

NASA Technical Reports Server

LOOPUS Mob-D: System Concept for
a Public Mobile Satellite System providing
Integrated Digital Services for the
Northern Hemisphere from an
Elliptical Orbit
H.Kuhlen, P.Horn
MBB - DEUTSCHE AEROSPACE (DASA)
P.O. Box 80 11 69; D 8000 Munich 80, FRGermany
Phone +49- 89 - 607 23542 FAX: +49 - 89 - 607 28998
ABSTRACT SYSTEM REQUIREMENTS
The market for Mobile Communications
Services is presently in a process of
revolutionary development not only in terms of
quantity but also in terms of quality of services.
Sophisticated end user equipment allows
intelligent signalling and comfortable user
interfaces at reasonable prices.
In 1988 the Groupe Sprcial Mobile (GSM),
a group jointly sponsored by European PTTs
and industry, submitted a set of standards for a
fully digital mobile communications system to
be implemented in the 90s in at least 18
European countries. However, providing
Mobile Services at any place within Europe
implies a major investment on one hand and a
considerable implementation period on the
other. Satellite systems, inherently, would not
only offer an immediate availability in the entire
anticipated service area once they are
implemented, but also provide options to extend
a regional service into a European or even
worldwide accessability.
This paper will introduce a new concept for a
satellite based public mobile communications
system, LOOPUS Mob-D, where most of the
"classical" problems in mobile satellite systems
are approached in a different way. The
LOOPUS system will offer a total capacity of
6000 high rate channel in three service areas
Europe, Asia and North America, covering the
entire Northern Hemisphere, with a set of GSM
compatible mobile services eventually providing
the "office-in-the-car".
This paper we will briefly highlight special
characteristics of the LOOPUS orbit and the
communications network architecture.
Different from the links in the fixed satellite
service, mobile links suffer generally from the
fact that user terminals are in motion and hence,
great variations in terms of signal strength are
induced by frequency selective multipath
propagation and other degrading effects.
Consequently, the system requirements for a
Mobile Satellite System differ from those for
the fixed services. A mobile satellite system can
be characterised by a great number of individual
users accessing directly the satellite from
non-stationary locations. All this with
preferably small and low cost user terminals. In
common with the fixed services is the fact that
also in the mobile environment a firm demand
for voice and data services exists.
Because of the real-time nature of "voice",
real-time transmission channels are required.
This is considerably less critical in the case of
data services. Absolute Bit Error Rates (BER)
can to a certain extend be improved by
appropriate error correction. However, taking
into account the inherent propagation delay of
the satellite channel the overall time delay really
becomes the critical issue for voice services if
too complex FEC procedures, e.g. interleaving,
has to be applied. This requires a
communications channel with a basically
uninterrupted direct line of sight characteristic.
These conditions are particularly well satisfied
from orbits with high inclination. LOOPUS has
been designed to provide the equivalent of all
services offered by the GSM 1 hence including
voice.
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International Mobile Satellite Conference, Ottawa, 1990
https://ntrs.nasa.gov/search.jsp?R=19920014841 2020-03-17T12:30:10+00:00Z
The maximum number of user channel has
been evaluated as 2000 32kbit/s (full-rate) or
4000 16kbit/s (half-rate) respectively. The
channel granularity reflects a 32kbit/s channel
for a voice signal with speech encoding i.a.w.
a Regular-Pulse Excitation combined with
Linear Predictive Coding (RPE-LTP) resulting
in bitrate of 13kbit/s, i.e. 260bit per 20ms. A
detailed descrj,plion of this encoding process is
given in refs.Z, _. This capacity is determined
by a number of system parameters i.e. the size
of the satellite (thermal dissipation), the DC
provisions of the solar generator, the efficiency
of the DCJRF conversion in the power amplifier
and, last but not least, by the overall gain of the
spacecraft and user antenna.
One of the interesting aspects of an inclined
elliptical orbit is that the satellites appear to be
quasi-stationary during a long period around the
apogees. It actually remains visible for several
hours at a very high elevation angle for the user
terminals particularly for those in high northem
lattitudes. This fact led to the name for this
program LOOPUS: (Quasi-Geostationary)
Loops in Orbit Occupied by Unstationary
Satellite.
Due to the relative movement of the
spacecraft with respect to the Earth the track of
the subsatellite points creates two cross-over
points. The elapsed travelling time of each
satellite between the one closer to the apogee,
through the apogee itself and back to the same
cross-over point takes about 8 hours of 14.4
hours for an entire orbit. The coherency in the
travelling of all nine satellites in their orbits
creates for an earth bound observer the effect
shown in Fig. It seems as if all satellites
would follow each other on the same ground
track. One satellite after the other becomes
"visible" until after 71h 46min the number one
satellite shows up again.
By appropriate selection of the six
parameters defining such orbits, the hand-over
from the descending to the ascending satellite,
referred to the cross-over points, can be
handled by one earth station. It is further
ensured that any of the five resulting apogee
loops is occupied by exactly one satellite at a
time ref. 4. Since all satellites are only active
transmitting within the defined loop, the
remaining time of the orbit can be used for
attitude and orbit corrections by means of an
electrical ion thruster system.
System Design Rationale
Not particularly for mobile systems, but here
this aspect is more critical than in fixed
systems, is the generic conflict of illuminating
large service areas such as one third of the
Northern Hemisphere with sufficiently high
power flux density (PFD). Large coverage and
high PFD are basically conflicting
requirements. As a result from a first analysis
on maximum antenna size, a footprint of about
1000km in diameter has been determined as
optimum. This spot corresponds to an antenna
gain of 38.6dBi which would also be in line
with the requirements from link calculations.
In terms of frequency selection, the
restrictions induced by the rather limited
bandwidth of the L-band for mobile satellite
services, the investigations for LOOPUS have
led to a different solution. It is expected that in
the mid 90s the entire L-band is almost
worldwide occupied by geostationary satellites,
particularly by 2nd and 3rd generation
INMARSAT. An interference analysis showed
that frequency re-use (e.g. by polarization
decoupling) can not be guaranteed under all
mobile link propagation conditions. We,
therefore, came to the conclusion, that the
Ku-band provides a good compromise between
sufficient bandwidth for further services on one
side and a reasonable link attenuation on the
other.
Today, the Ku-band already receives much
attention mainly in the field of direct
broadcasting and VSAT services. The growing
market for Ku-band equipment in consumer
technology has therefore not only significantly
increased their availability at reasonable prices
but also improved their performance (noise
figures).
The application of Ku-band implies the use
of high gain antennas for mobile terminals.
This requires on one side a minimum tracking
capability of the mobile antenna but on the other
side offers the advantage of inherent frequency
re-use due to large angular decoupling between
satellites on inclined orbits from those on the
geostationary orbit.
International Mobile Satellite Conference, Ottawa, 1990
79
SYSTEM CHARACTERISTICS
Highly-inclined Elliptical Orbit
Main characteristic of the full scale system is
that nine satellites will eventually travel along
three highly inclined (63.4 ° ) elliptical orbits
(HEO), three on each orbit. The selected orbit
configuration with heights of 41450km and
5784km for the apogee and perigee respectively
provides a simultaneous 24h-visibility in three
major economic regions of the Northern
Hemisphere: Europe, North America and Asia.
The large potential visibility of the entire
Northern Hemisphere and the resulting Earth
coverages due to the high inclination is shown
in Fig.
However, to cover the anticipated service
area would still require 75 spots of this size. It
is obvious that this approach for a fixed spot
configuration is not only technologically but
also economically unfeasable. Moreover, many
of the provided spots would quite probably
never require any communications at all. On the
other side also areas of less frequent
communications should not be discarded from
the beginning (e.g. distress communications).
A comprehensive market survey particularly for
the LOOPUS system has not yet been
performed. Comparison with or extrapolations
from the terrestrial mobile market is inherendy
incomplete because the provisions of the
potential service areas offered by this system
are unique and are therefore hard to compare to
other systems.
The conflict of high gain antennas with a
large service area is solved in the LOOPUS case
by means of an electrically steerable (flying)
spotbeam implemented by an active phased
array antenna. Thus, only one physical spot is
used but capable to be directed and dwell on
each required spot position on demand. Hence,
no area is excluded per se but also no energy is
wasted for unused spots.
Two active phased array antennas, one for
receive one for transmit, are controlled by an
on-board computer, allowing flexible beam
hopping in accordance with a permanently
updated, i.e. demand oriented, scan pattern.
Communications System Architecture
A number of up to 40 MSCs have been
considered for each of the three regions to
provide multiple access points to the terrestrial
networks(ISDN or PSTN). This would provide
one MSC in any significant area avoiding long
and costly landlines. For a mobile call from
Manitoulin Island e.g. into Ottawa it would be
uneconomic to downlink via the Vancouver
network and route by landline into Ottawa.
Functionally, these MSCs are gateways
between the satellite system protocol used on
the space links and the required signalling
protocols of the connected networks.
The operation of the satellite payload is
performed in close cooperation with the
Network Control Station (NCS). In the NCS all
network management functions are
implemented such as the
- configuration management
o fault management
- performance management
- security management
- accounting management
Also the special functions, Home Location
and Visitor Location Register (HLR, VLR), are
implemented in the NCS.
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International Mobile Satellite Conference, Ottawa, 1990
The spotbeams and the link concepts have
briefly been outlined before. Now, Fig.
shows the elements of the system architecture in
one of the three defined regions. The satellite
plays an active role in the operation of the
system. It actually interconnects the mobile
subscriber as well as the Mobile Switching
Centers by providing all communications links.
The Communications System
The characteristics of the mobile channel,
taking into account the high nominal elevation
angle for mobile terminals, is expected to be
dominated by direct propagation components.
Particulary the degradations introduced by
multipath propagation are less significant than
those in L-band.
The flying spot concept makes a traditional
FDMA approach impossible. Therefore, the
existence of a time"slicer" (flying spot) leads to
the application of TDMA, correlated and
synchronized with the hopping of the beam.
The resulting high bitrate for the indicated
system summes up to 69Mbit/s, i.e. 2048
channel @ 32kbit/s each. The actual capacity of
each virtual spot position is directly related to its
dwell time.
Admittedly, at first sight, the high bitrate has
a frightening impression due to its extraodinary
magnitude. Theoretically, there is no difference
in applying TDMA or FDMA to a given
communication channel, it is simply not more
than an exchange of time for frequency.
However, a number of technical aspects do
require a closer look before a final conclusion
on the applicability can be taken. It is out of the
scope of this paper to discuss the technical
implications of using high bitrates, but a few
remarks shall be _given at this point.
The system is operated in Ku-band, i.e.
14GHz. At this frequency the ratio bitrate vs.
bandwidth is moderate, hence power amplifiers
and other frequency selective devices only have
to provide a moderate bandwidth capability. As
a further consequence, the unavoidable Doppler
shift due to the relative movement of the
spacecraft to the users becomes negligible
against the necessary bandwidth of the
broadband signal. The applied powerful
modulation scheme is Rectangular Spectrum
Modulation (RSM ref. 5) providing excellent
spectrum efficiency, fast acquisition combined
with technological feasability.
The intelligent payload comprises a
regenerative transponder with processing and
routing capabilities and active phased array
antennas. As m_ntioned before, these two
antennas of lm2 each are reserved for the
reception and transmission of the mobile
signals. A third combined receive/transmit
antenna provides four spots for the Mobile
Switching Centers.
All links are interconnected on baseband
level, i.e. channel by channel, in the on-board
switching processor. Control and the demand
updating of the dynamic timeplan is performed
in the NCS and permanently communicated to
the on-board processor. The execution of the
timeplan is then performed by the on-board
processor. The updated timeplan reflects the
actual change in demand in terms of channel
capacity, dwell period and beam scan pattern of
the system. With the availability of intelligent
on-board facilities, many other functions in
support of the overall system operation can be
performed leading to autonomy in the overall
system operation. This includes time
management, present position calculation,
information dissemination, initial access
coordination monitoring and others.
[ '==\ 2 PLMN -Pubkc Land Mob01o Nelwo_kMSC- Mol_( Switching ContorNCS -Network Coordination Station
HLR. Homo Locahzlng Rogistef
VLR-Visitor Localizing RegislJr
SUMS - S_t_le User Mobil Sy$1em
MUMS - MullJple User Mobde System
Flying Spot Antenna System
=> 93 Vi_'tu_d Spot= at tOOOkm diam
40 MSCs
Total System Ca.paclly: 2000 Channel
SYSTEM ARCHITECTURE
OF ONE LOOPUS Mob-D PMSN
International Mobile Satellite Conference, Ottawa, 1990
81
From looking at the overall topology of the
LOOPUS system it becomes obvious that due
to the excellent inherent coverage of practically
any area of the Northern Hemisphere including
the Northpole the offered services can be
comprehensive. The clear cut separation of
land, aero or maritime mobile diminishs in such
a system environment. It has been pointed out
that originally the orbits have been optimized
for a coverage of the Northern Hemisphere. An
extension into the Southern Hemisphere is
hampered by the fact that the re-use of the
Ku-band frequencies is achieved only as long
as sufficient angular decoupling from systems
in the geostationary orbit is ensured.
SYSTEM OPERATIONS
A beacon signal is transmitted via a global
antenna from each active satellite to enable
automatic tracking of the ground antenna. By
receiving this signal, the system availability,
system timing, direction of satellite and other
system information can be obtained. In addition
this beacon also serves as a return channel to a
service request from a user terminal. The user
terminal is inhibited to transmit during times
that the beacon signal is not present. This
beacon transmits on a separate frequency. It is
important to note that the beacon can be
received everywhere in the entire region and not
only in the areas where the spot beam dwells.
A call can be initiated at any time the user
sees the "ready" message on his terminal. After
dialing, a short initial access burst carrying
origin and destination information is transmitted
from the user terminal on a separate frequency
(slotted ALOHA coordinated by the beacon).
This request is routed to the NCS. The NCS
will in response acknowledge the request,
check the authentication of the user card and
will eventually allocate a channel in the
on-board switch together with an update in the
antenna scan pattern control. As a consequence,
the spot will either extend its dwell time for
another channel period or a new virtual spot
position will be included in the pattern for the
time of communication. At the same time the
NCS determines the optimum gateway, i.e. the
Mobile Switching Center closest to the desired
destination of the call.
In analogy to the GSM, the call, its duration,
service category and so on will be registered in
the Visitor Location Register of the satellite
system. The VLR function is located in the
NCS. All information is then automatically
communicated to the Home Location Register
of the caller. So, one bill where all calls are
listed is received at the end of the month.
Any satellite user terminal receives its
network identity only after a smart card has
been inserted. The insertion, even without an
immediate call set-up, can be communicated to
the system if desired and will create an update
in the VLR. From that moment on, the user can
be reached by any other subscriber in the
system or caller from any other telephone in the
world.
CONCLUSIONS
In this paper the HEO satellite based mobile
communications system LOOPUS has been
briefly described. It has been shown how the
LOOPUS mobile subscriber can, via satellite,
either communicate (voice, data, FAX, etc.)
directly to each other or have access to any
national public network (ISDN or PSTN)
within three defined service areas: Europe, Asia
and North America. Through the use of a
sophisticated signalling in line with
recommendations of the GSM group home and
visitor location register (HLR/VLR), any
subscriber in the entire LOOPUS system ( i.e.
Northern Hemisphere) would be within reach
of any public telephone throughout the world.
REFERENCES
1. GSM Recommendation 02.02:
Bearer Services supported by a GSM PLMN
2. Vary, P. et.al. 1988. Speech Codec
for the European Mobile Radio System.
Proceedings International Conference on
Acoustics Speech and Signal Processing,
ICASSP-1988, pp. 227-230
3. GSM Recommendation 06.10: Full
Rate Speech Encoding and Decoding
4. Nauck, J.; Giinther, H.; Plate, H.
1987 A new type of orbits for
INMARSAT's 3rd generation, Proceedings
IAP-87-481
5. United States Patent No: 4 646 323
Feb. 24, 1987. Method and System for
Digital Data Transmission; Inv.: K. Meinzer
82
International Mobile Satellite Conference, Ottawa, 1990


LOOPUS Mob-D: System concept for a public mobile satellite system providing integrated digital services for the Northern Hemisphere from an elliptical orbit

LOOPUS Mob-D: System concept for a public mobile satellite system providing integrated digital services for the Northern Hemisphere from an elliptical orbit

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