This document analyzes system parameters for low earth orbit (LEO) and intermediate circular orbit (ICO) satellite communication networks. It discusses: 1) The necessary number of satellites and orbits to provide global coverage based on minimum elevation angles and footprint sizes, estimating a minimum of 48 satellites for Iridium and 15 for LEONET. 2) Intersatellite links (ISL), which increase network autonomy but also complexity. It models example ISL topologies for LEONET and Iridium constellations. 3) Time-varying pointing angles for intersatellite links between orbit planes, which generally require antenna steering unlike intraplane links.

1. IEEE JOURNAL ON SELECTEDAREAS IN COMMUNICATIONS, VOL. 13, NO. 2, FEBRUARY 1995 371
Number of
spot-beams
per satellite
Analysis of System Parameters for
48 16 37 37
LEODCO-Satellite Communication Networks
Markus Werner, Member, IEEE, Axel Jahn, Member, IEEE, Erich Lutz, Member, IEEE, and Axel Bottcher, Member, IEEE
Abstract- Currently many efforts are undertaken to develop
and install communication networks based on low earth orbit
(LEO) and intermediate circular orbit (ICO) satellites. However,
many problems are to be solved until the final operation of
such networks. This paper deals with basic design problems
of LEODCO-based networks. In the first part, the topology of
the satellite network is considered and estimates for the neces-
sary number of satellites,orbits and number of communication
channels per satellite are derived. Features and consequences
of intersatellite links are discussed. In the second part of the
paper, the number of communicationchannels per link is derived
with a more elaborate model. This includes the radio links
from the satellites to mobile users and to gateways, as well as
intersatellite links and terrestrial lines. We introduce a formal
model for LEO/ICO-based networks and propose a method for
the evaluation of link capacities,given the network topology and
the trafflc requirements. As an example, two constellations are
investigated in detail. One of these constellations is the Iridium
system proposed by Motorola, the other one is the LEONET
concept developed in an ESA study. Finally, the influence of
unequal traffic distribution is discussed.
I. INTRODUCTION
XISTING terrestrial radio networks (GSM, AMPS, etc.)
Eprovide mobile communications services within limited
regions. In order to supplement these terrestrial systems, a
number of LEOACO-satellite systems for global personal
communications have been proposed (Globalstar [11, Iridium
[2],Odyssey [3], etc.). Mobile users will be able to alterna-
tively access a terrestrial or a satellite network through dual-
mode handheld terminals, in this way achieving worldwide
roaming capability. In the future, third generation mobile
telecommunications systems (UMTS, FF'LMTS) with a fully
integrated satellite component will globally provide seamless
personal communications.
For the design of LEODCO-satellite systems many require-
ments have to be taken into account. For reasons of link
quality, global coverage must be achieved with a sufficiently
high satellite elevation. Together with the (restricted) choice
of orbit height and inclination, this requirement leads to the
necessary number of satellites and orbits. A larger number of
satellites might be chosen to increase the link availability by
means of multiple satellite visibility. The number of satellites
may be reduced by choosing high satellite orbits, but this
will increase propagation delay and required transmit power.
Usually, the footprint of a satellite is divided into a number
Manuscript received January 15, 1994; revised June 30, 1994. This work
The authors are with the German Aerospace Research Establishment (DLR),
IEEE Log Number 9407509.
was supported in part by ESAESTEC under Contract 9732/91/NL/RE.
Institute for Communications Technology, D-82230 WeSling, Germany.
TABLE I
SYSTEM PARAMETERS OF SOME PROPOSED LEOACO-SYSTEMS
Number of I 66 I 48 I 12 I 15
of cells, thus reducing power requirements and, by means of
frequency reuse, increasing bandwidth efficiency. The gain
in bandwidth efficiency may be reduced, however, when the
mobile users are unequally distributed within the satellite
footprint.
For a few examples of LEODCO systems, the main param-
eters of the respective satellite constellations are summarized
in Table I. LEONET is an IC0 concept developed in the
framework of an ESA study [8].'
In addition to the above mentioned requirements related
to the satellite constellation, several networking aspects have
to be considered. The demand for telephony and its global
distribution, together with upper limits for blocking probabil-
ity and speech delay, essentially determine the requirements
for network capacity and connectivity. These characteristics
depend on the number of satellites and gateway earth stations
and on the number of links between mobile users, satellites,
and gateways, including intersatellite links (EL'S).For a high
degree of connectivity, various routing alternatives are possible
and a good distribution of the traffic flow can be achieved.
Moreover, the flexibility of the network to cope with link or
node failures is enhanced. On the other hand, manufacturing
and positioning of a large number of satellites and gateways
means high fixed costs, and the permanent supply of large
capacities causes high recurring costs.
The discussions above show that because of the manyfold
interrelation of a large number of parameters the basic design
of a LEOACO-satellite system is an involved task and must
be tackled already in the initial state of system planning.
The subject of this paper is the analysis of basic param-
eters of the satellite constellation and the communications
'The name LEONET is historically derived from the title of this study.
0733-8716/95$04.00 0 1995 IEEE

2. 312 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 13, NO. 2, FEBRUARY 1995
Fig. 1.
isatellite
I
Ernin
Earth'sCentre
Illustration of the geometrical considerations.
Equator
~
Fig. 2. Hexagons inscribed into the footprints
of six isosceles spherical triangles, each one with an angle of
60" at the center of the footprint and two identical angles
network. In Section 11, we derive estimates for the necessary a = arctan (s) (2)
number of satellites and orbits and discuss the features and
consequences of intersatellite links. In Section 111, we analyze
the connectivity requirements in more detail. To this end, we
introduce a formal model for the description of the system
at the periphery of the footprint. With
(3)
2
1
T
3
< = 2 a - -
structure. Based on this, we investigate different constellations
with regard to link capacity requirements. Some constellations
are compared on the basis of numerical results. Finally,
the influence of unequal traffic distribution within a satellite
denotingthe spherical
hexagon is
of the triangles, the area of the
A = 6p2[. (4)
footprint is discussed.
11. LEO~~CO-SATELLITE
NETWORK
TOPOLOGY To cover the total surface of the earth, at least
A. Number of Satellites and Orbits
In this section, we derive estimates for the number of
satellites and orbits which are necessary to achieve a global
coverage.
The service area of a single satellite, called footprint, is a
spherical segment of the earth's surface in which the satellite
can be seen under an elevation angle E, equal or greater than
a certain minimum elevation angle The extent of the
footprint is determined by €,in and orbit height h, which
therefore are the most crucial system parameters. The half-
sided center angle Q of the footprint (cf. Fig. 1) is given by
[41
7l
@ = - -
2
arcsin -
( P L
with p denoting the mean radius of the earth.
By simply considering the extent of a footprint without
resorting to a specific satellite constellation, a lower limit
for the necessary number of satellites can be derived. For a
complete coverage of the earth's surface, it is inevitable that
the footprints overlap. The largest possible effective footprint
of a single satellite is then equivalent to the largest hexagon
inscribed into the footprint (cf. Fig. 2). This hexagon consists
(5)
4lTp2 - lT
n = - - ~
A 3a-IT
satellites are necessary. This lower limit holds for any type
of satellite constellation. Fig. 3 shows the number of re-
quired satellites versus orbit height for different values of
the minimum elevation. The number of satellites for proposed
systems (Table I) are also indicated. It can be seen that for the
systems Globalstar and Odyssey in relation to their designed
minimum elevation angles (20" and 30°, respectively) the
planned number of satellites is only slightly greater than
the required minimum according to (5). On the other hand,
LEONET uses 15 satellites, which is substantially more than
the theoretically required 9 satellites. Consequently, most of
the time a LEONET user would see two or more satellites
simultaneously, which increases the link availability in built-up
areas.
Besides the number of satellites, also the number of orbits,
0, is important, because usually a dedicated launch is neces-
sary for each orbit. In order to determine the required number
of orbits, it is sufficient to consider the satellite coverage at the
equator or at any other great circle. The best case for equatorial
coverage is depicted in Fig. 2. On the condition that in every
orbit there are at least two satellites, each orbit covers 3p@ of

3. WERNER et al.: ANALYSlS OF SYSTEM PARAMETERS 313
20” elevation
30” elevation
0 Globalstar
1000 10000
orbit height (km)
Fig. 3. Least number of satellites required for global coverage.
the equator. Therefore, at least
n=
orbits are necessary to guarantee global coverage. In Fig. 4, R
is shown versus the orbit height. Again, the values of proposed
systems are included for reasons of comparison. Similar to
Fig. 3, the values for Globalstar and Odyssey correspond very
well to the estimations. For LEONET the number of orbits is
also close to the estimated value. With 6 orbit planes, Iridium
achieves a minimum elevation of 8.2”.
Starting from the required number of orbits, R, a more
realistic estimation of the required number of satellites can
be derived for satellite constellations with polar orbits. For a
continuous belt of hexagons along an orbit track (Fig. 2)
(7)
satellites are necessary, giving a total number of R . n1
satellites. For example, assuming 8.2” minimum elevation for
the Iridium system, the above estimations yield R =6 and
n/ =11, resulting in 66 satellites, as actually planned for
Iridium. A comparison with Fig. 3 reveals that constellations
with polar orbits are not very efficient with regard to the
required number of satellites. For inclined orbit constellations,
(7) is not useful, because the concept of a continuous belt of
footprints along the orbit tracks is usually not followed here.
B. Intersatellite Links
The connectivity of a LEOACO-satellite network substan-
tially depends on the presence of intersatellite links (ISL’s).
The possibility to route long-distance traffic via ISL’s adds
to the autonomy of the system, reduces uncontrollable costs
for terrestrial PSTN links, and may decrease communications
delay. Also, vast areas can be served in which a LEO-
satellite cannot see any gateway station (e.g., the Pacific
15 I I
10” elevation
- - .
20” elevation
l
n
c
.-
n
L
0
;10
L
W
n
5
t
.
0 5
l
n
W
U
W
c
0
30” elevation
’ proposed systems: ’
- 0 Globalstar
- -
-- V I r i d i u m
v LEONET
1000 10000
orbit height (km)
Fig. 4. Least number of orbits required for global coverage.
Ocean). Generally, the requirements of satellite handover can
be loosened because the path to a certain fixed gateway can
be maintained as long as the mobile user is served by the
current satellite, whereas without ISL’s the satellite has to
“see” the gateway and the mobile user simultaneously.Finally,
the ISL network is well suited to carry signalling and network
management traffic.
On the other hand, the introductionof ISL’s entails a number
of consequences, such as additional weight, complexity, and
cost of the satellite payload. This comprises ISL antennas,
transmitters, and receivers, as well as switching capabilities
on board the satellite, which are inevitable when ISL’s are to
be used. The required ISL pointing, acquisition, and tracking
(PAT) also increases satellite complexity and demands steer-
able ISL antennas (in addition to steerable gateway antennas
on board the satellite). A serious problem for the negotiation
of national landing rights may be that PTT’s could consider
ISL’s a rival to their terrestrial networks.
Roughly speaking,LEO satellitesystems intended for global
coverage, such as Iridium or Calling [5] includeISL’s, whereas
for IC0 systems the advantages of ISL’s seem to diminish.
There are two types of ISL’s, namely intraplane ISL’s
connecting satelliteswithin the same orbit plane and interplane
ISL’s connecting satellites in adjacent orbit planes.
Two satellites on different orbit planes “see” each other un-
der time-varying pointing angles. Therefore, interplane ISL’s
generally require antenna steering, whereas intraplane EL’S
can be maintained with fixed antennas. Moreover, interplane
ISL’s may not be permanently maintained, because as the
satellites follow their orbits, their distance may vary within
a large range and the earth may interrupt the line of sight. In
this case, the interplane ISL would have to be switched off
and on again, requiring the formidable task of PAT.
Fig. 5 shows possible topologies of satellites and ISL’s for
the LEONET and Iridium constellations, respectively. The
LEONET constellation serves as an example for a typical IC0
concept, Iridium represents a well-known LEO system.

4. 314
LEONET
IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS,VOL. 13, NO. 2, FEBRUARY 1995
Iridium
8000 I 1000
'
'
1
.
0 sec
~000+70&
Distance range
Loss variation
Horizontal pointing
Vertical oointina
1 -
2000
I
3000
intraplane interplane
ISL 1 ISL
12.2 dB
-106". ..106"
-10". ..- 46'
-36"
5 -20
.E -30
intraplane interplane
ISL 1 ISL
34" ...65"
f -40
2 -50
k -60
-70
-80
I
I I
I 1
-16" I -16"
~
80
60
40
F 20
- 0
f -20
-40
-60
-80
-150 -100 -50 0 50 100 150
Longilude 8" deg
(a)
80
60
40
d 20
0 0
g -20
-40
-60
-80
-150 -100 -50 0 50 100 150
Longitude in dsg
(b)
Fig. 5. Topology of satellites and ISL's for (a) LEONET at t = 240 s and
(b) Iridium at t = 0 s. Subsatellite points -intraplane ISL's - - - - -
interplane ISL's.
For the LEONET constellation, intraplane ISL's and perma-
nently maintainable interplane ISL's are considered, Fig. 5(a).
Each satellite must be equipped with four bidirectional ISL
ports (ISL antenna, transmitter, and receiver); two of the four
ISL antennas must be steerable.
For the Iridium constellation, intraplane ISL's and three
equatorial rings of interplane ISL's are shown in Fig. 5(b).
It is assumed that for each time those interplane rings are
maintained which are closest to the equator. For the north-
bound satellites new interplane ISL's south of the equator
are switched on when the oldest interplane ISL's north of
the equator are switched off. The corresponding procedure is
followed for the south-bound satellites. For the boundaries
between north-bound and south-bound satellites we assume
appropriate interplane ISL's at any moment of time.
The above mentioned ISL topology only serves as an
example which will be used in Section 111 for the evaluation
I / I I V
6000
2 -40
z -50 l l o ~ ~
13000
I
I I I I 1
-50 0 50 100
-90 -
-100
Homonloi Poinling ~n deg
(b)
Fig. 6.
(b). Pointing from satellite 1 to satellite 12, for both constellations.
Time-varyinginterplanepointing angles for LEONET (a) and Iridium
of traffic flows. As discussed below, however, the considered
ISL topology seems to be a reasonable choice.
Fig. 6 gives an impression of the time-varying interplane
pointing angles in azimuth and in elevation,related to the flight
direction of a satellite. Fig. 6(a) shows that for the LEONET
constellation a large range of pointing angles is necessary,
causing rather challenging PAT requirements.
From Fig. 6(b) it can be seen that the PAT requirements
for Iridium are easier. The flat shape of the loop suggests to
keep the vertical antenna pointing fixed and to move the ISL
antenna only in horizontal position, as indicated in the Figure.
This approach is already mentioned in the literature [6].Here, a
vertical two-sided 2 dB-beamwidth of 5" is assumed, in which
the ISL can be maintained. With a 1:2 beam ellipticity, this
corresponds to approximately 36 dBi gain at 23 GHz. Then,
from the vertical pointing angles, Fig. 6(b), it can be concluded
that interplane ISL's can be maintained between latitudes of
approximately 60" south or north, respectively. This agrees
very well with the example ISL topology discussed above.
The remaining horizontal steering range amounts to 31O.
In Table 11, the geometric characteristics of ISL's are
compared for the LEONET (KO) and Iridium (LEO) con-

5. WERNER et al.: ANALYSIS OF SYSTEM PARAMETERS 315
stellations. The free space loss variation defined by 20 lg
max./min. distance is also included.
In the following we assume that ISL’s are used for Iridium;
for LEONET we consider ISL’s as a system option and
investigate both, the LEONET constellation with and without
ISL’s, respectively.
111. CAPACITY REQUIREMENTS
AND
TRAFFIC
ENGINEERING
ASPECTS
In this section we will first discuss the main network
elements and give an estimation of the number of channels
to mobile users. After that we develop a possible approach
to network capacity design on the basis of an elaborate
network model. The underlying capacity evaluation procedure
is discussed in detail and representative results are presented.
The main network nodes are communications satellites in
low earth or intermediate circular orbits (polar or inclined)
with periods of a few hours and a regular phasing between the
satellites within the same orbit. They have transmission and,
v)
(D
K
5
0
.
K
0
-
Y-
O
Q
L
n
5
x
0
v)
v)
0
0
0)
c
. . . . . . .
. . . . . . .
. . . . . . . ,
. . . . . . . .
. . . . . .
. . . . . . .
. . . . . . .
. . . . . . ,
. . . . . .
6000 j I
.. proposed
........:
........:
.
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.
.
.
.1
.
.
.
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.
.
.
.
.
.
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.
. I................
. . :
5000
4000
3000
2000
1000
0
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
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. ;.......>.
.
.
.
.
i.....j ....i...; ................
I
. . . . . .
. . . . . .
,
. . . . . .
. . . . .
1.. v Iridium ( 8 . 2 O ) . . . . . : :
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
..........a.. .....i
.
.
.
.
.
.
i
.....I ...‘.._
i_
30” elevation
. . . . . .
. . . . . .
1000 10000
orbit height (krn)
optionally (with onboard processing), a switching function.
The to existing public networks is guaranteed by
Fig. 7. Necessary number of channels on the mobile user link. Lines
represent estimates accordingto (8), symbols represent numerical resultsfrom
detailed analysis.
terrestrial gateway stations. Besides this interface function
their main tasks are switching and network management. The
third group of network nodes are the terminals of mobile and
fixed users, representingthe sources and destinations of traffic.
The network nodes are connected by several kinds of links:
Mobile user links (MUL’s).
Links between satellites and those mobile users within
their footprint, who communicate via the satellite.
Links between satellites and gateways in the coverage
area of the satellite.
Links through the public switched telephone net-
worWpublic data networks (PSTNPDN’s).
Totality of existing telephone and data networks,
thus providing the possibility for all fixed users to
communicate with mobile LEOACO system users via
gateway stations.
Direct connections between satellites; their use in a
LEOACO satellite system is optional. ISL’s may connect
satellites within the same orbit (intraplane) or satellites
in adjacent orbits (interplane).
For the routing of long-distance traffic (i.e., traffic between
users in different coverage areas), there exist two major
alternatives, depending on the use of ISL’s. In a system
providing ISL’s this traffic may be transported as far as
possible through the space segment. In connection with a
terrestrial backbone it is possible to significantly reduce the
number of worldwide necessary GW stations. Theoretically,
one GW could be sufficient for global connectivity. If the
system does not provide ISL’s, then the whole long-distance
traffic has to be transported through public lines. In this case
it is necessary for global connectivity that every satellite
has connection to at least one GW station at any instant of
time. A reasonable realization of a LEOnCO-satellite based
communicationssystem should of course combine the positive
Gateway links (GWL’s).
Intersatellite links (ISL’s).
features of both alternatives. To optimize configuration and
connectivity in this sense, a computer aided analysis-based
on an appropriate mathematical model-is a promising ap-
proach.
Due to the low antenna gain of the mobile terminals, the
power consumption of the MUL’s is high. Therefore, these
links are the most crucial ones with respect to the power
budget.
A. Estimation of the Number of Channels to Mobile Users
In the following we assume one million mobile users
worldwide and a traffic intensity of 5 mErl per user, resulting
in an overall traffic Tnet= 5000 Er1 to be transported within
the network. Additionally, we assume that Tnetis completely
generated on the land masses of the earth, which correspond to
a fraction of 26% of the total surface. With these assumptions
we can calculate an estimate for the number IC of required
MUL channels per satellite. In the worst-case, the footprint of
a satellite is completely over land; then
(8)
is an estimation for the number of channels needed.to carry all
the traffic generated in that footprint. This relation is plotted
versus orbit height in Fig. 7, together with results from a more
detailed analysis (Section 111-B).
k = - . -
1 Tnet
0.26 n
B. Network Capacity Design
The previous subsection gives a rough estimate for the
number of required MUL channels. Since the number of
communications channels per satellite (including GWL and
ISL’s as well as the MUL) determines the power subsystems
and thus the satellite mass and cost, this parameter will now
be discussed in more detail. Especially, a tool is presented and

6. 316 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 13, NO. 2, FEBRUARY 1995
used to calculate the necessary number of channels per satellite
based on a given traffic intensity distribution on the earth.
Additionally, with respect to traffic routing considerations also
the number of required PSTN lines will be discussed.
The main task of this analysis is the investigation of link
capacity requirements for various constellations, under some
basic conditions like permanent global connectivity and upper
limits for blocking probability and delay. The dimensioning of
the network should be user-oriented, i.e., adapted to the world-
wide expected demand for mobile communications and its
distribution (considering a realistic market share for LEOIICO
systems).
1)Network Model: As a base for the capacity investiga-
tions we present a graph-theoretical model describing the
network configuration at a certain instant of time [7]. (Time
variance of network topology and traffic are taken into account
on the computer calculation level and will be discussed later.)
The model consists of the following components:
1) n satellites SI,.. ..sn.
2) n traffic sources/destinations ml....,m,; mLrepresents
all mobile users communicating via satellite s,.
3) n network nodes fl,. ...f n in the footprints of the
corresponding satellites. The whole amount of PSTN
traffic “belonging” to the footprint of s, is assumed
to be concentrated in node f,. Furthermore, the fL are
assumed to be completely connected with each other
via the PSTN. The presence of at least one GW in the
footprint of s, results in a direct connection between fz
and s, via a GWL; therefore, such a GW can be regarded
as physically representing the fictitious node f7.
The whole traffic generated at an instant of time can be
described by a traffic matrix T of dimension 2n . 2n:
(no traffic to
In this matrix, the elements t i j describe the amount of
traffic between nodes i and j in Erlang (Erl). Traffic between
two fixed users fi and f j is assumed as pure PSTN traffic
and therefore not included in the analysis. In case only voice
communication over full duplex channels is considered, the
traffic matrix T becomes symmetric.
The totality of connections within the network is described
by a symmetric connectivity matrix C of dimension 371 . 371,
where “1” denotes an existing connection between adjacent
nodes:
n l . ... .. . ,m,,
0
0
E
f l , . . . .. .,Jn
0
I
(PSTN)
C&J,
’ I , . . . ...,S“
E
Cl,$,
cs,s,
(ISLS)
Here, E = ( e z J )denotes the unit matrix, i.e., eZJ= 1 for
i = j and eZJ= 0 for i # j. The c , , ~ ~
describe connections
with ISL’s, and the cszf,and cfZs,mark a connection between
a satellite and a GW station in its footprint, i.e.,
1,
0, for i = j and no GW present in the footprint
0, for i # j .
for i = j and a GW present in the footprint
(9)
2) Capacity Evaluation Procedure: Any LEO/ICO satellite
system is on principle highly dynamic. The movement of
network nodes-satellites and mobile users-on the one hand,
as well as the dynamic user activity due to the time zones
on the other hand, lead to time variance of network topology
(switching of ISL’s, changing GW’s in a footprint, etc.) and of
demand for link capacities. In order to take this time variance
into account, investigations are made for several successive
instants of time, in which the system is assumed as static.
By choosing the time interval small enough, it is possible
to gain reliable worst-case and average results for required
link capacities and for delay values. The generalized flow
chart of the C computer program developed for this capacity
evaluation in Fig. 8 illustrates the evaluation procedure for a
certain instant of time.
Starting point is a certain satellite constellation, which is
characterized through orbit height (respectively orbit period),
through number and inclination of orbits and throagh number
and phasing of satellites in these orbits. With a given minimum
elevation angle for the connection to the satellites the coverage
areas can now be calculated.
With respect to communications traffic, the underlying re-
quirement is to provide voice service with a blocking probabil-
ity less than 5% for 1million subscribers which are assumed to
be distributed within six regions according to Table 111. These
(land mass) regions are visualized in Fig. 9. Furthermore, it
is assumed that all users are permanently generating traffic
with 5 mErl, no matter which time zone they are actually in.
This assumption of a “permanent worldwide busy hour” seems

7. WERNER et al.: ANALYSIS OF SYSTEM PARAMETERS
Region
North America
311
Percentage Absolute #
25% 250 000
INPUT
- __ -
r i r - 7
Connectivitymatrix C
Traffic matrix T
N.America
CALCULATION I I
Europe Asia %America Africa Australia
PROCESS
- variationof routing strategies
OUTPUT
- Mean utilizationof different kinds of links
- Propagation delay statistics
- Statisticsabout lengthsof required PSTN lines
Fig. 8. Schematic description of network connectivity analysis.
to be quite legitimate if one considers that the main interest
with respect to system capacity design, and especially satellite
design, is in worst-case traffic requirements on the different
links in the network.
The traffic flow between the six regions is assumed accord-
ing to the regional traffic flow matrix in Table IV. In addition, a
distinction is made between two types of connections, mobile-
to-fixed and mobile-to-mobile. Assuming 10%of the users to
be mobile, yields a ratio of mobile-to-fixed traffic : mobile-
to-mobile traffic = 18:l.
For further network analysis the regional traffic is allocated
to the different satellites according to their percentage of
coverage of the respective land mass. This mapping of land
mass regions onto coverage areas is indicated by the hatchings
in Fig. As a result we get a traffic matrix T for the specific
configuration. Together with a given GW distribution, also the
connectivity matrix C can be determined.
Now the central aim is to transport all the generated
traffic from source to destination, considering the underlying
conditions (blocking probability, delay, etc.). That is, we
have to determine paths from source i to destination j with
t i j > 0. From connectivity matrix C it can be seen that
*This approach assumes footprints to be centered around the subsatellite
points, which does not hold for the Odyssey concept. Nevertheless, in this
mapping approach spot-beams are not taken into account, and the possible
land mass coverage of any satellite is clearly limited by the minimum elevation
angle; therefore, by applying the same approach to Odyssey with the minimum
elevation angle foreseen for this system, the comparison of the respective
numerical results with those of other systems seems to be legitimate.
TABLE I11
REGIONAL
DISTRIBUTION
OF SUBSCRIBERS
Europe 11 25% I 250000
Asia 11 20% I 200000
South America 1 N
I
: 1 :iiCMlii
Africa
Australia/NZ 10% 100000
Fig. 9.
constellation LEONET.
Traffic regions on earth; footprints and example GW distribution for
Africa 11 24 Er1 I 36 Er1 I 19 Er1 I 10 Er1 1 436 Er1 I 5 Er1
Australia 11 24 Er1 I 12 Er1 I 39 Er1 I 5 Er1 1 5 Er1 I 441 Er1
no direct connections (c;j = 0) exist between source and
destination nodes (mobile and fixed users). The basic criterion
for path search is a generalized cost function, and “cheapest”
paths are selected for the traffic transport; this is performed
with the well-known Dijkstra algorithm [9]. On the basis of
different cost functions several strategies for the routing of
long-distance traffic are possible:
ISL’s are preferred, i.e., high pseudo-costs are attached
to PSTN links and low ones to ISL’s.
PSTN links are preferred, i.e., high pseudo-costs are
attached to ISL’s and low ones to PSTN links.
If not each satellite has connection to at least one GW
station during the whole time, then a certain amount
of ISL capacity is necessary to maintain global connec-
tivity. Then it is desirable to use this ISL capacity in
the whole network as far as possible and route only the
traffic exceeding this amount through PSTN.
Paths with smallest possible propagation delay are cho-
sen.
If the cost function is 1 for each link, the algorithm
yields shortest paths in the sense of hops.

8. 37R IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 13, NO. 2, FEBRUARY 1995
80
60
40
20
0
-20
-40
-60
-80
Fig. IO. Land mass coverage and traffic mapping for constellation LEONET
In the computer program the first three routing strategies
were implemented. As output the algorithm yields the re-
quired (worst-case) link capacities in Erlang for the different
connections (MUL, GWL, ISL, PSTN). Furthermore, the av-
erage utilization is calculated for every link type, so that the
calculation of average capacity requirements is possible, too.
For network dimensioning the maximum number of required
channels on any link is of importance. Therefore, channel
numbers are calculated according to the Erlang B formula [101
(B:blocking probability; C: number of channels; A: traffic
in Erl).
In addition, from all point-to-point-connections the maxi-
mum propagation delay is extracted and an average value for
the whole network is calculated.
C. Results
First of all, we present results for the MUL. In Fig. 7,
the estimated value IC (cf. (8)) is plotted versus the orbit
height and compared with results from the more detailed
analysis. In this context it should be noted that the estimation
is based on the theoretically minimum number of required
satellites w.r.t. global coverage, whereas the exact figures are
derived from the proposed systems. Furthermore, in contrast
to the estimation, an unequal userhraffic density over land
mass is assumed for the numerical analysis, as presented
above. Especially, the region of Europe is characterized by
a high traffic density. For the LEO systems Iridium and
Globalstar, with footprint areas in the order of magnitude
of the European land mass, the calculated figures are close
to or even larger than the estimation. On the contrary, for
LEONET and Odyssey the exact figures are significantly
below the estimation. This is mainly due to the following two
reasons: First, the comparatively large footprints do in reality
not cover only land mass, but-ven in the worst-case-they
include quite a share of oceanic regions (cf. Fig. 9). Second,
bigger overlapping zones of the satellite footprints increase the
2ooo k
I
0 ‘ I I I A I
0 2 4 6 8 1 0 1 2 1 4
Number of gateway stations
Fig. 11. Required full duplex channels per ISL. ARS = Advanced routing
strategy (cost function #3). Constellarion: LEONET with 4ISL’s per satellite.
multiple satellite visibility, and thus the mobile user traffic can
advantageously be shared among satellites.
In order to evaluate the long-distance traffic requirements,
all constellations were investigated for three different routing
strategies and for several GW distributions, with ‘‘reasonable”
locations for the GW stations (e.g., preferably in most industri-
alized regions, on dry land). Figs. 11 and 12exemplarily show
worst-case channel requirements per ISL and in the PSTN,
respectively, for the LEONET constellation with 4 ISL’s per
satellite as presented in Section 11-B.
Fig. 11 shows the maximum required number of full duplex
channels per ISL. With an increasing number of GW stations,
the worst-case channel requirement for the ISL is decreasing
monotoneously to zero, if all long-distance traffic is preferably
routed via PSTN. With 7 GW stations reasonably distributed
over dry land, the LEONET system is able to guarantee global
connectivity without use of ISL’s. This is due to the fact
that every satellite has connection to at least one GW station
during the whole time, so that long-distance traffic can always

9. WERNER er al.: ANALYSIS OF SYSTEM PARAMETERS 319
I I I I I
I
E 1500
a
ln
.-
?
! loo0 t
Prefer PSTN
“ L o Prefer lSLs
I I - 6 I A I
0 2 4 6 8 1 0 1 2 1 4
Number of gateway stations
Fig. 12. Required full duplex PSTN lines for long-distance traffic in the
whole network. ARS = Advanced routing strategy (cost function #3).
Constellation: LEONET with 4 ISL’s per satellite.
be routed via PSTN. The corresponding GW distribution is
depicted in Fig. 9. If ISL’s are preferred, a roughly constant
amount of required ISL capacity remains for more than 3 GW
stations.
Fig. 12 illustrates the number of public full duplex lines that
are required to transport the long-distance traffic. If existing
public networks are preferred, then these requirements remain
relatively constant with a varying number of GW stations. On
the analogy of Fig. 11 it can be seen, that with 7 reasonably
located GW stations it is possible to avoid the use of PSTN
lines for transport of long-distance traffic.
Comparing the numerical results for LEONET in Figs. 11
and 12, the importanceof the applied routing strategy becomes
obvious: For a number of 6 GW stations (one in each region)
for example, in the case of ISL preference the maximum
required ISL capacity is high, whereas the demand for public
lines is low. If public networks are preferred, the amount of
required ISL channels can be reduced by more than 50%, but
at the same time the PSTN channel requirements enormously
increase. With the advanced routing strategy (cost function 3
in Section 111-B)
the demand for both, ISL channels and PSTN
lines can be clearly reduced at the same time.
From these results, it can be seen that a number of 6
or 7 GW stations is a clear lower limit for a reasonable
LEONET system realization; considering some other relevant
aspects as well (e.g., regulatory and political issues, multiple
visibility between satellites and GW’s, robustness w.r.t. failure
or regional traffic overload), one may regard a number of
10-20 GW stations as good choice for LEONET. However,
the 7 GW constellation is well suited to show the fundamental
trade-off between the use of ISL’s and the amount of required
public lines, in this context also stressing the importance
and prospectives of adapted strategies for the routing of
long-distance traffic. Corresponding numerical results for a
LEONET constellation without and with 4 ISL’s per satellite,
respectively, are given in Table V.
The Iridium system concept is essentially based on the
presence of an extensive ISL infrastructure;nevertheless, both
the GW distribution and the supplementaryuse of public lines
for transportof long-distancetraffic are importantmainly w.r.t.
the task of reasonably interfacingmobile and fixed users in the
global communications system. Therefore, detailed investiga-
tions for Iridium were performed similarly to LEONET. As
one expects, due to the smaller footprints the number of GW
stations should be higher than in LEONET, in order to use
the installed ISL infrastructure efficiently while reducing the
amount of required PSTN capacity. A number of 55 GW’s,
compared to GW distributions including 12-20 GW’s, yields
significantly lower worst-case channel requirements for MUL,
GWL, and PSTN; additionally, the utilization of installed
ISL links can be increased. Generally, the scope for traffic
flow optimization by means of adapted routing strategies is
quite limited for Iridium in comparison to LEONET. The
numerical results show that a preference of ISL’s for long-
distance traffic routing is the best choice. The ARS approach
is not suitable for Iridium in order to decrease ISL and PSTN
channel requirements to reasonably low values at the same
time, as can be seen in Table V.
The figures discussed so far refer to long-distance traffic
requirements. For the channel requirements on the GWL
the routing strategy’s influence is not significant. Due to a
reduction of traffic concentration, the worst-case demand for
GWL channels monotoneously decreases with an increasing
number of GW stations until all satellites have a direct
connection to at least one GW station.
Based on the worst-case channel figures for MUL (down-
link), ISL, and GWL, link budget calculations can be per-
formed to determine peak RF power requirements on board
the satellite, which is of basic importance for satellite design.
Results for LEONET and Iridium are given in Table V. The
budget was calculated with the same assumptions (e.g., link
margins, user antenna gain, satellite antenna pattern. .. ). The
average power values are calculated by considering the mean
utilization of the respective links. The figures show that the
power consumptionon board the satelliteis mainly determined
by the MUL. Consequently,the overallpower requirementsare
significantly lower for Iridium satellites.
In the last two lines of Table V, figures for maximum
and average propagation delay are denoted, showing a better
performance for Iridium due to the lower orbit altitude. As far
as voice service is concerned, an upper limit of 400 ms for the
end-to-end delay is set by CCITT, where signal processing and
switching delays are included as well as propagation delay.
Considering a typical packet length of 20 ms and current
switching technology, the sum of additional delays through
codec, assembling/disassembling processes and switch-
inghuffering operations should be in the range of 100 ms for
a typical point-to-point connection, so that the CCITT limit
for voice transmission can be guaranteed within either system.
D.Partition of Satellite Footprints into Spot-Beams
By use of multibeam antennas, the satellitefootprints can be
divided into smaller cells. Typical numbers of spot-beam cells
are NZ = 7, 19, 37, 48, or 61. An advantage is the increase

10. 380
Constellation
Gateway distribution
per satellite
ISL'S intra-orbit
inter-orbit
IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS,VOL. 13, NO. 2, FEBRUARY 1995
Iridium Iridium LEONET LEONET
55GW's 55 GW's 7 GW's 7 GW's
2/4 2/4 4 0
2 2 2 0
012 012 2 0
TABLE V
COMPARISON
OF NUMERICAL
RFSULTSFOR DIFFERENT
CONSTELLATIONS
Min. elevation angle (mob. user)
Min. elevation angle (GWs)
Results:
Required channels per MUL
(5 mErl, worst-case) per ISL
8.2" 8.2" 20" 20"
5" 5" 5" 5"
-
488 488 1047 1047
902 82 248 0
11 Pref. ISL's I ARS I ARS I Pref.PSTN
Routing strategy
Peak power MUL (1.6 GHz) 81 W
per satellite ISL (23 GHz) 7 W
(5 mErl) GWL (4 GHz) 3.9 W
Average power MUL (1.6 GHz) 13 W
per satellite ISL (23 GHz) 2 W
(5 mErl) GWL (4 GHz) 0.6 W
Max. propagation delay 171 ms
Aver. propagation delay 46 ms
Elevation parameters:
81 W 393 W 393 W
0.7 W 11.5 W 0
4.2 W 8.3 W 8.5 W
13 W 124 W 124 W
0.1 W 5.5 W 0
0.7 W 2.7 W 2.7 W
171 ms 265 ms 198 ms
32 ms 95 ms 75 ms
of bandwidth efficiency due to the reuse of frequency bands
in sufficiently separated cells. In a narrowband system, the
number of different frequency bands is typically NF = 3,4, 7 ,
or 9. For a uniform traffic distribution within the footprint, the
resulting reduction of required system bandwidth is Nz/NF.
Another advantage is the reduction of RF power for the MUL's
due to the concentration of the power within smaller areas,
equivalent to a higher satellite antenna gain. The reduction of
power is approximately Nz. The use of multi-beam antennas
requires high technical efforts, but they are planned for all
relevant LEOACO systems in order to reduce the transmission
power of the handheld terminals.
For many areas within the system coverage, we must take
into account that the mobile user traffic is unequally distributed
within a satellite footprint. Then, the advantages of spot-beams
will partially diminish.
In the extreme case, all traffic channels may have to
be concentrated in a single spot-beam, requiring sufficiently
flexible satellite onboard processing. Then, for a narrowband
system, the gain in bandwidth efficiency is completely lost, but
the reduction in necessary transmit power of the satellite and
the mobile terminals remains. For a spread spectrum system,
the interference within the heavily loaded cell approximately
rises by a factor of Nz. Since spread spectrum systems are
interference limited, this causes a link degradation of 10
lgNz dB. As a countermeasure, the processing gain could
be increased by increasing the chip rate (and the bandwidth
requirement, correspondingly) or by decreasing the service bit
rate (and voice quality).
A detailed investigation of the significant influences of
unequal traffic distribution represents a rewarding subject of
further research.
IV. CONCLUSION
Several basic system parameters of LEOACO systems have
been discussed. Estimations for the number of satellites, num-
ber of orbits and number of transmission channels have been
presented. A major part of the paper was devoted to the
discussion of the number of communication channels per satel-
lite. To this end, a formal model for mobile communications
networks based on LEOACO satellites was introduced and a
method for the analysis of network connectivity requirements
was proposed. This evaluation procedure can efficiently be
used in the initial process of planning and dimensioning
LEOACO networks. The concrete application in form of a
specially developed software tool was shown exemplarily
for a comparison between two proposed systems, Iridium
and LEONET. Numerical results of network connectivity
investigation show that the system constellation basically

11. WERNER et al.: ANALYSIS OF SYSTEM PARAMETERS 381
influences the link capacity requirements. Thus the presented
of network components and for comprehensive system cost
calculations [111.
Axel Jahn (M’90)received the Dipl.-Ing. degree in
1990 from the University of Karlsruhe, Germany.
Since September 1990, he has been a Research
Scientist with the Institute for Communications
Technology of the German Aerospace Research Eb-
tablishment (DLR) at Oberpfaffenhofen, Germany
results provide important input information for the design
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New York: Addison-Wesley, 1969.
New York: Wiley, 1975.
Markus Werner (M’92) received the DipL-lng.
degree in electrical engineering from the Technical
University Darmstadt, Germany, in 1991.
Since 1991,he has been a Research Scientist with
the Institute for Communications Technology of the
German Aerospace Research Establishment (DLR)
at Oberpfaffenhofen, Germany. His research inter-
ests are network aspects of communication systems.
Currently he is working on routing and congestion
control in dynamic satellite networks. He is also a
lecturer at the Carl-Cranz-Gesellschaft.
his main tields of
also a Lecturer at
From 1990-1993, he was a Scientific Assistant at
the FemUniversitat Hagen, Germany. His research
interests are simulation methods and the analysis of
communication networks. LEO satellite systems and
the influence of land-mobile channels are currently
interest. He is doing Ph.D. work on these subjects. He is
the Carl-Cranz-Gesellschaft.
Erich Lutz (M’92) was born in Augsburg, Ger-
many, in 1950 He received the Ing grad degree
from the Polytechnic Augsburg in 1972, the Dipl:
Ing degree from the Technical University Munich
in 1977, and the Dr -1ng degree from the Military
University Munich in 1983
From 1977to 1982he was a Research Assistant at
the Technical University and the Military University
Munich, working in the field of digital transmis-
sion over cables and optical fibers. Since 1982,
he has been with the Institute for Communications
Technology of the German Aerospace Research Establishment (DLR) in
Oberpfaffenhofen, Germany Since 1986, he has been head of the Digital
Networks section of this institute. His current research interests include
mobile radio channel characterization, error control techniques, multiple
access techniques, and networking aspects, in particular, for mobile satellite
communication networks
Gesellschaft fur Betrieb
at the Carl-Cranz-Gese
Axel Bottcher (M’89) received the Dipl.Math.
degree in 1987 from the Ludwig-Maximilian-
Universitat, Munich, and the Dr.1ng. degree from
the University of the Armed Forces, Munich, in
1992.
From 1988 to 1994, he worked as a Research
Scientist at the Institute for Communications
Technology of the German Aerospace Research
Establishment (DLR) at Oberpfaffenhofen, Ger-
many. Since July, 1994, he has been Head of the
radio planning group at T B&D Telekommunikation
und Dienstleistungen in Munich. He is also a Lecturer
Ilschaft.
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