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INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS, VOL. 11,229-240 (1993)
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VSAT NETWORKS IN THE INTELSAT SYSTEM
J . ALBUQUERQUE, L. BUCHSBAUM, C. MEULMAN, F. RIEGER A N D X. Z H U
INTELSAT, 3400 International Drive NW. Washington DC 20008-3098, U.S.A.
SUMMARY
This paper describes how VSAT networks currently operate in the INTELSAT system. Four classes
of VSAT networks (data transaction; circuit-switched; data distribution; microterminals) are identified,
and it is verified that all of them can operate with INTELSAT satellites. Most VSAT networks in
operation on INTELSAT today operate in fractional transponder leases. Fractional transponder capacity
estimates are presented for a wide range of scenarios and different INTELSAT satellite series. These
estimates clearly show increasing bandwidth utilization efficiencies for newer generations of INTELSAT
satellites. Provided that VSAT and hub sizes are appropriately selected, efficiencies are already
significant with existing satellites.
Two possible ways of increasing the utilization of satellite resources are examined in the paper:
demand assignment multiple access (DAMA) and multiple channel-per-carrier (MCPC) techniques.
The impact of using DAMA in circuit-switched VSAT networks is quantified.
K I ~ YWORDS
VSAT
INTELSAT Capacity estimates DAMA
1. INTRODUCTION
Several types of VSAT networks are now in operation, both domestically and internationally. Any
of these network types can. in principle, be operated
through an INTELSAT satellite. At present, there
are C-band and K,,-band VSAT networks in the
INTELSAT system and others are planned to begin
operation in the near future. This paper describes
how VSAT networks are currently operated in the
INTELSAT system and also offers transponder
capacity estimates for different types of networks in
present and future INTELSAT satellites. This paper
also discusses measures aiming at increasing the
efficiency of earth-station and satellite resources in
the context of VSAT networks.
In Section 2 a review is presented of the different
types of networks currently available in the market.
Advanced baseband processing techniques, such as
voice. facsimile and data compression/packetization, are discussed in Section 3 as a means of
increasing transponder capacity and enhancing user
flexibility. Section 4 describes the general conditions
under which these networks can operate in the
INTELSAT system. Transponder capacity estimates
are considered in Section 5 , both for the situation
in which a full transponder is dedicated to VSAT
operation and for the situation in which only a
portion of the transponder will be used by VSAT
networks (fractional transponder use). Section 5
includes an illustration of the increase in utilization
efficiency of satellite resources resulting from a
demand assignment capability in the network.
Finally, Section 6 presents some general conclusions.
2.
MCPC
IDENTIFICATION O F TYPES OF VSAT
NETWORKS
There has been considerable discussion in international fora (e.g. CCIR Working Party 4B, Task
Group 412, Task Group 4/3 and C C I R K C I l T Joint
Ad-Hoc Group on ISDN/Satellite Matters) referring to the meaning of the expression VSAT (very
small aperture terminal) network. Interpretations
range from very broad to very narrow ones. Among
the former, it has been suggested that a VSAT
network be interpreted as any network which
includes terminals with small antennas. At the other
extreme, a narrower interpretation sees a VSAT
network as a private network with a star topology,
comprising one hub station and a reasonably large
number of terminals with small antennas. In this
paper, the broader interpretation is adopted so that
all potential applications can be considered. However, this wide interpretation encompasses very different types of networks, and it has been found
useful to categorize these networks. Following a
review of network products currently available in
the market, four classes of VSAT networks have
been identified. These are:
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0 1993 by John Wiley & Sons, Ltd.
1. Data transaction (packet switched) networks.
2. Circuit-switched networks.
3. Video/audio/data distribution networks.
4. Microterminal networks (portable communications applications).
2.1.
Data trunsuction V S A T networks
Data transaction networks constitute the most
common class of VSAT networks. Two-way data
Received April 1993
230
J . ALBUQUERQUE
transmission (both interactive and batch) is the main
application. Other applications such as voice, video
and facsimile (fax) may be present in some cases,
but are usually considered as additional benefits of
the VSAT network. These networks have a star
topology in which a central hub station performs
both the functions of a ‘network control center’
(NCC) and of a ‘traffic gateway’. These are usually
packet switched networks in which terminals have
protocol processing capability and can support the
most common data protocols. Protocol processing
allows for adequate network response time and
more efficient use of the satellite channel.
Outbound transmission (hub to VSAT) is usually
made via a continuous digital carrier BPSK (or
QPSK) modulated by a convolutionally encoded
(typically at rate 112) TDM baseband signal. Information rates per outbound carrier are typically
between 56 and 512 kb/s. Outbound carriers are
preassigned to the hub and contain a framed baseband signal which includes timing and control information, as well as asynchronous data packets,
addressed to specific VSATs.
On the other hand, in-bound transmissions
(VSAT to hub) are made via BPSK (or QPSK)/
TDMA carriers, with satellite capacity, or at least
portions of it, shared through a contention scheme
(Aloha) or assigned on demand. Rate 1/2 convolutional encoding is generally used. The choice of
BPSK as opposed to QPSK for the in-bound link
is often dictated by off-axis emission contraints.
Information rate per in-bound carrier (burst rate)
is typically between 56 and 128 kb/s, whereas the
maximum information rate per port of a VSAT
terminal is 64 kb/s. In-bound and out-bound carriers
share the satellite capacity in FDMA mode, the
majority of the transponder power resource being
required for the out-bound link.
It is also possible that the in-bound transmissions
use BPSKKDMA and that spectrum spreading be
also used for the TDM/BPSK out-bound carrier.
This latter technique is for energy dispersal purposes
since transponder sharing between the set of inbound carriers and the out-bound carrier is still
done in FDMA. Products using CDMA are usually
restricted to lower data rates as compared to those
that use TDMA for in-bound transmissions.
Since many applications have low duty-cycle
traffic requirements, fixed assignment is inefficient
for in-bound transmissions. As a consequence, some
demand assignment capability for in-bound transmissions is required. In order to fulfil this requirement, each equipment manufacturer employs a proprietary algorithm for satellite capacity assignment
which may be a combination of
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et
al.
assigned to a VSAT as a result of an explicit
or implicit request
(c) contention scheme-VSATs contend for satellite capacity.
In general, requests for satellite capacity are transmitted in the in-bound frame in a contention mode
(slotted Aloha), either as a separate packet or along
with a data packet being currently transmitted by
the requesting VSAT (‘piggybacking’). Messages
assigning satellite capacity are contained in the outbound TDM frame. For CDMA in-bound transmissions, the demand assignment feature is, in general, not present, since CDMA intrinsically offers
a random access capability. In this situation, some
form of channel overload control may be used.
VSAT products in this class generally have extensive network management capabilities. Typically,
this function is at a network control centre (NCC)
co-located with the hub station and includes:
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( a ) fixed assignment-satellite capacity is permanently assigned to a given VSAT or a given
port in a VSAT
( b ) demand assignment-satellite
capacity is
(i) monitoring of link operation and performance at the VSAT or the port level
(ii) network configuration
(iii) enabling and disabling of VSATs
(iv) assignment of link protocols and interface
rates at the port level
(v) software downloading
(vi) gathering of network statistics including the
generation of reports and the creation of
independent customer accounts.
The network control centre also performs satellite
capacity assignment functions and, in some systems,
may also act as a packet switch.
2.2. Circuit-switched VSA T networks
In general, circuit-switched networks have a mixture of preassigned circuits and circuits assigned
on demand, with the demand assignment capability
limited to voice circuits. The capability of changing
preassigned connections without traffic interruption
can also be encountered. Mesh or star topologies
are commonly used. Voice transmission plays a
major role in these networks, with data transmission
having secondary importance. Assignment of voice
circuits on demand is done either from a network
control centre or via a distributed control procedure.
Data circuits are constituted of point-to-point clear
channels which are generally preassigned. Video
conferencing applications may be also available.
Traffic carriers are either digital SCPC/FDMA
or TDMA carriers. Modulation is either BPSK or
QPSK, with convolutional encoding of different
rates (e.g. 1/2, 3/4 or 718) and Viterbi or sequential
decoding. For SCPUFDMA systems, the information rate per carrier is often limited (up to 32 or
64 kb/s), but information rates up to 2.048 Mb/s
per carrier are also encountered. For TDMA carriers, burst rates are commonly in the range 1 to
15 Mb/s with port rates up to 2.048 Mb/s. For
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VSAT NETWORKS IN THE INTELSAT SYSTEM
SCPC/FDMA systems, it is possible to increase the
utilization efficiency of preassigned satellite circuits
by the inclusion of multi-channel per carrier
(MCPC) equipment in the VSATs. This approach
is examined in more detail in Section 3.
These VSAT networks have an NCC which performs monitoring and control of traffic terminals,
network configuration control, generation of call
records, software downloading and data recording.
Satellite capacity assignment can also be performed
by the NCC or can be accomplished via a distributed
control procedure, with a busy/idle table being kept
by each traffic terminal which is updated by control
messages exchanged among them. Therefore, in
addition to traffic carriers, control carriers are also
transmitted in the network. In SCPC/FDMA systems these control carriers share the transponder,
in FDMA mode, with traffic carriers. In TDMA
systems, control and traffic messages share the
TDMA frame.
2.3.
23 1
figuration is also possible. In the latter case, the
satellite capacity assignment function can either be
performed from a central point or be distributed
among the ‘traffic gateways’. Satellite capacity
assignment in this context may include the assignment of a specific CDMA code for accessing the
satellite or assignment of a time-shifted version of
a single code sequence.
Direct sequence spread spectrum code division
multiple access (DSSS/CDMA) is employed both
for in-bound and out-bound traffic carriers, with
BPSK modulation. FEC coding can be used. Often,
the sets of in-bound and out-bound traffic carriers
occupy different bands in the transponder (i.e. they
share the transponder in FDMA). In addition to
traffic carriers, control carriers are also transmitted
in the network to convey control and monitoring
messages and information pertaining to satellite
capacity assignment. For a star network, the outbound control carrier is spectrum spread and shares
a frequency band using CDMA with out-bound
traffic carriers. Transponder capacity (power) and
a specific spreading code are permanently assigned
to this carrier. In-bound control carriers are also
spread and also share a band in CDMA with inbound traffic carriers. However, all in-bound control
carriers use the same code and, therefore, collisions
occur when more than one in-bound control carrier
is transmitted (in this sense, in-bound control carriers share the transponder in an S-Aloha mode).
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Videoluudioldatu distribution networks
Very often, broadcast capabilities are superimposed on two-way star networks, as described in
section 2.1 above, and most data transaction network products include this feature. However, there
are network products which are exclusively intended
for one-way operation. These have. in general, a
single star configuration although their network
management systems can also control a multi-star
configuration (several ‘traffic gateways’).
In particular, digital audio and data distribution
networks often employ a TDM carrier (BPSK or
QPSK) similar to the out-bound carrier in a data
transaction network. Products using a BPSK carrier
which is spread by a PN sequence are also encountered. Note that this is not a CDMA system, and
spreading here has the purpose of rendering the
emission less interfering to adjacent satellites or
terrestrial radio relay systems. Typically, information rates are as high as 256 kb/s for data distribution and 384 kb/s for audio distribution.
2.4. Microterminal networks (portable
communicatioris upplicutions
The distinguishing characteristic of these networks
is the portability of the terminals (antenna diameters
less than 60 cm). Because of the wide antenna
beamwidths CDMA is used to mitigate interference
problems and to cope with off-axis e.i.r.p. density
limitations (e.g. the ones contained in CCIR Recommendations 524 and 728). Voice is expected to
be the basic application for microterminal networks.
Data and, to a lesser extent, low rate imagery applications can also be accommodated. Information
rates per remote terminal are limited (usually up to
19.2 kb/s). Microterminal networks are, typically,
circuit-switched networks and have a star (single
hub) Configuration. Operation in a multi-star con-
3 . BASEBAND PROCESSING TECHNIQUES
AS A MEANS OF INCREASING
TRANSPONDER TRAFFIC AND OF
ENHANCING USER FLEXIBILITY
The provision of economical thin-route satellite services with voice, data and fax capabilities has
remained one of INTELSAT’s targets since the mid
1980s. The Vista service introduced in 1983’ was
predicated upon the use of a 4.5 m antennas and
single-channel-per-carrier companded frequency
modulation (SCFC/CFM) technology, which leads
to a relatively costly earth segment for thin-route
services. The combination of VSAT technologies
with advanced digital baseband processing techniques, mainly through voice, data and fax compression and packetization can now allow the introduction of less costly thin-route services using
terminals with multi-channel capabilities.
These technologies have been under close evaluation at the INTELSAT Technical Laboratories
since 1989. The results of the subjective evaluation
of various processing techniques carried out during
1990 under simulated satellite link conditions are
described in Reference 2. It was found that a voice
quality better than currently obtained with SCFC/
CFM can be achieved at around 8 kb/s. using algorithms such as codebook excited linear predictive
(CELP) coding and time domain harmonic scaling
(TDHS). This allows a 64 kbls carrier using BPSK
232
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J . ALBUQUERQUE
and rate B convolutional encoding with sequential
decoding at a CIN, in the range of 54 to 55 dB Hz
to yield up t o ten 8 kbls voice channels. For comparison purposes, it is worth noting that SCPUCFM
requires a GIN, of 54.2 dB Hz for a single voice
channel.
Two multiplexing schemes are used for the combination of voice, fax and data channels: time division
multiplexing (TDM), and packet and statistical multiplexing. Although the latter technique is capable
of yielding a higher number or channels than TDM,
it implies a somewhat more complex terminal. The
selection of one multiplexing technique over another
depends primarily on the actual traffic requirements
and the operational and maintenance capabilities
available at the remote site(s), since on a per-channel basis, the hardware costs are quite comparable.
Similarly to voice compression, considerable progress has been accomplished by the industry in data
and fax compression in recent years. Equipment to
digitize C C I n Group 111 fax and compress data by
an average ratio of 4:l is commercially available
and has been tested and demonstrated in the
INTELSAT Technical Laboratories. Still-image
transmission systems which can be considered as a
subset of data transmission is another area which is
experiencing rapid rates of progress and hardware
miniaturization and has substantial synergism with
thin-route VSAT applications. Combined with the
capability offered by the multiplexers to reconfigure
the bit rates allocated for voice, data and fax, these
baseband processing techniques can enhance the
user flexibility and substantially increase transponder traffic throughput.
3.1. Multi-channel thin-route C-band VSA T field
trial
The INTELSAT Technical Laboratories have
conducted a field-trial of these baseband processing
techniques between a 1.8 m C-band VSAT and its
Washington, D.C. Headquarters K,,-band earthstation, using the cross-strapped capabilities of
INTELSAT satellites. When the field-trial was
initiated in March 1992, an inclined-orbit
INTELSAT V satellite at 325.5"E was used, which
was subsequently replaced by an INTELSAT VI
satellite at the same orbital location. In the experiment various baseband packages were tried.
A block diagram of a typical configuration for a
multiple channel VSAT satellite link is shown in
Figure 1. For example, a 64 kb/s information rate
carrier can carry 10 voice channels, or if fax and
data are also desired, combinations such as six voice
channels, one CCITT Group 111 fax and one 9.6
kb/s data channel which could be used for image
transmission (other configurations also available).
Data compression of the data channel can increase
its throughput for file transfers to about 38 kb/s.
Alternatively, by adding a statistical multiplexer,
several lower rate users (e.g. one 4.8 kb/s and two
et al.
2.4 kb/s) can be accommodated through the 9.6 kb/
s data channel. Although the 64 kb/s information
rate appears to be adequate for most thin-route
applications, most multiplexers are able to support
higher rates, such as 128 kb/s, further enhancing
the flexibility of users with growing traffic patterns.
Although a K,-band hub was used in this field
trial strictly for convenience reasons, it is expected
that thin-route VSAT networks of this type will
make use of C-band hubs (such as Standard A, B,
and F3) in a star configuration. BPSK was selected
for two major reasons:
the off-axis emission (CCIR Rec. 524) from
the remote VSAT to the hub station link is
the limiting factor for reducing the size of the
VSAT antenna, and BPSK offers a 3 dB natural power density spreading relative to QPSK.
Transponders carrying traffic to VSAT terminals will normally operate in a power-limited
condition, and therefore the extra bandwidth
required for BPSK transmissions (compared to
QPSK) is not relevant for the overall system
efficiency.
Convolutional encoding with sequential decoding
was selected over Viterbi decoding due to the 1 dB
higher coding gain it provides at 64 kb/s.
4. VSAT NETWORKS CURRENTLY
OPERATING IN THE INTELSAT SYSTEM
All categories of networks described in Section 2
are currently operating in the INTELSAT system.
These are closed networks operating in transponder
capacity which can be obtained from INTELSAT
through different leasing arrangements. The most
flexible of these arrangements is known as the
Intelnet service in which satellite capacity can be
leased in bandwidths varying from 100 kHz to a
full transponder, in increments of 100 kHz. VSAT
networks can also be accommodated in 100 kHz
incremental bandwidth allocations in domestic or
international leases, within what is known as the
multi-use transponder services. Further, transmissions to TVRO (television receive-only ) VSATs
can be made within INTELSAT broadcast services.
A large number of VSAT networks are currently
in operation in the INTELSAT system. Thirty of
these networks operate through Intelnet leases.
Among these, seventeen are data distribution networks (one-way transmission from hub to VSATs),
seven are data transaction networks (two-way transmissions in star configuration) and six are circuitswitched networks with mesh topology. Several
other networks are in operation through leases pertaining to multi-use transponder services.
For Intelnet, as well as for multi-use transponder
services, technical aspects pertaining to access to
the INTELSAT space segment can be found in Reference 3. As explained above, for both situations,
leases can correspond to a fraction of a transponder
INTELSAT
SATELLITE
HUB STATION
(C or Ku-Bend)
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Figure 1. Block diagram for multiple channel VSAT
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234
J . ALBUQUERQUE
or to a full transponder, with Intelnet leases being
commonly of the fractional type. For fractional
leases, the user is entitled to the fraction of the
transponder down-link e.i.r.p. (equivalent isotropically radiated power) corresponding to the
lease fractional bandwidth. The operating point of
any fractionally leased transponder is predetermined
by INTELSAT, and users cannot exceed their allocated up-link power-flux density (p.f.d.), as this
would alter the transponder operating point and
therefore disrupt the conditions under which a transmission plan has been analysed and approved. On
the other hand, a full transponder lease allows the
user to choose the transponder operating point and
gain setting, and as such optimize satellite resource
utilization from the leaseholders point of view.
For leases both within Intelnet and multi-use transponder services, earth-stations have to satisfy the
specifications of a Standard-Z earth-station' for
domestic applications or those of a standard43
earth-station5 for international applications. These
two INTELSAT standards include requirements
pertaining to antenna sidelobe performance,
antenna polarization (senses and axial ratios),
antenna steering, e.i.r.p. stability, frequency bands
of operation, carrier frequency tolerance, off-beam
e.i. r. p. density, spurious emissions, intermodulation
products and carrier spectral sidelobes.
From this list it is seen that INTELSAT requirements refer merely to the antenna system and RF
characteristics (mostly related to the transmit side).
In particular, no specifications are given for earthstation transmit gain, earth-station receive GIT,
maximum e.i.r.p. per carrier, carrier characteristics
(e.g. modulation, coding, information rate), transponder access technique, performance parameters
(e.g. threshold bit error ratio, availability). Substantial flexibility is given to service providers leasing
capacity from INTELSAT, and, as long as antenna
and RF specifications are met, VSAT network products can be freely chosen among those available
in the market.
For Intelnet leases, it is further possible that
earth-stations not meeting the specifications of a
Standard-G or Standard-Z be approved by
INTELSAT as non-standard earth-stations. In
addition, earth-stations can be type-approved, precluding, therefore, the necessity of testing each unit.
More than twenty commonly used RF earth-station
products have already been type-approved, and
establishing a VSAT network using such earth-stations only requires minimum testing.
5 . TRANSPONDER CAPACITY ESTIMATES
OF INTELSAT SATELLITES FOR VSAT
NETWORKS
The total information rate which can be transmitted
through an INTELSAT satellite transponder in the
context of VSAT networks is, of course, dependent
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et al.
on a large number of parameters. These can be
roughly grouped into:
1. Satellite parameters-saturated
e.i.r.p., C / T ,
saturation p.f.d., gain settings, cross-polarization isolation.
2. Earth-station parameters-antenna diameter,
output power, maximum permissible e.i.r.p.,
cross-polarization isolation.
3. Carrier parameters-information
rate, modulation, coding scheme, transponder access
technique, threshold bit error ratio, required
availability.
4 . Link parameters-propagation margins, interference allowances.
In addition, network topology, as well as the proportion of different kinds of links in a given network
(e.g. the ratio between out-bound and in-bound
transmission rate requirements in a star network),
also affect transponder capacity. Several scenarios
are considered here and, although far from being
exhaustive, they certainly allow that capacity estimates be obtained for many situations likely to be
encountered.
The most relevant satellite and earth-station parameters considered in the calculations are presented
in Tables 1-111. The 'typical' G / T values cited in
Tables I1 and I11 are based on LNA temperatures
of 55 K at C-band and 120 K at K,-band and are
probably typical of VSATs in the field at this time.
Advances in HEMT FET technology now makes
possible uncooled LNAs having noise temperatures
of 35 K at C band and 80 K at K,,-band.
Concerning carrier characteristics, BPSK modulation has been used whenever it leads to powerlimited operation or off-axis emission constraints
preclude the use of QPSK. When employing BPSK
carriers, energy spreading (spreading factors 2, 4 or
8) has been included, whenever it became necessary
for the carrier to meet the CCIR off-axis emission
limit.
Two different values of threshold BER (bit error
ratio) have been Considered: lo-", deemed to be
appropriate for voice applications; and 10Vhfor data
applications. Throughout the calculations, rate 1/
2 convolutional encoding with Viterbi decoding is
assumed, and required values of E,IN,, are then 4.6
dB (voice) and 6.5 dB (data). Link availability
values adopted are 99.96 per cent for C-band and
99.6 per cent for K,, band. When performing link
calculations, these requirements are assumed to be
met with system margins of 3 dB (C-band), 4 dB
(K,,-band) and down-link margins of 4 dB (C-band)
and 7 dB (K,,-band). As a result, for a threshold
BER of 10 ',clear-sky BEK values are better than
for C-band and better than 10 for K,,-band.
On the other hand, for a 10 ' threshold BER, t h e
corresponding clear-sky values are better than 10 'I
and better than 10 "' for C-band and K,,-band.
respectively.
Unless stated otherwise, occupied bandwidth i5
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235
VSAT NETWORKS IN THE INTELSAT SYSTEM
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Table I . INTELSAT satellite coverages and beam-edge saturated e.i.r.p. values
INTELSAT
satellite
~~
VIV-A
VI
VII
VII-A
K
VIII
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C-band
global beam
e.i.r.p. (dBW)
C-band
hemi-beam
e.i.r.p. (dBW)
K,-band
spot beam
e.i.r.p. (dBW)
~
23.5
26.5
26.0
29.0
NIA
29.0
36.5
44.7
43.0
45.8
47.0
44.0
29.0
31.0
33.0
33.0
NIA
34.5
Table I t . C-band earth-station parameters
Antenna diameter
(m)
0.5
1.8
2.4
3.5
9.0 (INTELSAT STD F-3)
16.0 (INTELSAT STD A)
Receive gain
(dB)
Transmit gain
(dB)
Cross-polarization
isolation
Rx & Tx
24.7
35.3
37.8
41.1
28.1
38.7
41.2
44.5
52.7
S8.2
17.7
17.7
17.7
17.7
17.7
30
49.3
54.8
'Typical' GIT
(dB1K)
4.8
14.4
17.7
20.8
20.8
35.0
Table 111. K,,-band earth-station parameters
Antenna diameter
(m)
1.2
1.8
24
5.5 (INTELSAT STD E-2)
Receive gain
(dB)
Transmit gain
(dB)
Cross-polarization
isolation
Rx & Tx
'Typical' GI T
(dB1K)
40.7
44.3
46.8
55.6
42.8
46.4
4x4
30
30
30
30
17.2
21.2
23.6
29.0
taken t o be 0.6 and 1.2 times the symbol rate for
QPSK and BPSK, respectively. Under this assumption the bandwidth limited capacities are 0.714
(b/s)/Hz for QPSK and 0.357 (b/s)/Hz for BPSK.
In addition. losses associated with antenna pointing
or tracking. power amplifier instability and earthstation equipment noise are also taken into account,
and 10 per cenl of the total noise is allocated t o
interference from terrestrial systems. Capacity
estimates are generally presented without considering interference from other satellite networks.
For one particular situation, 20 per cent of the
total noise is further allocated to adjacent satellite
interference in order to illustrate the satellite
capacity reduction expected to occur under such
conditions .
Bearing in mind the different types of networks
described in Section 2. transponder capacity estimates for several network configurations are presented in what follows. These capacity estimates are
564
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generally given by the information rate per unit
bandwidth attainable in each case and are expressed
in (b/s)/Hz.
5.1.
Data trarisactiori networks
As mentioned above, a 1 0 F threshold BER is
assumed for these networks, since data communications is the main application. The network has a
star configuration, and the hub is either a Standard
F3 (C-band) or a Standard E2 (K,,-band) earthstation with the characteristics given in Tables I1
and 111. The situation considered here encompasses
the typical data transaction network with TDM/
BPSK (or OPSK) out-bound carrier and TDMA/
BPSK (or QPSK) in-bound carrier. The results are
also valid for any configuration in which BPSK (or
QPSK) carriers access the transponder in FDMA,
TDMA. or mixed FDMA/TDMA mode.
Capacity estimates are presented in Figures 2-5.
236
J . ALBUQUERQUE
0.6
0.4
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et al.
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..............................
0.3
0.2
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ld
2 4 3.6
1.1 2 4
V-VA
36
1 1 24
1 3 2 4 S.6
S.6
Satellite
VlWlM
Vl
Figure 2. Capacity estimates expressed in (b/s)/Hz for data transaction networks
(9.0 m hub) and full transponder utilization (C-band hemi-beam): BER better
than 10 - h for 99.96 per cent of the time; clear-sky BER better than 1 0 - O
0.5
-:--
1:s
E2 1:l
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............................
0.4
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...............................
0.3
................................
0.2
0.1
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1 4 2 4 .d
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ld
24
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Figure 3. Capacity estimates expressed in (b/s)/Hz for data transaction networks
(9.0 m h u b ) and partial transponder utilization (C-band hemi-beam): BER better
than 10 for 99.96 per cent of the time; clear-sky BER better than l o - "
o.irl'i
-:-m
m1:a
0.8
B1:1
0.6
0.4
0.3
0.2
0.1
n
"
11
11
K
u
1 1
14
VI
u
1 1
1 1
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u
11
11
u
Vlll
Figure 4. Capacity estimates for data transaction networks ( 5 . 5 m hub) and partial
for 99.6 per cent
transponder utilization (K,-band spot): BER better than
of the time; clear-sky BER better than lo-'"
z
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zyxw
z
VSAT NETWORKS IN THE INTELSAT SYSTEM
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0.4
0.3
0.2
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a
237
12
11
1:1
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Figure 5. Illustration of the effect of adjacent satellite interference on capacity
estimates (5.5 m hub)
Note that Figures 2 and 3 compare full and fractional
transponder uses for a C-band hemi-beam. Figure
4 refers to fractional use of a K,,-band spot beam
transponder. Finally, Figure 5 compares capacity
estimates without and with an allocation for interference originating in other satellite networks.
The parameter ‘out-bound to in-bound ratio’
appearing in these Figures refers to the ratio of the
total number of bits flowing in the R F satellite
channel outwards from the hub to the total number
of bits flowing towards the hub. Note, in particular,
that in general this ratio will be different from the
ratio between the corresponding user data rates.
This may happen, for instance, because a contention
scheme is being used in the in-bound direction and,
as a result, the in-bound user data rate is only a small
fraction (e.g. 5 or 10 per cent) of the corresponding
information rate in the satellite channel.
5.2. Circuit-switched networks
Since voice is the main application envisaged for
this type of network, the corresponding transponder
capacity calculations are based on a threshold bit
erro ratio of 10VJ. Capacity estimates are given
in Figures 6 and 7 for star and mesh networks,
respectively. These estimates are presented for networks with preassigned circuits as well as for those
with DAMA (demand assignment multiple access)
capability.
When the traffic generated by or destined to each
VSAT in a network is low, the number of required
channels in a fixcd assignment scheme significantly
exceeds the overall traffic load expressed in Erlangs.
If a DAMA system is used to access a pool of
satellite resources, the required number of satellite
channels is dctermined by the blocking probability
and by the average traffic load at each VSAT. The
first parameter. which is the probability that a user
request will be rejected due to unavailability of
satellite channcls. characterizes the performance of
the DAMA system. Other performance criteria
(e.g. connection response time) may be also used,
but only the blocking probability is considered here.
The traffic load can be expressed in Erlangs and
gives the percentage of time that, on average, a
satellite channel of a certain capacity is expected to
be in use. For the calculations presented here, the
blocking probability is set at 0.1 per cent and 64
kb/s channels have been considered with 10 per cent
traffic load at each of these channels.
5.3.
zyx
Data distribution networks
Capacity estimates for a data distribution network
are presented in Figure 8. Such a network is an
extreme case for the situations considered in subsection 5.1, with all traffic flowing in the out-bound
direction. As a consequence, capacity estimates in
Figure 8 are a lower bound for the corresponding
estimates appearing in Figure 3 . when different
ratios between in-bound and out-bound information
rates are considered.
5.4.
Microterminal networks
Capacity estimates are presented in Figure 9 for
a star network with a Standard A ( 1 6 4 m) hub and
microterminals with a 50 cm diameter antenna. Outbound and in-bound transmissions each use half of a
36 MHz transponder in CDMA format. Information
rate in each direction of the two-way communication
between the hub and any of the microterminals is
19.2 kb/s. The chip rate is 6.14 Mchip/s. Note that
transponder capacity in Figure 9 is expressed as the
number of 19.2 kb/s circuits.
6. CONCLUSIONS
This paper has described how VSAT networks currently operate in the INTELSAT system. Four
classes of VSAT networks (data transaction; circuitswitched; data distribution; microterminals) have
been identified, and it has been verified that all of
238
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et al.
J . ALBUQUERQUE
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8
6
6
4
4
s
a
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2
1
1
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8.6
24
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24
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1.1
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24
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3.6
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Figure 6. Capacity estimates for circuit-switched star networks with a 9.0 rn hub
(C-band hemi-beam): BER better than l W 4 for 99.96 per cent of the time; clearsky BER better than 10 +. For comparison between networks with and without
D A M A refer to the vertical axis on the right side expressing capacity in number
of 64 kb/s channels. Energy spreading is not used here
3.5
3
2.5
2
1.5
1
0.5
0
1.)
2 4
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1.6
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Figure 7. Capacity estimates for circuit-switched mesh networks (C-band hemibeam): BER better than 10 for 99.96 per cent of the time; clear-sky B E R
better than 10 ". For comparison between networks with and without D A M A
rcfer to the verical axis on the right side expressing capacity in number of 64 kbi
s channels. Energy spreading is not used here
them can operate with INTELSAT satellites. Most
VSAT networks in operation on INTELSAT today
operate in fractional transponder leases. Fractional
transponder capacity estimates have been presented
for a wide range of scenarios and different
INTELSAT satellite series. These estimates clearly
show increasing bandwidth utilization efficiencies
for newer generations of INTELSAT satellites. Provided that VSAT and hub sizes are appropriately
selected, efficiencies are already significant with
existing satellites.
Two possible ways of increasing the utilization of
satellite resources have been examined in the paper:
demand assignment multiple access (DAMA) and
multiple channel-per-carrier (MCPC) techniques.
The impact of using DAMA in circuit-switched
VSAT networks has been quantified. As an illus-
tration it can be said that a 36 MHz C-band hemibeam transponder of an INTELSAT VI would be
able to carry more than 1500 64 kb/s DAMA channels in a star network in which circuits are established between 2.4 m VSATs and a 9.0 m hub. Concerning MCPC techniques, subjective evaluation
and field trials at the INTELSAT Technical Laboratories have demonstrated that voice processing techniques using TDHS packetized voice and CELP
algorithms at around 8 kb/s can provide a voice
quality only slightly inferior to PCM (64 kb/s) and
ADPCM (32 kb/s), but superior to CVSD, CFM,
etc., while at the same time demanding 7 to 10 dB
less satellite power o n a per-channel basis. Voice,
data and fax compression when associated with
VSAT RF technology are particularly attractive
techniques for thin-route satellite applications which
zyxwvu
z
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239
VSAT NETWORKS IN THE INTELSAT SYSTEM
0.2 f
l
...........................................................
.............
..............................
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WA
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am
MI
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Figure 8. Capacity estimates expressed in (b/s)/Hz for data distribution networks
(9.0 m hub) and partial transponder utilization (C-band hemi-beam): B E R better
than lo-" for 99.96 per cent of the time; clear-sky B E R better than l W y
.................................................................
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. . . . . . . . . . . . . . . . . . . . .....
. . . . ................ . . . . .
8
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6
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4
2
n
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. . . . . ............... . . . . .
.....
.....
.....
.....
zyxwv
od
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91t.1111.
Figure 9. Capacity estimates expressed in number of 19.2 kbls circuits for
microterminal star networks ( 16.0 m hub) and full transponder utilization (36
MHz C-band global-beam): BER better than
for 9996 per cent of the time;
clear-sky BER better than 10
can increase the transponder channel capacity by a
factor as high as 10 as well as enhance user flexibility. Advances in RF and modem technologies can
also be expected to yield significant capacity gains
in the near future over those indicated here.
REFERENCES
1. L. Buchsbaum. 'System design for VISTA-The
INTELSAT
service for low density traffic routes', Proc. 12th AIAA International Communications Satellite Systems Conference, Arlington, Virginia, U.S.A.. March 1988.
2. L. Buchsbaum. N. Kusmiri and W . Karunaratne, 'Technological developments for the provision of thin-route satellite services using C-band VSAT terminals', Proc. 9th International
Conference on Digital Satellite Communications, Copenhagen,
Denmark, June 1992.
3. INTELSA T Earth Station Standard IESS-410, 'INTELSAT
space segment leased transponder definitions and associated
operating conditions', 9 December 1991.
4. INTELSAT Earth Station Standard IESS-602, 'Standard Z :
performance characteristics for domestic-earth stations
zyxwvut
accessing the INTELSAT leased space segment', 9 December
1991.
5. INTELSAT Earth Station Standard IESS-MI, 'Standard G :
performance characteristics for earth stations accessing the
INTELSAT space segment for international services not
covered by other earth station standards', 9 December 1991.
6. J. Phiel and F. Rieger, 'VSAT networks in the INTELSAT
system, present and future', Proc. 1992 Microwave Workshop
and Exhibition, Tokyo, September 1992.
Authors ' biographies:
Jose Paiilo A. Albuquerque was born in Rio de Janeiro,
Brazil, on 23 June, 1944. He received the Diploma de
Engenheiro and the M.Sc. degree, both in electrical engineering, from Pontificia Universidade Catolica do Rio de
Janeiro (PUCIRJ) in 1966 and 1968, respectively, and the
Ph.D. degree in electrical engineering from the Massachusetts Institute of Technology in 1973. From 1967 to 1970
he was an Assistant Professor at PUC/RJ and from 1970
to 1973 he was a doctoral student at MIT with fellowships
from Conselho Nacional de Pesquisas (CNPq/Brazil) and
240
zyxwvut
zyxwvu
zyxwv
J . ALBUQUERQUE et
PUC/RJ. From 1973 to 1984 he was an Associate Professor, and since 1984 he has been a Professor PUCIRJ,
teaching in the Electrical Engineering Department and
doing research in communications within the Center for
Studies in Telecommunications (CETUC). From 1979 to
1982 he was Director of CETUC. From March 1982 to
March 1984 he was on leave from PUC/RJ working in
the Communications Engineering Department of
INTELSAT, Washington, DC, within the INTELSAT
Assignee Program. From April 1984 to January 1987 he
was Vice President for Academic Affairs at PUCIRJ. In
January 1992 he has again joined INTELSAT where he
is now Coordinator for Radiocommunication Standards
in the Orbital Resources Department.
Luiz M. Buchsbaum received a degree in Communications
Engineering from the Catholic University of Rio de
Janeiro, Brazil in 1972. He has continuing education in
Engineering Economics and Business Administration
from Santa Ursula University, Rio de Janeiro, Brazil
during 1977-1978. From 1973 to 1979 he worked for the
Satellite Engineering Department of EMBRATEL, the
Brazilian Signatory to the INTELSAT agreement with
growing responsibilities in both international and domestic
earth-station projects. He was system engineer for the
first INTELSAT TTC&M station at Tangua and Program
Manager for the Tangua 3 (domestic) hub station and the
second INTELSAT TTC&M. In 1979 he joined
INTELSAT where he has occupied several positions in
the Communications Engineering and R&D Department.
He has had major responsibilities in the development of
INTELSTAT services such as IBS, IDR, VISTA, SCPC,
TV, etc. and associated IESS performance requirements.
Currently, Mr Buchsbaum is a Principal Engineer in the
INTELSAT Technical Laboratories, leading a group
which is mostly involved with the development of VSAT
and TV services.
Christopher B. Meulman received his B.Eng. (1985)
degree with honours in electrical engineering from Sydney
University, Australia. He joined Aussat, Australia’s domestic satellite operator, in December 1984 as a Communi-
al.
cations Systems Engineer and was involved in the
implementation of the national satellite system communication network and from 1986 in the development and
implementation of a national VSAT network. In January
1990 he joined INTELSAT where he is currently a Senior
Communications Engineer in system planning and network evolution. His areas of interest and activity include
VSAT networks, LAN interconnection, ISDN and onboard processing as well as the use of &-band for future
satellite applications.
Frederic Rieger received the B.S.E.E. from New York
University in 1970, an M.S.E.E. from Cornell University
in 1971 and a D.Sc from the George Washington University in 1991. From 1971-1974 he was at Bell Telephone
Laboratories involved in the development of microwave
components for use in circular overmoded waveguide.
From 1974 to 1990 he worked at COMSAT Laboratories
where he was involved in the design and testing of earthstation systems. He was the principal microwave designer
of the &-band NASA ACTS master control earth-station.
Since 1990 he has been with the Transmission Engineering
and Modeling Department of INTELSAT where his
responsibilities include the development of earth-station
performance requirements for INTELSAT spacecraft and
the modelling of VSAT networks.
Xiaobo Zhu received the B.S. degree in electrical engineering from The National University of Defense Technology, Changsha, China, in 1982, the M.S. degree in
spacecraft reliability from the Chinese Academy of Space
Technologies, Beijing, in 1985, and the M.S. degree in
communications from George Washington University,
Washington DC, in 1991. From 1985 he worked in Beijing
Institute of Control Engineering where he was involved
in designs of telemetry signal processing system, industry
control processors and databases. Since 1987 he has been
in INTELSAT. His current responsibilities are in the
areas of digital communications systems (TDMA, VSAT,
DCME etc.). He is also involved in ITU C C I l T Study
Group 7 activities concerning data communications in
satellite systems.