Vsat Networks in the Intelsat

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS, VOL. 11,229-240 (1993)

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 ~ Y WORDS VSAT INTELSAT Capacity estimates DAMA MCPC

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. 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) refer- ring to the meaning of the expression VSAT (very small aperture terminal) network. Interpretations range from very broad to very narrow ones. Among the former, i t 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. How- ever, 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:

1. Data transaction (packet switched) networks. 2. Circuit-switched networks. 3. Video/audio/data distribution networks. 4. Microterminal networks (portable communi-

cations applications).

2.1. Data trunsuction V S A T networks

Data transaction networks constitute the most common class of VSAT networks. Two-way data

0737-2884/93/040229- 12$11 .OO 0 1993 by John Wiley & Sons, Ltd.

Received April 1993

230 J . ALBUQUERQUE et al.

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. Infor- mation rates per outbound carrier are typically between 56 and 512 kb/s. Outbound carriers are preassigned to the hub and contain a framed baseb- and 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 in- bound 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 require- ment, each equipment manufacturer employs a pro- prietary algorithm for satellite capacity assignment which may be a combination of

( 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

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 out- bound 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 exten- sive network management capabilities. Typically, this function is at a network control centre (NCC) co-located with the hub station and includes:

( i ) monitoring of link operation and perform-

(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.

ance at the VSAT or the port level

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 mix- ture 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

VSAT NETWORKS IN THE INTELSAT SYSTEM 23 1

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. Videoluudioldatu distribution networks

Very often, broadcast capabilities are superim- posed 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 Rec- ommendations 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-

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 out- bound 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 in- bound 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).

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


and rate B convolutional encoding with sequential decoding at a CIN, in the range of 54 to 55 dB Hz to yield up to 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. trial

Multi-channel thin-route C-band VSA T field

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 earth- station, 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 exper- iment 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

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 natu- ral 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 circuit- switched 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 Ref- erence 3. As explained above, for both situations, leases can correspond to a fraction of a transponder

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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 iso- tropically 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 earth- station 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). Substan- tial 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, i t 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, pre- cluding, 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

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 pro- portion 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 power- limited 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 10Vh for 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 K,,-band. On the other hand, for a 10 ' threshold BER, the 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

for C-band and better than 10

VSAT NETWORKS IN THE INTELSAT SYSTEM 235

Table I . INTELSAT satellite coverages and beam-edge saturated e.i.r.p. values

INTELSAT C-band C-band K,-band satellite global beam hemi-beam spot beam

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Table I t . C-band earth-station parameters

Antenna diameter ( m )

Receive gain Transmit gain Cross-polarization 'Typical' GIT (dB) (dB) isolation (dB1K)

Rx & Tx

0.5 24.7 28.1 17.7 4.8 1.8 35.3 38.7 17.7 14.4 2.4 37.8 41.2 17.7 17.7 3.5 41.1 44.5 17.7 20.8 9.0 (INTELSAT STD F-3) 49.3 52.7 17.7 20.8 16.0 (INTELSAT STD A) 54.8 S8.2 30 35.0

Table 111. K,,-band earth-station parameters

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taken t o be 0.6 and 1.2 times the symbol rate for QPSK and BPSK, respectively. Under this assump- tion 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 earth- station 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 con- sidering 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

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) earth- station 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.


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VSAT NETWORKS IN THE INTELSAT SYSTEM

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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. 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 subsec- tion 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. Out- bound 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; circuit- switched; data distribution; microterminals) have been identified, and it has been verified that all of


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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; clear- sky BER better than 10 +. For comparison between networks with and without DAMA refer to the vertical axis on the right side expressing capacity in number

of 64 kb/s channels. Energy spreading is not used here

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0.5

0 1.) 2 4 w 1.6 2.4 3.3 1.8 2.4 36 1.8 2 4 3.5

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Figure 7. Capacity estimates for circuit-switched mesh networks (C-band hemi- beam): BER better than 10 for 99.96 per cent of the time; clear-sky BER 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. Pro- vided 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 hemi- beam 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 estab- lished between 2.4 m VSATs and a 9.0 m hub. Con- cerning MCPC techniques, subjective evaluation and field trials at the INTELSAT Technical Labora- tories 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

VSAT NETWORKS IN THE INTELSAT SYSTEM 239

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W A

1.a 24 s.a 1.a t 4 51 1.a 0 4 am MI 9.t.llll. vl WWM

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): BER 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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.....

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" od 0.0 0.6 od-

W A vl VlWM vlll 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 Inter- national Communications Satellite Systems Conference, Arling- ton, Virginia, U.S.A.. March 1988.

2. L. Buchsbaum. N. Kusmiri and W . Karunaratne, 'Technologi- cal 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

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 Massachu- setts 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


PUC/RJ. From 1973 to 1984 he was an Associate Pro- fessor, 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-

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 on- board 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 Univer- sity 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 earth- station 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 Tech- nology, 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 CCI lT Study Group 7 activities concerning data communications in satellite systems.

Vsat Networks in the Intelsat

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