VSAT for Business Systems from the Book: Satellite Communications Systems, 3rd Edition

22: VSATs for Business Systems

22.1 VSATs for Business Systems

VSATs, very small aperture terminals, are small earth stations capable of receiving from and

sometimes transmitting to satellites. They represent an important addition to the

telecommunications world because they can provide a service directly to the user at virtually any

geographic location covered by a suitable satellite beam. They do not require any support from a

local terrestrial communications network and can even be run from portable or alternative power

supplies.

The term VSAT appears to have been coined around 1979 and was first used in connection with

the systems offered by Equatorial in the USA. These early systems operated at C-band and

represented a radical departure from the normal usage of satellite communications links.

VSATs can be used individually or, more usually, in networks of related users and they are often

used in conjunction with a hub station. A hub station is usually a larger earth station at the centre

of a star network. The small size of VSATs brings with it special system and equipment design

problems, both for the VSAT itself and for the hub to which it operates. These problems are

discussed later in this Chapter. The deregulated telecommunications policy in the USA meant

that VSATs could be used at an early stage. In Europe, regulation prevented their use for a

number of years. However, there are now many VSAT systems in Europe and the UK lottery is a

well known example of a network which uses approximately 2500 VSATs.

Although the term VSAT is generally used in connection with small, fixed-location terminals for

business use, there are comparable developments in related fields. For example, low-cost TVROs

could offer VSAT-type services at higher data rates and lower cost than traditional designs. The

various INMARSAT systems can provide different types of voice and data service anywhere in

the world and are widely used by people who need to stay in touch.

The communications scenario has been changing in Europe since the initial UK Green Paper on

the development of the common market for telecommunication services and equipment of June

1987. Most countries have liberalised their internal markets and have taken steps towards

opening up their telecommunications sectors to foreign competition. These policies have had a

major effect on the use of VSATs, SNG terminals and other satellite-related services.

The small size of a VSAT is the key to understanding its importance. Before the advent of

VSATs, satellite earth stations were large, expensive facilities run by national PTTs and large

corporations. Antennas ranged in size from 30 metres down to around ten metres. Such facilities

are, to this day, rather expensive to procure and expensive to run.

A VSAT, in contrast, is relatively cheap to procure, can be installed more or less anywhere and

has low, predictable running costs.

Thus, over the last ten years, VSATs have become a tool for business and a means of bypassing

existing terrestrial infrastructure. This has been particularly true in the USA where the liberalised

regulatory environment allowed people to use VSATs at a very early stage. In the rest of the

world, the situation was quite different. In the 1970s, most countries still regarded

telecommunications as a state monopoly and very little use of VSATs was allowed.

Things have moved on. We are now moving to a phase where VSATs can almost be considered

as commodities. VSAT vendors are now marketing products suitable for any business and we are

on the verge of seeing these products in use by the private individual.

22.2 Applications of VSAT Systems

22.2.1 One-Way and Two-Way VSATs

VSATs are frequently classified as either one-way or two-way devices. One-way VSATs can

receive but not transmit; two-way VSATs can receive and transmit. In the broadest sense, the

term one-way VSAT includes the data terminals and TVROs of different types.

There is no inherent limit on the transmit capability of a VSAT but two-way VSATs usually

transmit at rates up to a couple of hundred kbit/s. The low directivity of VSAT antennas is a

practical limit on the power, and hence the boresight EIRP, which may be transmitted. The

transmit equipment is usually completely solid state and highly integrated.

22.2.2 Data Distribution

There are many kinds of system where data originates consistently at one source and must be

transmitted to many sinks. One example is a library, which has a store of documents (an

archive), some of which are copied to users spread over a wide area. The data flow is primarily

one way. The user can request a copy of a document using a few characters (e.g. a British

Library request form) but the document itself may be very large. This is illustrated in Figure

22.1.

Figure 22.1: Data Distribution

Conventionally, paper copies are posted to the person making the request; however they could be

transmitted by satellite to a VSAT. Of course they could also be transmitted by facsimile or by

telex, and often are, but the inherently-wide bandwidth of a satellite transponder is ideally suited

to this kind of bulk data transfer. Another common situation concerns the transfer of data

between computers. For many years some research establishments have transferred computer

tapes by road on a regular basis. The transmission of such data by telephone link using modems

is certainly possible, but at a transmission rate of a few kilobits per second the transfer time is

unacceptably long. However, on a satellite link, many megabits of data can be transferred in a

few seconds, with a quality of transmission and a tariff which are almost independent of location.

Another example of this type of traffic is seen on the Internet. A search request requires only a

few bytes of data, but the response can be thousands of document references. Even to download

one or two of the documents can require very considerable data transfer to the user. The Hughes

DirecPC system and other systems can be applied to this situation and provide the user with satellite-based Internet and much higher data rates than dial up customers would normally

receive.

22.2.3 Rural Applications

In some countries the terrestrial infrastructure is still underdeveloped and access to telephone

lines is scare. Some estimates suggest that over half the world's population has still never made a

single telephone call! VSATs offer a means of providing the necessary communications

capability without the massive financial investment which is normally required for a terrestrial

infrastructure. In India, a network called AGROMET has been used to collect and distribute both

agricultural and meteorological data. This system uses VSATs with the capability to transmit

information from rural areas to a central location (Figure 22.2) where the data could be

processed; this may bring significant benefits to a large, widely dispersed, rural community.

Figure 22.2: Rural Applications

22.2.4 Disaster Monitoring

There is sometimes a need to provide reliable communications to remote sites for the emergency

services; this can be readily achieved using VSATs almost regardless of the terrain and location.

In Florida, USA, Hughes has provided an emergency management system of 106 sites. The

availability of the remote terminals is quoted as 99.887% for September 1995 to December

1996.1

22.2.5 Business Applications

In the business world VSATs are of interest because of their potential for providing companies

with a competitive edge. International agencies, such as INTELSAT, EUTELSAT, INMARSAT

and ESA, have all undertaken extensive research in connection with VSATs and offer services or

technology. Businesses can use two-way VSATs to provide a range of services both internal and

external. The major motor companies, such as GM, Renault and VW, all have VSAT systems.

Such systems are used for parts ordering, stock control and many other purposes.

A study by Logica for the Commission of the European Communities (CEC) forecast an annual

market size of US$351 million by the year 2000 for two-way VSATs. Although at present there

are many thousands of one-way VSATs in operation in Europe, there are only a few thousand

two-way VSATs. In general, businesses want to establish private networks so that they can link

their locations and move their information about in a safe manner and for the lowest possible

cost. In the USA there are many such networks, some of which are extended beyond national

boundaries and span the globe. The applications are numerous, some examples are:

supermarkets;

airline reservations;

car rental companies;

conference facilities;

insurance companies;

newsgathering;

law firms;

motor manufacturers.

22.3 Regulatory Issues

22.3.1 Licencing

Despite deregulation of the markets, it is still necessary and important that VSATs are operated

in a manner which does not cause excessive interference to other users. To this end, VSATs in

Europe must be licensed with the appropriate national radiocommunication agency.

There are two main issues:

i. The VSAT must not interfere with other users. ii. The VSAT must comply with the satellite operators' requirements.

The broad conditions under which all satellite equipment operate are laid down by the ITU. The

ITU is a body of the United Nations and it operates through the World Radio Conferences which

take place every two years. In the UK, the Radiocommunications Agency (RA) is responsible for

interpreting and implementing the regulations. The RA also uses the telecommunications acts

which embody the relevant UK legislation.

European directives now require member states to deregulate all telecommunications services,

including satellite services. Telephony was excluded from some of the legislation but member

states, with the exception of Spain, Ireland, Greece and Portugal, had liberalised voice in January

1998. However, all states are required to be fully open to competition by 2003.2

22.3.2 Frequency

The electromagnetic spectrum is a precious resource and must be used wisely. One of the ways

in which this is done is by splitting the spectrum into bands which can be used for similar

purposes. The main split of spectrum is:

i. L-band is for mobile applications (primarily the INMARSAT system) and also for forthcoming satellite cellular systems such as Iridium, GlobalStar etc.

ii. C-band is in principle for large and small fixed services. It is very crowded but still important where propagation losses are high. Some terrestrial systems plan to use C-band

for cellular telephony; an example of this is the Freedom system.

iii. X-band is used for military systems. iv. Ku-band has been used for most VSAT systems to date. v. Ka-band is emerging as a likely band for future systems, but high rain losses have always

made it less attractive.

22.3.3 Performance Specifications

The European Union has attempted to produce a set of standard specifications for VSATs. The

standards are produced by ETSI.

Each satellite operator produces its own standards for both receive and transmit performance and

it is necessary to comply with these standards to be allowed to use a satellite system. VSAT

vendors seek to have their equipment type approved so that they can install and operate a VSAT

with the minimum of formality.

22.4 Service Provision

22.4.1 Procuring a VSAT System

Most people, and companies, like to deal with a one-stop shop when procuring satellite services.

The customer wants a ready-made solution which includes: equipment supply and installation,

approvals (satellite operator, local planning authority, RA), safety legislation (EMC, low voltage

directive etc.), network services (routers, interface standards, management, disaster recovery

etc.).

In exactly the same way as we expect to procure a telephone, plug it in and find that it works (in

Europe at least), customers want to be able to do that with VSAT systems as well. The fact that

it's a satellite system is irrelevant to the user; he or she simply wants a system that works, with

well defined performance and price parameters.

This sort of service is available from several different vendors.

22.4.2 Vendor-Provided Services

Most service providers are actually equipment manufacturers as well. After a period of

development and growth, the main equipment providers are now almost all large corporations

with vertical integration of all the resources necessary to provide a VSAT system. Some of them

build and launch the satellites, build the VSATs and hubs (where appropriate) and provide all of

the network equipment and application software to run on them.

They have hub stations which are shared so that a user does not have to provide any of the

infrastructure and only pays for the part of it that he uses.

Each vendor has built equipment to its own standards and tried to make it universally acceptable.

Much of this equipment is highly versatile and can support many different types of interface.

22.4.3 Private Services

Some large companies have built their own networks for intracompany use and started to sell

spare capacity to other companies. Shared hubs have evolved in this way.

22.4.4 Service Features

A service provider has to produce a service with desirable features. Some of the features which

are required are:

a single point of contact for the user;

the provision of suitable equipment;

the procurement of space segment;

installation, commissioning and maintenance services;

the integration of the system with customer-interface equipment;

network-management services;

local regulatory approval;

disaster-recovery services.

22.5 Voice and Data

22.5.1 Speech

Speech can be transmitted via VSAT if it is suitably encoded to reduce the data rate required.

High-quality speech can be carried on satellite channels with a data rate of 9.6 kbit/s using

modern voice encoders and modems capable of operating at low C/N values. As work on speech

encoding progresses the data rate required for high-quality speech will continue to fall. Systems

operating at 4.8 kbit/s are now in use. It is important that the VSAT data protocol is designed for

such services as otherwise its data-framing mechanism may prevent the voice encoders from

functioning correctly. Modern VSAT systems are designed to integrate voice and data.

A speech circuit set up between two VSATs via a hub will include a double hop and accordingly

an end-to-end delay of at least 0.5s. Outlying areas want to connect to the cities, both for speech

and data, and a hub located in a city can provide both services to many villages. The provision of

speech circuits to a rural community can strongly affect its economic development. Although the

delay can exceed the ITU recommendations for speech circuits, the provision of good quality

echo compensation can render the service acceptable. An ITU report on the subject estimated the

ratio of benefit to cost in third-world countries to be between 85: 1 and 200: 1.

22.5.2 Bulk Data

Bulk-data transfer may require large files, databases for example, to be transmitted with very low

error rates. Some applications will tolerate no errors at all. The transfer time for such data is

often not critical so that it can be transmitted with low priority or in the quiet hours.

At the same time, it is always desirable to minimise transmission times.

22.5.3 Transactional Data

Some activities are transactional in nature. A classic example of this is creditcard verification

and associated activities: cash balance, cash withdrawal etc.

The volume of data in both directions is small, the data must be absolutely error free and the

transaction must be as quick as possible. Nobody wants to stand by a cash machine for ages

waiting for a response.

Other types of data in this category include airline reservation systems and booking systems in

general. In these systems also, errors cannot be tolerated.

22.5.4 Store and Forward

Other data may fall into an intermediate category where the message size is not large and time is

not critical; nevertheless delivery within a few tens of minutes is desirable. E-mail could be

considered in this category.

22.6 Example Systems

22.6.1 Hughes Olivetti Telecom

Hughes, like the other major players in this field, has a range of systems for different

applications. See the company's web site for the latest data. The DirecPC system provides

Internet access at rates of up to 2 Mbit/s; Turbo Internet access service is said to cost from $16 to

$40 per month; the HOT (Hughes Olivetti Telecom) system can transmit a 675 Mbyte CDROM

over a satellite in under 30 minutes and provide a constant data stream to an individual at 400

kbit/s.3

Market data indicates that Hughes has approximately 70 per cent of the world market for

VSATs.

22.6.2 Matra-Marconi Space

Matra-Marconi supplies the Skydata VSAT system in Europe; three photographs of Matra's

equipment are included in this Chapter (Figures 22.3, 22.4 and 22.5). The Skydata system can

use frame relay to provide a very flexible, high data rate solution. Frame relay is capable of

handling disparate types of source material such as voice and data (which have very different

system requirements).

Figure 22.3: A 2.4 Metre VSAT for Videoconferencing

Figure 22.4: A POLYCOM Receive-Only VSAT

Figure 22.5: A 1.8 Metre VSAT

Some of Matra's equipment is shown in the following photographs. Figure 22.3 shows a system

with a 2.4 metre antenna; this is used for videoconferencing. Figure 22.4 shows a small terminal

developed in collaboration with POLYCOM for data broadcasting. More than 3000 reception

terminals are in operation. Figure 22.5 shows another videoconferencing terminal with a smaller

antenna, illustrating the integration of the feed, LNA and SSPA at the prime focus of the

antenna.

22.6.3 GE Spacenet

GE Spacenet provides a wide range of VSAT and hub equipment;4 its Skystar Advantage private

hub system can operate with networks of 50 to 5000 remote sites.

In 1997, GE Spacenet acquired AT&T Tridom, and it now has over 70 000 VSAT sites around

the world.

22.6.4 AT&T (Now Part of GE Spacenet)

The AT&T Clearlink system incorporates TCP/IP, SNA/SDLC and X25, and data broadcast,

packetised voice and digital voice overlay are offered. Prices for Ku-band systems start at $6000

and for C-band systems $9000.5

22.6.5 Scientific Atlanta

The Skyrelay VSAT network can support voice, data and video. Prices are said to start from

$4900 per remote terminal.

There are now many companies involved in the provision of equipment and services for VSAT.

Some others are:

SIS;

Multipoint;

Orion;

GlobeCast;

22.6.6 DVB and Satellite Interactive Terminals

DVB will have a major effect on VSATs and VSAT standards. An ad-hoc group hosted by ESA-

ESTEC has prepared specifications for SITs (satellite interactive terminals) taking into account

the use of DVB. The SIT could be thought of as a VSAT or as a TV receiver only terminal but

with a satellite return channel. Clearly, the aims are to provide workable standards and to drive

down costs.

DVB can be used for data broadcasting as defined in the draft ETSI Standard EN 301 192. Data

broadcasting includes the transmission of IP datagrams as well as interactive TV and software

downloading. These facilities may be some of the most important aspects of DVB in the middle

future. The transmission of ATM and IP over DVB are now major issues of research around the

world. DVB carries MPEG2 video and audio signals as well as program information signals in a

prescribed but flexible format. It is an international standard which reflects many years of

research and development by all of the world's leading players in the broadcasting business.

DVB has been defined for all current transmission media.

SITs could be used for many purposes including Internet connections. The astonishing growth of

the Internet and multimedia services provide new opportunities for business and domestic users.

See, for example, the SES web site, for information about their plans for SIT usage on the

ASTRA satellite system.7 The growth of use of multimedia-capable personal computers to

access the Internet and the World Wide Web (www) for the transfer of image, video and audio

information has resulted in a higher demand for Internet bandwidth. The acceptance of DVB has

resulted in the development of digital platforms to provide television, radio and data services

over satellite and terrestrial networks. There is a convergence of technology which will lead to

lower costs and higher performance for small terminal satellite users.

22.7 Design Considerations

22.7.1 Antennas

Hub stations generally have antennas in the range 5 to 8 m diameter; although some systems

have hubs as small as 2.4 m. The size required depends on the type of VSAT system, the satellite

being used, the location and all the other parameters which are involved in a link budget.

The VSAT antenna usually has a diameter of 60 cm to 90 cm for Ku-band. However, antennas as

small as 45 cm would be adequate close to beam centre for some services. Antennas as large as

2.4 m could be required for other services. A diameter of 1.8 m would be used beyond the

normal beam edge of 4 dB; this is the case with television transmissions from current

EUTELSAT transponders. The efficiency of small receive-only antennas can be high (about 70

per cent); they are light and inexpensive.

Despite its small size and low cost, the VSAT antenna must meet demanding specifications. The

relevant parameters in Europe are in the specifications produced by the ETSI technical

committee on satellite earth stations (TCSES).

Standard DE/SES-2002, draft pr ETS 300 159, May 1991 covers this area. It deals with two

types of specification:

i. Type approval requirements such as: o off-axis EIRP;

o tx antenna polarisation discrimination;

o antenna pointing accuracy capability;

o polarisation plane alignment capability.

ii. Recommendation on receiving quality.

The most common type of VSAT antenna is the front-fed paraboloid (i.e. a section of a

parabola). Two forms are used, the offset and the axisymetric.

The offset antenna has no blockage and its efficiency can be high at between 60 and 70 per cent.

It also has low sidelobes owing to the absence of blocking (and diffracting) material in the wave

front.

Hub antennas can also be of offset or axisymetric design. Some hubs will use Cassegrain

antennas.

For Ka-band, which is a likely candidate for some future VSAT systems, the antenna could be

even smaller. Some flat, phased-array-type antennas have been developed for this application

and they could become more common.

22.7.2 Receive-Only VSATs

Typically, an offset antenna will be used for receive-only VSATs. This can have an RF front

end, comprising an LNA and a downconverter, mounted at the prime focus behind the feed. The

Cassegrain has the same items mounted on the feed behind the main reflector. In both cases, it is

common to use an integrated front end containing the LNA and downconverter. The front end

housing on some units also incorporates the feed (i.e. the feed is part of the casting or moulding).

The received signal must be processed to extract the user data, and the stages of processing

depend on the design. The first stage is demodulation, whereby a signal is extracted from the

carrier. The demodulator will most probably be either BPSK or QPSK and may be followed by a

deinterleaver, a decoder and a descrambler. The INMARSAT-C system, which provides a

service to very small vehicle-mounted terminals, uses all of these processes to compensate for

the very poor channel conditions under which it has to operate.

The user data can then be passed directly to a device (such as a PC or a fax machine), or it can be

passed to a demultiplexer which will split it into several streams and supply them to different

users or applications.

22.7.3 Two-Way VSATs

A block diagram of a two-way VSAT is shown in Figure 22.6.

Figure 22.6: A Two-Way VSAT

The receive side is essentially the same as for the one-way VSAT but the antenna performance

may be changed. In order to provide optimum performance at both receive and transmit

frequencies the antenna efficiency may be lower, a value of 60 per cent would be normal. If the

receive efficiency reduces from 75 to 60 per cent the received signal level drops by 10 log(60/75)

= 0.97 dB. The carrier-to-noise ratio will also drop by approximately the same value.

An ortho-mode transducer (OMT) is necessary to separate the receive and transmit paths and to

ensure that they are on orthogonal polarisations. The power amplifier produces about 1 W of

output power at saturation and is a compact solid-state device; these are now readily available at

low cost. The transmit chain can be realised in several ways.

22.7.4 Size Constraints

Traditionally, the earth stations in any particular network have all been of approximately the

same size or, more importantly, the same specification. This approach balances the requirements

of the transmitting and receiving facilities and ensures that all participants in the network share

the costs on an equitable basis. However, in a VSAT network, the distribution of facilities is

highly asymmetrical with many VSATs and only one (or very few) hubs. In such a network the

main requirement is to minimise the cost of the VSAT terminal and to transfer, as far as is

possible, the technical complexity and cost to the hub station. Because the VSAT has a small

aperture antenna, it must receive a correspondingly higher power from the satellite in order to

provide an adequate carrier-to-noise ratio for error-free demodulation. If, however, the signal

level from the satellite is arbitrarily increased to provide the necessary carrier-to-noise ratio for a

VSAT, it is most likely that the limit on allowable flux density per kilohertz at the earth's surface

will be exceeded. At present there is a limit for downlink power spectral density of 6 dBW/kHz

in the USA and this may also be adopted in Europe.

22.7.5 Phase Noise and Carrier Recovery

The local oscillators in earth-station frequency converters are usually generated using phase-

locked oscillators. The cost of these components may be significant compared with the overall

price of a VSAT and it is preferable to use a dielectric resonator oscillator (DRO) if possible.

However, this may affect operation with low data rates, where close to carrier phase noise is

most important. A DRO, which has poor phase noise compared with a PLO, can be used, but the

effects on carrier and clock recovery and on the overall stability must be considered. Phase

demodulation can be achieved by regenerating a carrier or by using a pilot signal. Carrier

regeneration can be accomplished by one of the following methods:

squaring loop;

Costas loop;

decision feedback.

Figure 22.7: Phase Noise and Low Data Rates

In a system operating with a low C/N (such as a VSAT), a Costas loop has advantages.

If the phase-noise bandwidth is similar to that of the signal it may be impossible to demodulate

the signal. This is because any circuit which tracks out the phase noise will also track out the

modulation on the signal and the information content will then be lost. Even if the information

on the signal is not lost, this effect will increase the probability of an error in the demodulator.

22.7.6 VSAT Advantages

The small size of a VSAT has several inherent design advantages. As satellites are generally

maintained on station to an accuracy of +/-0.05 in both north/south and east/west directions,

there is no need to track the antenna. However, it is worth noting that satellite operators,

including INTELSAT, have considered ways of tracking small antennas. This is due to the

shortage of transponders and the need to continue operating satellites even when there is

effectively no fuel left for north/south station keeping. Another advantage of very small antennas

is their lack of sun outages. The beamwidth of a VSAT is such that the increase in system noise

temperature when pointed directly towards the sun is negligible.

22.8 Modulation and Coding

22.8.1 Modulation and Coding Methods

There are many trade offs in achieving a desired level of performance. The modulation and

coding techniques used can be traded against parameters such as:

antenna diameter;

received power;

EIRP;

bandwidth;

transmission time;

transmission rate;

cost and complexity.

Unfortunately, most of these trade offs work against each other. For example, since it is not

necessary to conform to common standards one could use an advanced modulation scheme such

as coded 8PSK. This has advantages in terms of C/N required and bandwidth occupied.

However, modems using this scheme are expensive and sensitive to phase noise (as are most

higher-order modulation schemes).

A commonly-used forward-error-correction (FEC) technique, such as half-rate convolutional

encoding with Viterbi decoding, will improve the user BER by several orders of magnitude but it

will also double the occupied bandwidth. BPSK and QPSK with some form of coding or

spectrum spreading are often used for transmissions to the VSAT.

The use of BPSK enables a particularly low-cost demodulator to be used, even if it is combined

with spectrum spreading. BPSK is resistant to phase noise since phase shifts of up to 90 will

not (in the absence of thermal noise) cause an error. BPSK has only one decision threshold,

which means that the distance to that threshold is maximised; it is therefore also resistant to

thermal noise (see Chapter 9 for details). Transmissions from VSATs to the hub also often use

FSK since this can be easily implemented by, for example, direct modulation of an oscillator.

There is no requirement for spectrum spreading, since the EIRP of the signal is low. It may,

however, be necessary to use coding so that the data can be accurately recovered.

There are several types of code in common use which are suitable for implementation in a VSAT

system; some of these are:

linear block coding;

convolutional coding with Viterbi or sequential decoding;

concatenated coding e.g. Reed-Solomon/biorthogonal or Reed-Solomon/convolutional.

Reed-Solomon codes are the best known codes in the family of linear block codes. The BER

achieved is proportional to the length of the code for a given C/N; a long code can correct a

larger number of errors. The number of correctable errors is given by:

t = (n - k)/2 = n(1 - k/n)/2

where t is the number of correctable symbols and k/n is the code rate (see Chapter 10 for more

details).

Convolutional encoders, being constructed from shift registers, are simple to implement. The

simplest decoders can also be made from shift registers but they are some 3 dB less efficient than

Viterbi or sequential decoders. Many current VSAT systems and conventional earth stations use

rate 1/2, constraint length seven Viterbi decoders. These can provide a BER of 1 106 with an

Eb/No of 5.5 dB and are available as integrated circuits for inclusion in OEM equipment.

Sequential decoders are powerful and their complexity is not dependent on constraint length as is

the case with Viterbi decoders. They suffer because the processing time is variable for each

output bit and a large buffer memory is needed when the C/N is poor.

A very powerful code can be realised by concatenating two codes. With this technique, an

effective code can be achieved with relatively simple implementation because the decoder is

constructed from two simpler decoders. Many modems can use two concatenated codes. The

result is a data stream which is almost literally error free as long as the C/N of the link is above a

threshold. However, if the C/N falls below the threshold the performance of such systems will

degrade rapidly.

22.8.2 Modulation Techniques

In general, the same considerations apply to VSATs as to all other satellite communications

systems. However, there are some differences. VSATs systems must be reasonably priced and

this has limited the complexity of the modem used. Equipment space is limited by the need to

have a compact installation and this also limits what can be used.

Modems have become highly integrated and the use of DSP-based designs means that modem

performance can be several dB better than in the past.

INMARSAT used OQPSK (offset QPSK) for its systems. OQPSK is now being used for other

systems which seek to exploit the advantages of a constant-envelope modulation scheme.

Applying a filter to PSK makes it nonconstant envelope; new schemes like MSK are constant

envelope and can have better spectral efficiency than PSK.

22.8.3 Coherent and Noncoherent Demodulation

Coherent demodulation gives the best performance and is based on the regeneration of a local

oscillator in the demodulator by phase locking to the received signal. However, noncoherent

demodulation can also be used. In the case of PSK, this is also called differential demodulation

since it uses the phase of the last symbol as the phase reference to demodulate the current

symbol. Noncoherent demodulation can be used in situations where the C/N is low and a low-

cost circuit is required. It has worse performance than coherent demodulation, as one would

expect.

22.8.4 Noise

Since satellite links are between earth and space, there is little scope for manmade noise to be

introduced. Most of the noise on a satellite link is thermal in origin; thus it is spectrally flat and

wideband. However, flat, wideband noise can cause bursts of errors in a decoder; an interleaver

can spread such error bursts and minimise their impact.

22.8.5 Coding

Most codes are designed for use on an AWGN channel and Shannon's information theory is

relevant to this type of channel. Modern coding schemes are getting close to the Shannon limit of

C/N which in fact limits the coding gain which it is possible to achieve.

BCH codes can be decoded by simple decoders using shift registers with logic gates, but soft-

decision decoding cannot readily be applied to block codes.

22.8.6 Spread Spectrum

In order to achieve an adequate received C/N at a VSAT (with its very small antenna), it may be

necessary to increase the power transmitted by the satellite to such an extent that the power flux

density (PFD) at the earth's surface is unacceptable high. One way of overcoming this problem is

by spreading the transmitted signal over a wider frequency range and thus reducing the PFD. The

spread-spectrum technique can be used for both digital and analogue modulation methods,

although the term spread spectrum is usually used only in connection with digital schemes.

With a spread-spectrum signal, the PFD at the earth's surface is held within allowable limits, but

the total power, equal to PFD multiplied by the occupied bandwidth, can be as high as required.

In this way the carrier power received by the VSAT can be increased to the level required to

provide adequate signal quality. In the case of a digital signal, the signal quality is measured in

terms of BER and the required BER is usually determined by the user's application. The effects

of spectrum spreading are illustrated in Figure 22.8.

Figure 22.8: The Effect of Spectrum Spreading

A spread-spectrum system spreads the energy of the data signal over a bandwidth which is 100 to

1000 times higher than would normally be used. The term processing gain is used to describe the

degree of spreading. If the occupied spectrum is increased by 1000 times, the processing gain is

30 dB (i.e. 10 log 1000). However, there is no real gain or loss in the system. Apart from a small

implementation loss, a few tenths of a dB, the overall end-to-end link budget in terms of received

C/N is the same whether spectrum spreading is used or not.

Because a VSAT is small, its beamwidth is high which means that it is capable of receiving

excessive interference from adjacent satellites. Also, the satellite may have to radiate high power

levels so that the VSAT has a sufficiently high C/N. Spread spectrum can avoid both of these

problems. Interfering signals received by the VSAT are themselves spread in the demodulation

process and their effect is made negligible.

Two common methods of producing a spread-spectrum signal are directsequence (DS)

modulation and frequency hopping (FH). Civilian systems use DS but FH is used in military

systems.

Spread spectrum has been used primarily for C-band applications where spectrum crowding is

most severe. C-band systems have lower-gain antennas and thus adjacent satellite interference is

worse. Spread spectrum makes a system relatively immune to multipath, ASI, ACI and sun

outages.

In a spread-spectrum system each bit of information is replaced by a pseudorandom (PN)

sequence; this is shown in Figure 22.9. Conventionally, the bits of the spreading sequence are

referred to as chips. The length of the sequence is system dependent but would typically be 1000

chips in a commercial system.

Figure 22.9: The PN Sequence

22.9 Data Transmission and Protocols

22.9.1 Quality of Service

The VSAT system must be designed to provide the quality of service which the application

requires in order to function in a satisfactory manner.

Quality of service involves factors such as bit error rate or packet error rate, link availability,

throughput delay and call set-up time. The VSAT system could be optimised for a particular

application or it could be designed to be flexible and support many different types of application.

Often the VSAT system will be connected to one or more data-processing devices and is

essentially part of a network. Therefore, network optimisation is an important part of VSAT

design.

22.9.2 Circuit and Packet Switching

Circuits can be packet switched or circuit switched. In a packet-switched system each packet of

data is individually routed from one end of the application to the other, possible over different

routes. In a circuit-switched system, circuits are allocated for the duration of a call, at least, and

all data travels along the same route from end to end.

Each type of switching has advantages for different types of traffic. The most flexible VSAT

system can actually switch between the two modes of operation depending on the traffic profile

at a given time. It can be very important to know what type of traffic is flowing at each part of

the day in order to optimise a system.

There are usually many parameters that can be altered to improve a network's performance.

Parameters such as buffer size, number of buffers and various time constants depending on the

network protocol.

22.9.3 Error Detection and Correction

Error detection and error correction are two different but related techniques. Errors can be easily

detected using a cyclic redundancy check (CRC). Most CRCs consist of a 16-bit word added to

the end of a data sequence. The CRC is capable of detecting almost all errors. In fact the residual

error rate, i.e. the error rate due to errors which the CRC does not detect, is about 1 in 1014

bits or

less.

Error correction can be performed in two ways. Either through forward error correction, or

through ARQ. With FEC, the data is encoded on transmission with many parity bits and this

information is used on reception to detect and correct errors. A common form of FEC uses half

rate, in which case there are as many parity bits as data bits, or three-quarters rate, in which case

there is one parity bit to every three data bits.

An FEC system alone can supply data which has an arbitrarily low error rate, but it does not

guarantee that the data is error free.

An ARQ system uses a mechanism such as a CRC to determine that something is wrong within a

packet or frame of data and arranges for that packet to be transmitted again.

If a system requires data which is error free then an ARQ system must be used.

22.9.4 ACKs and NACKS

End-to-end protocols usually function by sending acknowledgments (ACKS) and negative

acknowledgments (NACKS). An ACK indicates that a particular packet or frame has been

received. A NACK indicates that a packet or frame was not received within a particular time

frame.

A protocol can function with ACKS alone or with NACKS to speed things up but it cannot

operate with NACKs alone.

22.9.5 Frame Formats

The ISO model refers to packets and frames and these terms are often used interchangeably. In

principle, each system vendor could use its own frame format but there is merit in using a

standard format.

HDLC, in particular, is widely used as a model for the frame even if the data is not being used in

a proper HDLC environment.

22.9.6 VSATs and Interface Standards

The value of using standard interfaces between communicating devices, whether part of a

network or not, has long been recognised. Standards such as RS-232, X21 and X25 are widely

used and organisations around the world are developing their systems with the OSI model in

mind. One-way VSATs do not easily fit into such systems since they cannot directly (i.e. on the

satellite channel) provide acknowledgments of data received. This problem can be overcome by

spoofing at the hub or tunneling through a terrestrial link.

Two-way VSATs are able to provide acknowledgments but often, especially at Ku-band, cannot

receive their own transmission owing to their small size. This is typically a requirement of

distributed control systems, such as IEEE-802.3, where each device is responsible for ensuring

that it does not disrupt the communications of others. This problem can be overcome in systems

with a central control entity such as a hub station. A practical example of a distributed control

system is the operation of SATNET, which is part of the well known global network ARPANET.

SATNET is used by C-band INTELSAT earth stations of 30 metre diameter and cannot easily be

used by VSATs.

The way in which VSATs fit into the OSI model is illustrated by Figure 22.10.

Figure 22.10: VSATs and the OSI Model

When implementation of a VSAT system is being considered, the cost of ownership must be

compared with the cost of using existing communications services. The cost of ownership

includes the purchase price, which will normally be amortised over several years, plus running

and maintenance costs. It is impossible to give definitive prices for VSAT equipment because the

functions and services offered by different suppliers vary so much. However, the take up of

VSATs will depend strongly on cost of ownership since, from the user's point of view, they are

simply a means to an end, and the lowest-cost system with comparable performance will win.

In the USA, low-speed C-band two-way VSATs cost about $6000 when purchased in quantities

of several hundred. Two-way Ku-band VSATs may cost two or three times as much. The cost of

ownership of a $6000 VSAT, amortised over three years, would be about $400 per month (the

total cost of ownership is about two or three times the equipment purchase price).

The hub is an expensive item, the costs of which are generally shared by the network users either

directly or through an element of their connection charges. According to Morgan 6, the monthly

cost of a hub station, including operating and administrative overheads, is approximately $108

000. Therefore, in a network of 1000 users, the total cost per user would be $108 per month. In

this case the total running costs per user would be $580 per month. Whether or not this cost is

attractive depends on the service required and the value placed by the user on having a degree of

control over their own communications system. There are strong indications that many users

value this aspect of the system very highly.

Consider that a forward link data rate of 2 Mbit/s is available and that each user has 1/1000 of

the total channel time in each month. The volume of data which each could transmit or receive

is:

Get MathML

The cost of this data is then 5184 Mbit/$580 which is 9 Mbit/$. This is a highly simplified

example but it demonstrates that the cost of using a VSAT can be comparable to the cost of using

existing terrestrial services

22.9.7 DVB

DBV is associated with television but is now being used to carry Internet traffic. One of the main

advantages of this is the ready availability of equipment such as data inserters and PC cards. This

equipment can be used as part of a system which can deliver high data rates to the home or

office.

Possibly the greatest advantage of DVB is that it is an international standard which is clearly

going to be around for some time.

DBV signals can carry data which uses other protocols, and many organisations are examining

the use of ATM and TCP/IP over DVB.

22.9.8 ATM and Frame Relay

The terms ATM (asynchronous transfer mode) and frame relay refer to systems which route

packets of data over a network. The difference between them is essentially that ATM operates at

the OSI model level 2, the packet level, and frame relay operates at level 3, the frame level.

They are very popular at present not least because they appear to be able to handle both voice

and data. Routing devices (switches) are available that can be integrated into a VSAT system.

22.10 Satellite Access Techniques

22.10.1 Sharing the Satellite

A satellite is a relatively expensive and scarce resource. In traditional systems, the satellite

transponder was used in a very well defined and stable manner which did not change from year

to year. However, when VSAT systems are used, a single satellite transponder may be used by

many different VSAT networks. Each network may carry different traffic at different times of

the day. The environment is essentially dynamic and can change over a period of hours.

The VSATs must access the transponder in an efficient manner to avoid wasting the satellite

capacity and to reduce the cost of that capacity by allowing as many users as possible to access

it.

Satellite access concerns the means of utilising, and sharing, the satellite's resources.

The use of many VSATs means that terminal cost rather than satellite cost may be a limiting

factor, therefore, the most efficient use of the transponder may not be of greatest relevance.

22.10.2 Addressing and Channel-Access Techniques

In a receive-only VSAT network channel access is a minor problem which only concerns the hub

or hubs. The hub typically has exclusive use of a fixed-frequency channel and is able to transmit

as and when required. However, it may still be necessary to address information to each

individual VSAT or to groups of VSATs. Each VSAT receives all of the information on the

channel to which it is tuned; it only uses, however, the information which is addressed to it. The

address can be unique to a particular VSAT or it can be a group address which covers many

VSATs.

Each VSAT could have an individual address and, in addition, one or more group addresses. A

sophisticated system may also transmit, to the VSAT, information which will allow it to decrypt

the data and prevent others from doing so.

In a two-way system the situation is clearly quite different. There may be several hundreds or

even thousands of VSATs which wish to transmit to the hub. It is not feasible to consider having

a separate channel for each VSAT so they must share a channel or channels. Many schemes have

been proposed to deal with this problem but there is no clear favourite since system requirements

differ so much. However, there are many systems which need to share a single channel for

communication at essentially random times. A common technique for dealing with this problem

is to use ALOHA or a token-passing technique. Both of these methods are now used as part of

the IEEE-802 specification for local area networks and the principles also apply well to a VSAT

system. A possible channel frequency plan is shown in Figure 22.11.

Figure 22.11: Two-Way Frequency Plan

The satellite's capacity can be shared in one of two domains, namely, the time domain and the

frequency domain. The time domain can be shared by several VSATs in time-division multiple

access (TDMA) such that data from a given VSAT is compressed in time and transmitted so as

to interleave at the satellite with data from other VSATs. The transmission of bursts such that

they do not overlap and interfere with each other usually requires a mechanism for acquisition

and synchronisation. For this mechanism, the hub can usually provide a master timing reference

onto which all VSATs can lock. Although such a system may appear simple, there are problems

caused by satellite movement and VSAT geographic location to take into account.

In a frequency-division multiple access (FDMA) system, each signal accessing the satellite is

assigned its own carrier frequency. The frequency allocated is available all of the time and the

access method can be simple. However, FDMA would be very inefficient in terms of channel

utilisation for only one VSAT. The efficiency is increased by using time-division multiplexing

(TDM). In this case many VSATs can use the channel but the controlling mechanism is very

much simpler than with TDMA.

22.10.3 Access Methods

Satellite access methods include:

22.10.3.1 FDMA (Frequency-Division Multiple Access)

This is the simplest type of system. Each user has a different frequency allocated and transmits a

single carrier on that frequency.

22.10.3.2 TDMA (Time-Division Multiple Access)

Although generally considered to be very complicated, this method is nevertheless suitable for

VSAT systems. A reference burst can include a control and data channel to inform each VSAT

when and on what frequency it should transmit. A separate channel can be used by the VSATs to

request capacity. Usually there is a master station in such systems which is responsible for

capacity allocation.

22.10.3.3 FM2 (FM Squared)

This system uses a number of FM signals, each at different subcarrier frequencies, which are

modulated onto a single, TV like, carrier; this allows low-cost FM receivers to be used. Because

each VSAT receives the composite signal, which is high power and wideband, the carrier-to-

noise ratio can be high and demodulation relatively straightforward.

22.10.3.4 CDMA (Code-Division Multiple Access)

Several spread-spectrum signals can use the same frequency at the same time as long as each

uses a different spreading code. This is referred to as CDMA. As long as the different codes used

by each signal have low crosscorrelation coefficients they will not interfere with each other.

There is, of course, a limit to the number of signals which can be piled on top of each other. The

other spread-spectrum signals look like noise to the wanted signal. As more and more CDMA

signals are stacked on the same frequency, the C/N of the wanted signal degrades and the BER

will also degrade.

22.10.3.5 Sharing Transponders

In the inbound direction, i.e. from the VSAT to the hub (if there is a hub), the signal is usually of

very low level. If the VSATs are sharing the transponder with a much larger signal there is the

possibility of small-signal suppression which could badly affect the VSAT link budget (a single

small carrier in the presence of a large carrier suffers 6 dB suppression).

22.10.4 Multiple Access Strategies

In addition to deciding how the satellite transponder will be shared (frequency, time, code

domains), the system must handle the allocation of capacity to users (VSATs) when they require

it. Capacity can be assigned in several ways. Some common techniques are:

22.10.4.1 Fixed Assigned (or Preassigned)

The channels or time slots are preassigned and remain so indefinitely or for a significant time

(hours to days).

22.10.4.2 Demand Assigned

The channels or time slots are dynamically assigned as the VSATs request them.

22.10.4.3 Voice Activation

Each speech channel is used for about 50% of the time during conversation and it makes sense

for VSATs carrying voice traffic to switch off their transmitters during gaps in the speech and

thus save on carrier power. Some systems use the gaps in speech to insert data channels

rendering a very efficient speech and data system.

22.11 Network Configurations and Availability

22.11.1 Star Network

If the hub is affected by atmospheric attenuation then all of the VSATs in a network will be

affected. The availability of the hub must be very high, typically 99.995% (i.e. 26 minutes outage

per year) in order to prevent this.

22.11.2 Mesh Network

In a mesh network, atmospheric attenuation will affect only a part of the network. The

availability of each VSAT is determined by the service being provided.

22.12 Link Budgets

22.12.1 One-Way Systems

To illustrate the way in which VSATs are used in real systems, the link budgets in Tables 22.1

and 22.2 give examples of the system performance which can be achieved for one-way and two-

way cases using satellites of the ECS type.

Table 22.1: One-Way Link Budget

Open table as spreadsheet

Hub to VSAT No FEC FEC

Uplink at 14.25 GHz

Earth station EIRP 63.6 dBW 58.4 dBW

Spreading loss 163.6 dB 163.6 dB

Path loss 208.0 dBW 208.0 dB

Atmospheric loss (clear sky) 0.2 dB 0.2 dB

IPFD for saturation -76.4 dBW/m2 -76.4 dBW/m

2

IPBO at the satellite 23.8 dB 29.0 dB

IPFD at the satellite -100.2 dBW/m2 -105.4 dBW/m

2

Satellite G/T -5.3 dB/K -5.3 dB/K

Uplink C/N0 78.7 dBHz 73.5 dBHz

Downlink at 10.95 GHz

Saturated EIRP (-2 dB contour) 43.0 dBW 43.0 dBW

OPBO 17.8 dB 23.0 dB

Satellite EIRP 25.2 dBW 20.0 dBW

Path loss 205.8 dB 205.8 dB

Atmospheric loss (99.9% of the year) 4.0 dB 4.0 dB

Antenna gain 38.5 dBi 38.5 dBi

Receiver G/T 12.1 dB/K 12.1 dB/K

C/N0 56.1 dBHz 50.9 dBHz

Overall link budget

Overall C/N0 56.1 dBHz 50.9 dBHz

Overall C/N 12.9 dB 7.8 dB

Implementation margin 2.0 dB 2.0 dB

Eb/N0 10.9 dB 5.8 dB

BER 1.010-7

1.010-7

Table 22.2: Two-Way Link Budget

Open table as spreadsheet

Uplink at 14.25 GHz

VSAT EIRP dBW 40.5 dBW

Spreading loss 163.6 dB

Path loss 208.0 dB

Atmospheric loss (clear sky) 0.2 dB

IPFD for saturation -76.4 dBW/m2

IPBO at the satellite 46.9 dB

IPFD at the satellite -123.3 dBW/m2

Satellite G/T (-4 dB contour) -5.3 dB/K

C/N0 55.6 dBHz

Downlink at 10.95 GHz

Saturated EIRP (-2 dB contour) 43.0 dBW

OPBO 40.9 dB

Allowance for small carrier suppression 2.2 dB

Satellite EIRP 2.1 dBW

Path loss 205.8 dB

Atmospheric loss (99.9% of year) 4.0 dB

Receiver G/T 29.0 dB/K

C/N0 47.7 dBHz

Overall link budget

Overall C/N0 47.1 dBHz

Overall C/N 7.3 dB

Implementation margin 1.5 dB

Eb/N0 5.8 dB

BER 1.010-7

These link budgets were calculated to show how the system must be set up in order to obtain, in

each case, a user (as opposed to channel) BER of 1 10-7

. This is adequate for many

applications. The cases with and without half-rate FEC are illustrated and the benefit of having

FEC is clearly seen in the reduced EIRP required from the hub and from the satellite. In the FEC

case the output back off is 23.0 dB, thus the VSAT link is using only 0.5% of the transponder's

saturated power output. Assuming that the maximum power available from the transponder

under multicarrier conditions is 41.0 dBW, i.e. 2 dB down on saturation, then a further 63 such

VSAT links could be accommodated in the same transponder. Since each carrier can serve a

large number of VSATs on a timeshared basis, it is clear that a single transponder could actually

accommodate communications to many thousands of VSATs. This fact is important for the

system economics.

22.12.2 Two-Way Link Budget

The EIRP of the VSAT is quite low. This is advantageous for reasons of cost, safety and

reliability. The link budget assumes the use of rate half FEC and a user data rate of 9.6 kbit/s.

The link uses only 0.01% of the transponder power but is still able to provide a BER at the hub

of 1.0 10-7

.

22.13 References

1 'Network performance summary'. Hughes VSAT August 1996 report.

2 'Satellite services: the European regulatory framework'

(http://www2.echo.lu/legal/en/converge/satellite.html)

3 EUTELSAT Press Release, EUTELSAT at CEBIT 97

(http://www.eutelsat.org/press/release/press14.html)

4 GE Spacenet has a very comprehensive web site packed with VSAT information. Start at:

http://www.ge.com/capital/spacenet/prodserv/ssadat1.htm

5 Equipment show review

6 MORGAN, W.L., and ROUFFET, D.: 'Business earth stations for telecommunications' (ISBN:

0 471 635561)

7 SES Website: www.aia.lu/recept/arcs/index.html