`
`very small aperture terminals
`
`Edited by
`JOHN EVERETT
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`1
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`APPLE 1008
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`1
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`APPLE 1008
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`Published by: Peter Peregrinus Lid., London, United Kingdom
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`® 1992: Peter Peregrinus Lid,
`
`Apart from any fair dealing for the purposes of research orprivate study,
`orcriticism or review, as permitted under the Copyright, Designs and
`Patents Act, 1988, this publication may be reproduced, stored or
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`sent to the publishers at the undermentionedaddress:
`
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`Mishael Faraday House,
`Six Hills Way, Stevenage,
`Herts, SGQi 2AY, United Kingdom
`
`While the editor and the publishers believe that the information and
`guidance given In this work is correct, all parties must rely upon their own
`skifl and judgment when making use ofit. Neither the editor nor the
`publishers assume anyliability to anyone for any loss or damage caused
`by any error or omission in the work, whether such error or omission is
`the result of negligence or any other cause. Any andall such Iiabllity is
`discialmed.
`
`The moral right of the authors to be identified as authors of this work has
`been asserted by them in accordance with the Copyright, Designs and
`Patents Act 1988.
`
`British Library Cataloguing in Publication Data
`
`A CIP catalogue record for this book
`is available from the British Library
`
`ISBN 0 86347 2009
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`2
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`
`
`Contents
`
`
`
`Preface
`Acknowledgments
`The contributors
`
`}
`
`MOtinwisiyEe
`
`Introduction to VSATs J.L. Everett
`Historical perspective on VSATs
`What is a VSAT?
`Satellite communication frequency bands
`Space segment to support VSATservices
`Network configurations
`A representative VSAT system
`Earth terminals in a VSAT network
`1.7.) Hub earth terminal
`. La2 VSAT earth terminal
`1.8 Earth terminal sub-systems
`1.8.1 Antennas
`1.8.2 High power amplifiers (HPAs}
`1.8.3
`Solid state power amplifiers (SSPAs)
`18.4 Low noise converters {LNCs}
`1.8.5
` Up- and downconverters
`1.8.6 Modems and codecs
`1.8.7 Network interface unit “NTU
`1.9 Modulation and coding schemes
`1.10 The communicationof data across the VSAT network
`i.11 Multiple access
`1.11.1 Multiple access schemes
`L.11.1,1 Frequency division multiple acecss
`(FDMA)
`1,.11.1.2 Time divisicm multiple access (TDMA:
`1.11.13 Code division multiple access (CDMA;
`1.11.2 Selection of access scheme
`1.12 Network or mrultiaccess protocols
`1.13 Network management
`4 One- and two-way VSAT systems
`1.14.1 Dedicated one-way data systems
`1.14.2 Data distribution based on television broadcasting
`1.14.3 Two-waysystems
`1.15 Ka-band VSAT systems
`1.16 Military VSAT systems
`LI? Link budgets
`1.18 VSAT applications and services
`
`xxi
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`Chapter I
`Introduction to VSATs
`
`J.L. Everett
`Government Communications Headquarters, Cheltenham,
`Gloucestershire, UK
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`The multiaccess and broadcast capabilities of satellites have been historically
`recognised but it has proved difficult to realise their full potential because of
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`technologylimitations. The advent of the VSAT type of system, whether one-way
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`data broadcasting or two-way interactive, represents the congruence of recent
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`advances in several technological areas including higher gain and higher power
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`satellites, relatively inexpensive microwave and RF components, digital modems
`
`and protocol processing. This evolutionin satellite and earth terminal technology
`
`will ensure a role for the VSAT in most
`telecommunication architectures,
`
`whetherit is a business data network supporting a major company in the United
`
`States or as the backboneofabasic telecommunications service in a Third World
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`country.
`
`Mostof the early developments in VSAT systems and service concepts evolved
`
`in the United States encouraged bya liberal regulatory environment and the
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`availability of space segment at very competitive tariffs. These systems were
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`developed, mainly, to support business requirements for data distribution and
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`two-way interactive data communications where reconfigurability and rapid
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`deployment are important. Morerecently, the potential for VSATs in support of
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`communications in developing countries has been recognised.
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`A moreliberal interpretation of the term VSAT can include any form of small
`terminal system, irrespective of whether it is part of a dedicated business data
`network or a data network based onterrestrial television distribution or whether
`it is military or civil in application.
`While most commercially available VSAT systems have been designed to carry
`data, the underlyingtraffic can be digitised voice, facsimile, reduced rate video
`or even narrowstrands of data within the composite capacity of the system.
`The term VSAT is generally assumed to refer to fixed installation systems;
`while mobile systems also have small aperture antennas and are based onrelated
`technology, many aspects are outside the scope of this book.
`
`This chapter provides a broad overviewof the subject of Very Small Aperture
`Terminals (VSATs) in terms of technology, systems and related issues: it also
`provides a context for the succeeding chapters.
`
`1.1 Historical perspective on VSATs
`
`1.2 What is a VSAT?
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`Introduction to VSATsS
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`\_/
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`appeared on the scene. In addition to these international organisations, a number
`
`Frequency bands for all forms of radio spectrum usage are allocated by the
`International Telecommunications Union (ITU) with thefirst allocations for
`satellite communications made in 1959. These have been revised and extended at
`subsequent World Administrative Radio Conferences (WARCs) held in 1963,
`1971, 1977, 1979 and 1987.
`In general, fixed, commercial VSATsystemsuse satellite transponders operat-
`ing at C-band (uplink 6 GHz, downlink 4GHz) or Ku-band (uplink 14 GHz,
`downlink 11 or 12 GHz) within the Fixed Satellite Service (FSS). More recently,
`the Broadcast Satellite Service (BSS) in Ku-band has been used. Thereis also an
`allocation for fixed services at Ka-band (uplink 30-0 GHz, downlink 20-2 GHz).
`L-band {uplink 1-6GHz, downlink 1-5 GHz) is used extensively by Inmarsat
`for the provision of mobile satellite services.
`
`Fig. 1.1
`
`VSAT star configuration
`
`The VSAT,or microterminal as it is sometimes called in Europe,is generally
`assumedto be the remote terminal in a dedicated data network based on the star
`configuration depicted in Fig. 1.1. This configuration comprises a hub earth
`station, with a larger aperture antenna, controlling a cluster of VSATs, with
`small antennas, typically in the range 0-45 to 2 metres (m) diameter.
`
`1.3 Satellite communication frequency bands
`
`1.4 Space segment to support VSAT services
`
`Satellites used for communications are almost exclusively in the geostationary
`orbit, located on an arc 36000km above the equator.
`Space segment is available from organisations which have procuredsatellites,
`arranged launch and preliminary tests in-orbit and who then operate these
`satellites on a commercialbasis. Initially, these organisations were international,
`e.g. Intelsat offer satellite capacity at C- and Ku-band, but as requirements
`for satellite communications have increased, regional and domestic systems have
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`Introduction to VSATs
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`3
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`of companies, mainly in the USA, ownorlease satellites which are used to carry
`their own or their customer’s traffic. In most cases, the military use dedicated
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`satellites operating in different bands from civil satellites.
`
`Transponders currently operating in the FSS band typically extend from 36 to
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`72 MHzin bandwidth with EIRPlevels from 30 to 52 dBW. Equivalent isotropi-
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`cally radiated power (EIRP) is the power transmitted from thesatellite: it is the
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`productof output powerfrom thesatellite’s amplifier and antennagain. In most
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`VSAT systems power rather than bandwidth is the limiting resource in the
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`satellite transponder.
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`Frequencyre-use is usually achieved by using mutually orthogonal right hand
`
`circular (RHC) andleft hand circular (LHC) polarisation beams in C-band, and
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`by mutually orthogonal linear polarisation beams in Ku-band.
`
`Whenselecting space segment
`it is important to ensure that the charac-
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`teristics of the satellite transponders are suitable for operation with small
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`terminals. The Intelnet [1] service offered by Intelsat and the Satellite Multi-
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`Services (SMS)
`[2] from Eutelsat are particularly suitable for small terminal
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`applications.
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`
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`1.5 Network configurations
`
`
`
`Astar network comprising a hub operating in conjunction with a population of
`VSATsis the configuration upon which all one-way and most two-way systems
`are based. The use of the star configuration has been predicated bylimitations
`
`in the performanceofcurrently available components and systems: it requires the
`
`inclusion of a relatively expensive hub terminal.
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`Thestar configuration has the advantage that the hub can maintain effective
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`control ofthe networkandit is compatible with most business traffic requirements
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`with the hubeither colocated or directly connected to the company head office
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`with individual VSATsserving field offices or retail outlets.
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`Satellites with higher gain together with improvements in low noise amplifier
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`(LNA) and solid state power amplifier (SSPA) performance now offer the
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`prospect of direct VSAT-to-VSAT communications within a mesh architecture
`
`[3] as shownin Fig. 1.2. This offers the feature of reduced propagation delay
`
`(typically 0-25 s compared with 0-5s for the star configuration) whichis especially
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`advantageous when the link carries voice traffic.
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`It may be necessary to designate one VSATasthe ‘master’ terminal, possibly
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`on a rotational basis, to ensure the exercise of proper control of the network.
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`| Ultimately, a mesh network without a master terminal should becomeareality
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`whensuitable control protocol techniques have been developed.
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`
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`}
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`1.6 A representative VSAT system
`The configuration ofa typical two-way VSATterminalidentifying the constituent
`sub-systems is shown in Fig. 1.3 and a representative commercially available
`product (developed by Multipoint Ltd.) in Fig. 1.4. The earth terminals and
`
`
`their sub-systems are described in Sections 1.7 and 1.8. a
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`Fig. 1.2 VSAT mesh configuration
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`1.7 Earth terminals in a VSAT network
`a VSAT network are the hub and VSAT earth
`The principal elements in
`in terms of signal handling, the differ-
`terminals: they perform similar functions
`d and the performance in the receive
`ence being in levels of power transmitte
`mode.Antenna performanceis specified by EIRP in the transmitting mode and the
`figure of merit G/T in the receiving mode. Both of these parameters depend on
`antenna size;
`in general terms,
`the larger the antenna the better the system
`performance. These terms and their meanings are defined in Chapter 2
`1.7.1 Hub earth terminal
`The hub earth terminal can supporteither one or a number ofVSAT networks
`
`Transmitted Signal
`
`Reflector
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`TRANSMISSION PATH
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`t
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`Modulator /
`coder
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`t'I
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`l'41
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`Antenna unit
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`1
`Low Nolse Converter
`IF( TOMHz
`RECEIVING PATH Five)
`Fig. 1.3 Configuration ofa typical two-way VSAT terminal
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`Introduction to VSATs
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`Fig. 1.4 A representative commercially available VSAT system
`(courtesy Multipoint Ltd.)
`
`operating over a givensatellite: the shared hub conceptis attractive as it allows
`the high cost of a hub to be amortised over several networks, Earth terminals of
`this type are expensive, costing between $300 000 and $5 000 000 (at 1991 prices)
`depending on features and performance.
`A typical C- or Ku-band earth terminal used as a hub may have an antenna
`diameterin the range 5-6 to 11m.
`
`1.7.2 VSAT earth terminal
`A VSATearth terminalis characterised by a much smaller antenna, typicallyless
`than 2m in diameter. Consequently, the unit costis appreciably lower than that
`of the hub,typically below $12 000. The VSAT has both lower antenna gain and
`lower transmit power than a hub, with the power normally generated by semi-
`conductor devices of the type described in Chapter 3.
`It is not normally convenient to integrate all the electronic equipment into a
`single package, for engineering reasons outlined in subsequent chapters describ-
`ing individual systems. A compromiseis reached where the sub-systems carrying
`microwave frequency signals comprise an outdoor unit while all others are
`accommodated in an indoor unit, i.e. the interconnections are at an intermediate
`frequency (IF).
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`6
`Introduction lo VSATs
`1.8 Earth terminal sub-systems
`The sub-systems used in hub and VSATearth terminals are identified and their
`functions outlined below.
`
`1.8.2 High power amplifiers (HPAs)
`Travelling wave tube amplifiers (TWTAs) are generally used in hub earth
`terminals as they can give power output levels up to several kilowatts and have
`the capability of being tuned across an individual satellite uplink band. Various
`types of C- and Ku-band TWTs and TWTAs are reviewed in Chapter 4,
`1.8.3 Solid state power amplifiers (SSPAs)
`Improvements in semiconductor technology have yielded SSPAs providing
`output powers of 50 Win C-band and 20 Win Ku-band: these power levels are
`adequatefor final stage amplification in VSAT earth terminals.
`1.8.4 Lownoise converters (LNCs)
`Low noise converters perform the dual function of amplifying and downconvert-
`ing the received signal while minimising the noise added to the signal. Careful
`designis essential to ensure that no spurioussignals are generated and phase noise
`in theoscillators is kept to acceptablylow levels. Typical noise temperature values
`of 73 K (C-band) and 100K (Ku-band) can be obtained, corresponding to noise
`figures of 1-0 and 1-3 dB, respectively. Especially low noise devices are used in the
`
`1.8.1 Antennas
`‘The term antenna embracesthereflector, feed and a device to separate transmit-
`ted and received signals at the antenna. Any two-way earth terminal has two
`separate paths for the transmit and receive signals. These paths combine at the
`orthogonal mode transducer (OMT) which routes the transmit signal to the
`antennafor uplinking to thesatellite, while the signal received from the satellite
`bythe antennais passed into the receive chain. Thefeed has to transmit signals
`at one frequency and receive them on another, usually slightly lower, frequency;
`the feed/reflector combination Is designed as a composite unit to provide these
`required features.
`A major objective in antenna design is to achieve high efficiency and high gain
`in the direction of thesatellite commensurate with low radiated signal levels in
`other directions, i.e. the production of a narrow ‘main’ beam with lowlevel
`‘sidelobes’. Offset feed configurations are generally used in small terminals to
`avoid beam blockage, giving high efficiency together with low sidelobe levels.
`The antenna assembly must be of robust construction, adequate to survive
`underspecified wind loading conditions.
`Ku-bandlinks use linear polarisation so the antenna assembly must allowfor
`rotation to enable thefeed to be aligned with the polarisation ofsignals transmit-
`ted to and received from thesatellite. This feature is not required in the case of
`circular polarisation used in C-band.
`Antennas used in VSAT systems are described further in Chapter 2.
`
`first stage of the converter as the noise performance of this stage determines the
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`

`Introduction to VSATs=7
`overall noise performance ofthe converter unit. Low noise converters (sometimes
`termed downconverters) are described in Chapter5.
`1.8.5 Up- and downconverters
`Upconverters are used to translate the signal intended for transmission from an
`intermediate frequency (typically 70 MHz) to a microwave signal
`(6 GHz in
`C-band, 14GHzin Ku-band) where it is amplified in an HPA (or SSPA) for
`transmission to the satellite. Downconverters translate the microwave signal
`received bythe earth terminal (4GHzin C-band, 11 or 12 GHz in Ku-band) to
`@ similar intermediate frequency (IF).
`1.8.6 Modems and codecs
`As there is a requirementfor a modulator and a demodulator in each terminal they
`are usually incorporated in one unit for convenience, this unit being referred to
`as a modem. Similarly, the coder and decoder functions are built into another unit
`called a codec.
`A digital signal applied to the input of a modulator appears, typically, at the
`eutpul as a phase shift keying signal (PSK) centred on an IF around 70 MHz.
`A. demodulatorin the receive path accepts a PSK sienal at IF and converts it
`back to a baseband digital signal.
`Thefunctions ofmodems and codecs are outlined in Section 1.9 and described
`in Chapter 6.
`
`
`
`1.9 Modulation and coding schemes
`In a typical VSAT network a modulator is used to convert digital data to
`analogue form for transmission over the satellite, with the demodulator at the
`receiving end ofthe link used to extract the information even when the signal has
`been distorted and corrupted by the addition of noise.
`Phase modulation schemes are preferred for satellite communications applica-
`tionssince these require a constant powerlevel irrespective ofthe data transmit-
`ted: this avoids the need for transponderload adjustment and smoothing which
`would be required for a non-constant envelope modulation scheme.
`€ predominantly desi
`niably employed with phase modulation universally
`used, i.e. phase shift keying (PSK), This can take the form of binary phase PSK
`(BPSK)or quadriphase PSK (QPSK).
`In powerlimited systems (always the case with the downlink toa VSAT), the
`under utilised bandwidth in the satellite transponder can be made available for
`digital encoding. In encoding for forward error correction (FEC), redundantbits
`are added to a bit stream 5
`receiving end ofthe link. Som
`coders while others have int
`
`1.8.7 Network taterface unil (NIU)
`‘This unit is needed to implement the user protocol interface and access to the
`
`Satellite.
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` 8
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`Introduction ta VSATs
`
`
`O°
`different car
`{ access.
`
` Probability of error
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`
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`% Viterbi decoding
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`( % rate, constraint length 7 } without coding
`with coding * 20 22 (dB)
` Fig. 1.5 QPSK/BPSK with and without coding
`
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`codec combination.
`network
` 1.10 The communication of data across the VSAT
`
`The objective of the VSATis to provide an end-to-end communications link in
`a way whichis as transparent as possible to the user. From a user perspective,
`
`
`external interfacing is required to allow the protocol on the user’s terminal
`
`
`equipment to be understood and processed by the VSAT system, The network
`
`
`protocol employed by the VSATfacilitates the efficient transfer ofuser data over
`the satellite link, while the multiple access schemeallows many users to share the
`satellite transponder resource.
` 1.11 Multiple access
`
`
`
`
`
` QPSK/BPSK
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`
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`modulation and digital coding maynot be clearly defined. Bit error rate (BER)
`values obtained for various EF; {Nj ratios with and without FEC coding are shown
`in Fig. 1.5. This relationship indicates the performance of a modem or modem/
`
`
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`As one transponder maybe required to handle transmissions from a number of |
`
`ni HATTA
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`TMnTWW
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`|M
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`t
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`L.11.1 Multiple access schemes
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`Introduction to VSATs
`§69
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`different earth stations, it is necessary to use techniques which allow multiple
`access.
`
`The transponder resource can be shared by frequency ortimeallocation or by
`the use of non-interfering codes.
`
`
`
`
`In FDMA users share the
`LIL.L.1 Frequency division multiple access (FDMA)
`transponder byprior allocation of individual channels: this technique is applic-
`able to analogue anddigital services. Transpondersare operated in a linear mode
`to avoid non-linearities which would generate intermodulation products and
`cause interference to all users.
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`In TDMAeach useris assigned
`1.11.1.2 Time diviston multiple access (TDMA)
`the full bandwidth of the channel for a short period whichis then made available
`to another user for the next period, and so on. The attraction ofthis technique
`is that the transponderis operated at high powerlevels (and high efficiency), even
`running close to saturation without interference being caused to other users.
`‘TDMAis intended primarily for very high bit rate digital modulation schemes.
`Conventional TDMAhashigh capacity (typically 120 Mbit/s) which is a long
`way outside the capabilities of VSATs. However, partial transponder TDMA
`operating in the range 32-128 kbit/s is used in some VSATsystems.
`
`1.11.1.3 Code division multiple access (CDMA) CDMAis based on the properties
`of direct sequence spread spectrum. Spread spectrum is a meansof transmission
`in whichthe transmitted signal occupies a bandwidth in excess of that of the data
`signal. This ‘spreading’ is achieved by combining the data signal with a high bit
`rate code signal whichis independentof the data. Reception at the distant end
`of the link is accomplished by mixing the incoming composite data/code signal
`with a locally generated and correctly synchronised replica of the code to obtain
`the data. The codes used for this purpose have characteristics allowing any
`individualcodeto be distinguished from others:it is this property which permits
`several composite signals to share a common frequency band,
`i.e.
`the code
`division multiple access feature.
`There are practicalrestrictionsin the use of spread spectrum which meansthat
`it is only employed for interference rejection or for security reasons in military
`systems.
`
`ave
`
`FaREPT
`
`L.11.2 Selection of access scheme
`It has to be remembered that the multiple access schemeis simply providing a
`channel through the transponder for the traffic.
`Selection of an access scheme has to take accountof the requirements of (what
`may be) a changing population of VSATs to access a satellite in a way that
`optimises satellite capacity, satellite EIRP and spectrum utilisation in a flexible
`and cost effective manner. Clearly, since it is unlikely thatall these factors can be
`optimised, some compromiseis necessary.
`Further details of multiple access schemes and the basis of their selection for
`specific applications will be found in later chapters describing two-wayinter-
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`active VSATsystems.
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`Introduction to VSATs
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`1.12 Network or multiaccess protocols
`
`Not surprisingly, there are trade-offs between channelutilisation efficiency and
`the complexity and cost of implementing protocol schemes at the VSATstations.
`In satellite systems power and (to a lesser extent) bandwidth are constrained; in
`these power limited situations, a relatively inefficient protocol may be used
`without major adverse effect on system throughout. It should be noted that in
`those applications where the traffic is quasi-continuous (over 0:5s duration)
`preassigned access schemes are more efficient and have the added advantage of
`simplified implementation and consequent lower cost.
`The important aspects to be considered in selecting a channel sharing or
`network protocol for a VSAT system are:
`
`Once the multiple access scheme has been selected there is a need to define
`network protocols which provide the means of interface between the VSAT and
`hub station. A proprietary protocol layer is introduced between the remote
`VSATand the hub to support the end-to-end communicationslink.
`It will be apparentfrom later chapters that there is considerable interaction at
`the design stage between the multiple access schemes and network protocols
`selected for use in a system.
`Within a specified multiple access scheme consideration must be given to the
`proportion of capacity allocated to each earth terminal. Whenthis is decided in
`advance of transmissionit is fixed or preassigned access: whenit is in response to
`changing traffic demands it is demand access. Preassigned access schemes are
`appropriate for systems containing a limited number of VSATs where traffic
`levels are relatively constant. In contrast, demand access is used in systems with
`large populations of VSATs wheretraffic is highly variable in terms of density,
`origins and destinations.
`In those applications where traffic is bursty (i.e. short data bursts at random
`intervals) there are incentives for using channel sharing protocols which can
`supporta large number of remote terminals on each inbound(i.e. VSAT to hub)
`channel. This is achieved by allowing an earth terminal to transmit whentraffic
`is available and to wait for an acknowledgment: ifno acknowledgmentis received
`within a specified time period, due to interference orcollisions with other data
`bursts, the traffic continues to be transmitted at intervals until an acknowledg-
`mentis received. This random access approach avoids the need for the overhead
`required to support a demandaccess schemefor the supply of dedicated channel
`resources.
`
`
`
`ic)
`
`(a) Data type, transactional or quasi-continuous
`(6)
`Channel sharing efficiency(i.e. the percentage of time that data is carried
`on multiple access channels)
`Delayin transmitting data (average and peak) resulting from traffic depen-
`dent queueing delay
`Robustness when channel errors and equipmentfailures occur
`System operation
`start up and recovery factors
`Implementation complexity and cost
`
`(d)
`(é)
`(f)
`
`Many of the data communications protocols used in VSAT systems were
`developed originally for terrestrial links and have had to be modified to operate
`
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`

`1.13 Network management
`
`Network managementis an essential feature in any VSAT network in order to
`maintain system integrity. The management system must be integrated into the
`network at the design stage and be sufficiently versatile so that it can respond to
`changing requirements of users and be able to monitor the health and status of
`the system: the health monitoring function should be performed periodically in
`a manner transparentto the user. In general terms VSAT network management
`embraces administrative, operational and planning functions, several of these
`being identified and discussed in Chapter 10.
`
`Introduction to VSATs
`11
`with the propagation delayassociated with geostationarysatellites, ie. 0-58 for
`a system based on a star configuration.
`As will be apparent from succeeding chapters, a numberof user protocols can
`be supported by commercially available VSATnetworks.
`Networkprotocols (sometimes termed multiaccess protocols) and the protocol
`software used in VSATnetworks are described in Chapters 7 and 8 respectively,
`followed by an overview of system design in Chapter 9.
`
`not needed.
`
`1.14 One- and two-way VSAT systems
`In generic terms these systems can be considered in three forms:
`(a) Data distribution: one-way
`‘6) Data gathering: one-way
`‘e)
`Interactive: two-way
`In functionalterms these three formsare represented in Figs. 1.6a, 1.6 and 1.6.
`1.14.1 Dedicated one-way data systems
`The hub in a star configuration data distribution system broadcasts to a network
`of dispersed VSAT terminals with antenna diameters within the range 0-45 to
`2-0m. More sophisticated systems embodyselective addressing to the sub-group
`or even individual terminal level. BPSK js normally used to support data rates
`up to 1 or 2 Mbit/s [VSATs designed to handle a Tl carrier (1-544 Mbit/s) are
`sometimes termed TSATs].
`In applications where extremely small antennasize (typically below 0-6m
`diameter) is critical, direct sequence spread spectrum [4] is used to enable the
`receive-only terminal to reject unwanted signals (representing interference) from
`adjacent satellites on the geostationary arc. The small
`terminal scenario is
`represented in Fig, 1.7.
`The Equatorial Communications company madesubstantial progress in the
`1980s in developing and installing data distribution receiving systems, based on
`spread spectrum, in business premises across the United States.
`It should be noted that the unwanted signal problem with small terminals
`becomes less severe as the link frequencies are increased because the beams
`become narrowerand, ultimately, the protection afforded byspread spectrum is
`
`14
`
`

`

`12
`
`Introduction to VSATs
`
`Satellite
`
`another user, causing incurable interference. For this reason, principally, two-
`
`A numberof one-way systems have been developed and some are commercially
`available. Representative systems from Ferranti and Polycom are featured in
`Chapters 11 and 12, together with APOLLO,an advanced documentdistribu-
`tion system, developed by the European Space Agency (ESA), in Chapter 13.
`Data gathering requirements are satisfied by two-way systems. A scenario based
`on the use of dispersed VSAT units relaying data into a hub terminal without a
`return link is unacceptableto satellite operators as the hub would not beable to
`communicate to the remote VSATs nor exercise any control over them. This
`latter factor represents a serious flaw in the operating conceptsince a given VSAT
`could suffer a malfunction resulting in it operating on a frequencyallocated to
`
`Satellite
`
`Fig. 1.6
`
`(a) Data distribution system. (b) Data gathering system. (¢) Two-way inter-
`active system
`
`15
`
`

`

`gral
`
`
`
`wantedsignal
`
`VSAT antenna beam pattern
`
`Fig. 1.7
`
`Small terminal scenario
`
`way interactive VSAT systems are used for data gathering applications even
`thoughthis results in a more expensive implementation.
`
`Introduction to VSATs=13
`
`satellites on geostationary are
`
`interfering©
`
`ny
`
`—
`
`are broadcast over Europe from satellites operating in the FSS bands.
`
`1.14.2 Data distribution based on television broadcasting
`Data distribution based on television broadcasting can be considered in two
`classes: that based on conventional transmission schemes e.g. PAL (or NTSC,
`SECAM) and that based on Multiplexed Analogue Component (MAC) [5]
`packet systems. The formerare relayed oversatellites operating in the FSS band
`while MACsystemsare intended for use on dedicated direct broadcastsatellites
`(DBS) operating in the BSS band. However, a limited number of MAC signals
`
`16
`
`

`

`14
`
`Iniroduction to VSATs
`
`Data can be broadcast in conjunction with conventionaltelevision schemes by
`using either Vertical Blanking Interval (WBI) or sub-carrier techniques. In the
`VBI scheme data are transmitted at a rate of 9-6 kbit/s per line with up to 5 lines
`normally being used;
`this technique is well established and supports teletext
`services on terrestrial television. In the sub-carrier case data are combined with
`the television video and audio components in an FM modulator. Data rates up
`to several hundred kbit/s can be transmitted but the data signal ‘steals’ power
`from the television signal and so data rates are usually restricted to around
`100 kbit/s.
`The MACpacket concept represents an entirely new television broadcast
`system providing a highly flexible medium for the transmission of video, sound
`and data in separate fields in the MACpacket format.
`A proof-of-concept data distribution system based on the MACschemeis
`described in Chapter 14.
`
`The allocations in Ka-band are 27-50 to 30-00 GHzfor uplinks and 17-70 to
`
`1.14.3 Two-way systems
`A range of two-waysystems has been developed to satisfy different requirements
`and several are nowavailable as products. A representative realisation is based
`on a hub inastar configuration transmitting a time division multiplex (TDM)
`stream to all VSATs in the network. The multiplex contains network control
`instructions and messages for some orall of the VSATs. A VSAT with a message
`for the hub {or for another VSAT via the hub} will transmit a short duration
`burst on a ‘calling’ channel requesting access to a channel to transmit its message:
`the hub acknowledges the request, assigns a channel and the VSAT changes
`frequency and transmits its message. In a busy network there will be collisions
`between some access request bursts and the VSAT maynot get an acknowledg-
`ment from the hub. Underthese circumstances the VSATwill retransmit its burst
`request after a pseudo randomly determined interval, and continue doing so until
`it receives an acknowledgment and is assigned a channel, At the end of the
`message the transmission is terminated and the message channel becomes avail-
`able for reassignment.
`PSK modulation is normally employed for the TDMbroadcast and the VSAT
`transmissions, often in conjunction with FEC coding on the hub to VSAT link.
`A variety of multiple access techniques and multiaccess protocols are in use, most
`of them proprietary which means that systems from different vendors are not
`generally compatible with each other.
`Representative two-way systems available from AT&T Tridom, Hughes
`Network Systems, NEC and Multi