Technology development specialist

Copies of the Standards are available from ETSI and the EBU see Table 1: Specification Reference ETSI Status DVB-S ETS 300 421 Digital Broadcasting System for television, sound and data

Julio E Duran Technology Development Specialist

of applications have converted the DVB/MPEG-2 to attractive systems for the delivery of different kinds

of services and applications via satellite This paper explains in detail what the DVB standard is, its scope, and its technical parameters and characteristics The important Conditional Access System within the DVB is also explained DVB is intended and developed to provide DTH, multiprogram TV services in the BSS (Broadcasting Satellite Services) and FSS (Fixed Satellite Services) bands Additionally, DVB addresses consumer IRDs (Integrated Receiver Decoder) DVB is being used in different applications such as INTERNET, Multimedia Information Broadcasting, Tele-learning, Tele- medicine, Tele-shopping, and others The exploitation of the multiplex flexibility allows the use of the transmission capacity for a variety of TV service configurations In the next years, DVB will continue to provide internationally accepted standards for digital broadcasting The most contentious issues at present are the development of "Conditional Access System", "Data Broadcasting" and "Interactive Services" standards.

2.1 What is the Digital Video Broadcasting (DVB) Project?

The story of the DVB Project began in 1990 Experimental projects such a SPECTRE showed that the digitalvideo compression system known as "Motion Compensated Hybrid Discrete Cosine Transform Coding" washighly effective in reducing the transmission capacity required for digital television Up until then, digitaltelevision broadcasting was thought to be impractical to implement commercially In the United States, the firstproposals for digital terrestrial HDTV were made In Europe, the Swedish Television suggested that fellowbroadcasters should form a pan-European platform to develop digital terrestrial HDTV During 1991,broadcasters and consumer equipment manufacturers discussed how this could be done Broadcasters,consumer electronics manufacturers and radio-regulatory bodies agreed to come together to discuss theformation of a pan-European group that would oversee the development of digital television in Europe - theEuropean Launching Group (ELG) Over the course of about a year, the ELG expanded to include the majorEuropean media interest groups, both public and private, the consumer electronics manufacturers and commoncarriers It drafted a Memorandum of Understanding (MoU) establishing the rules by which this new andexciting game of collective action would be played The MoU was signed by all ELG participants in September

1993, and the Launching Group became the DVB (Digital Video Broadcasting) Project DVB provided the

forum for gathering all the major European television interests into one group It promised to develop acomplete digital television system based on a unified approach It was now clear that digital satellite and cabletelevision would provide the first broadcast digital television devices Fewer technical problems, and a simplerregulatory climate meant that it could develop more rapidly than terrestrial systems Market priorities meant thatdigital satellite and cable broadcasting systems would have to be developed rapidly

2.2 Why DVB?

Because DVB System provides a complete solution for digital television and data broadcasting across a range

of delivery media Careful planning and common sense have been the keys to success From the outset, it wasclear that the sound and picture-coding systems of ISO/IEC MPEG-2 should form the audio and image-coding.DVB would need to add, to the MPEG transport stream multiplex, the necessary elements to bring digital

television to the home through cable, satellite and terrestrial broadcast systems Interactive Television is also

an attractive service and DVB might help with the framework for the interactive television services of the future.The digital vision and sound-coding systems adopted for DVB use sophisticated compression techniques TheMPEG standard specifies a data-stream "syntax" The system designer is given a "tool-box" from which tomake up systems incorporating greater or lesser degrees of sophistication In this way, services avoid beingover-engineered, yet are able to respond fully to market requirements and are capable of evolution

The sound-coding system specified for all DVB systems is the MPEG audio standard The DVB Projectsuggests, in its current MPEG-2 Guidelines document, that MPEG Layer II (MUSICAM) should be used This isalso used for many other audio products and services throughout the world MPEG Layer II is a digitalcompression system which takes advantage of the fact that a sound element will have a masking effect onlower-level sounds (or on noise) at nearby frequencies This is used to facilitate the coding of the audio with lowdata rates [3] Sound elements which are present, but would not be heard even if reproduced faithfully, are notcoded The MPEG Layer II system can achieve a sound quality which is very close to Compact Disc Thesystem can be used for mono, stereo, or multi-lingual sound, and in future MPEG-2 audio can be used forsurround sound [3]

2.3 DVB Standards

DVB standards are open and inter-operable Following approval by the Steering Board, DVB specifications areoffered for standardization to the relevant standards body (ETSI or CENELEC), through theETSI/EBU/CENELEC JTC (Joint Technical Committee) and the ITU-R, ITU-T and DAVIC For each deliverysystem, a set of User Requirements is drawn up by the relevant Commercial Module These are used asconstraints on the specification For example, in the case of DVB-T, the Terrestrial Commercial Moduleproposed user requirements that outline broad market parameters for a DVB-T system (price-band, userfunctions, etc.) The Technical Module then developed the specification, following the user requirements Theapproval process within DVB requires that the relevant Commercial Module support the specification before it isfinally approved by the Steering Board

In addition DVB has developed DVB Receiver Guidelines, suggestions for Interfaces for the domestic receiver,and a Common Interface specification intended for conditional access applications Both interface documentswere agreed by the DVB Steering Board in March 1995 and are now with CENELEC for standardization [5].DVB-S has been acknowledged by the ITU in a Recommendation for the broadcast transmission of digitaltelevision for 11/12 GHz satellites DVB-C is also included in an ITU Recommendation Technical specifications

in the areas of subtitling, MVDS (Multipoint Video Distribution System) and interactive television are beingsubmitted to the appropriate standards body

Copies of the Standards are available from ETSI and the EBU (see Table 1):

Specification Reference

(ETSI)

Status

DVB-S ETS 300 421 Digital Broadcasting System for television, sound and data

services; Framing Structure, Channel Coding and modulation for 11/12 GHz satellite services

SB 3(93)9approved

DVB-C ETS 300 429 Digital Broadcasting System for television, sound and data

services; Framing Structure, Channel Coding and modulation for cable systems

SB 4approved

DVB-SI ETS 300 468 Digital Broadcasting System for television, sound and data

services; for Service Information (SI)

SB5(94)20approved

DVB-T ETS 300 744 Digital Broadcasting System for television, sound and data

services; for Digital Terrestrial Television

SB12(95)7approved

DVB-MS ETS 300 748 Digital Broadcasting System for television, sound and data

services; for Multipoint Video Distribution System at 10 GHz and above

SB12(95)15approved

DVB-MC ETS 300 749 Digital Broadcasting System for television, sound and data

services; for Multipoint Video Distribution System below 10 GHz 14(96)43SB

approved

DVB-SMATV ETS 300 473 Digital Broadcasting System for television, sound and data

services; Satellite Master Antenna Television (SMATV) distribution systems

SB7(94)26Rev 1

DVB-NIP ETS 300 802 Digital Video Broadcasting (DVB); Network Independent

Protocols for DVB Interactive services

SB14(96)44approved

DVB-RCC ETS 300 800 Digital Video Broadcasting (DVB); DVB interaction channel for

Cable TV distribution system (CATV) 14(96)45SB

15(96)4approved

DVB-CS ETR 289 Digital Video Broadcasting (DVB); The Common Scrambling

system description

SB14(96)9approved

DVB-CI ETR 289 Common Interface Specification for Conditional Access and other

Digital Video Broadcasting Decoder Applications

SBapproved

DVB-Data prTS 101 192 Digital Video Broadcasting (DVB); Specification for the

transmission of data services in DVB bitstreams SI-DAT

drafting

Table 1 (continue) DVB Specifications Status

2.3.1 DVB-S

Baseline System for modulation and channel coding for satellite digital multi-programme TV/HDTV services to

be used for satellite digital TV for primary and secondary distribution in FSS and BSS bands[9] The BaselineSystem is intended to provide Direct-to-Home (DTH) services for consumer Integrated Receiver Decoders(IRDs) as well as collective antenna systems (SMATV) and cable television head-end stations The BaselineSystem uses QPSK modulation and concatenated error protection strategy based on a convolutional code and

a shortened Reed-Solomon code The Baseline System is suitable for use on different satellite transponderbandwidths Compatibility with MPEG-2 coded TV services, with a transmission structure synchronous with thepacket multiplex, is provided The multiplex flexibility allows the use of transmission capacity for a variety of TVservice configurations, including sound and data services All service components are time multiplexed (TDM)

on a single digital carrier

2.3.2 DVB-C

Baseline System for a Digital multi-programme Television by Cable (DTVC) [10] Harmonized transmission

standard for cable and satellite, based on the MPEG-2 System Layer, with the addition of Forward ErrorCorrection (FEC) technique It is compatible with the modulation/ channel coding system used for digital multi-programme television by satellite (DE/JTC-DVB-6) The System allows for further evolution as technologyadvances and is capable of starting a reliable service as of now The Baseline System is based on QAMmodulation scheme It allows for 16, 32 or 64-QAM constellations and permits future extension to higherconstellation such as 128 and 256-QAM

2.3.3 DVB-SI (Specification for Service Information (SI) in DVB systems)

The provision of a solution for the "Seamless Transition Networks" with respect to the "Network IdentificationTable" in the case of SMATV and some CATV networks was not covered by the ETS 300 468 (SI) Ed 1 As aconsequence HISPASAT, RAI, DTI, SAT/Sagem, Sharp and the Spanish Administration proposed revision EWPitems on ETS 300 468 and ETR 211 Specification of Service Information (SI) data which forms part ofDVB/MPEG-2 bitstreams is defined, in order that the user can be provided with information to assist in theselection of services and/or events within the bitstream, so that the Integrated Receiver Decoder (IRD) canautomatically configure itself for the selected service SI is specified in ISO/IEC 13818-1 as ProgrammeSpecific Information (PSI)[6] This specification complements the PSI by providing data to aid automatic tuning

of IRDs, and additional information intended for display to the user, mainly in text form The form ofpresentation of the information is not specified, and IRD manufacturers have freedom to choose appropriatepresentation methods It is expected that Electronic Programme Guides (EPG) will be a feature of Digital TVtransmissions The definition of EPG is outside the scope of the SI specification, but the data contained withinthe SI specified here could be used as basis for an EPG The present specification describes ServiceInformation (SI) for use in broadcast MPEG-2 bitstreams The MPEG-2 System layer specifies SI which isreferred to as Programme Specific Information (PSI) The PSI data provides information to enable automaticconfiguration of the receiver to demultiplex and decode the various streams of programme within the multiplex

The basic building blocks which are part of the DVB System are the MPEG-2 data packets These are length containers with 188 bytes of data MPEG includes Program Specific Information (PSI) so that theMPEG-2 decoder can capture and decode the packet structure This data, transmitted with the pictures andsound, automatically configures the decoder and provides the synchronization information necessary for thedecoder to produce a complete video signal at its output [6] MPEG-2 also allows a separate ServiceInformation system to be used to complement the PSI DVB has prepared an open Service Information system

fixed-to accompany DVB signals It can be used by the decoder and the user, fixed-to navigate through the array ofservices offered

Key data, necessary for the DVB IRD (Integrated Receiver Decoder) to automatically configure itself, isavailable in the MPEG-2 PSI DVB-SI adds information that enables DVB IRDs to automatically tune toparticular services and allows services to be grouped into categories with relevant schedule information

In a DVB environment, the viewer of tomorrow will be receiving a multitude (perhaps hundreds) of channelswith his IRD These services could range from interactive television, to near video-on-demand, to specializedprogramming The viewer will need help DVB-SI provides the elements necessary for the development of theElectronic Programme Guides (EPG) that are likely to become a feature of the new digital television services.More elaborate EPGs may also be provided, perhaps as additional elements via a receiver interface

DVB-SI is based on four tables, plus a series of optional tables Each table contains descriptors outlining thecharacteristics of the services/event being described The four tables are:

NIT The Network Information Table groups together services belonging to a particular network provider It

contains all the tuning information that might be used during the set-up of an IRD It is also used to signal achange in the tuning information

SDT The Service Description Table lists the names and other parameters associated with each service in a

particular MPEG multiplex

EIT The Event Information Table is used to transmit information relating to all the events that occur or will

occur in the MPEG multiplex The EIT contains information about the current transport, and optionally coversother transport streams that the IRD can receive

TDT The Time and Date Table is used to update the internal clock of the IRD.

In addition there are three optional SI tables:

BAT The Bouquet Association Table provides a means of grouping services that might be used as a way an

IRD presents the available services to the viewer A particular service can belong to one or more bouquets

RST The sections of the Running Status Table are used to rapidly update the running status of one or more

events The Running Status sections are sent out only once, at the time the status of an event changes, unlikethe other SI tables which are normally repetitively transmitted

ST Stuffing Tables may be used to replace or invalidate either sub-tables or complete SI tables.

With these tools, DVB-SI covers the range of practical scenarios Seamless transition between satellite andcable networks, near video-on-demand, and all operational configurations, are possible

2.3.4 DVB-T

Digital terrestrial broadcasting is one of the cornerstones of DVB The DVB-T system specification, for the

terrestrial broadcasting of digital television signal, was approved by ETSI in February 1997 [4]

The Coded Orthogonal Frequency Division Multiplexing (COFDM)-based system, proposed in DVB-T, allowsthe use of either 1705 carriers or 6817 carriers Reed-Solomon outer coding and outer convolutionalinterleaving are used, in common with the other DVB standards The inner coding is the same used for DVB-S.The data carriers in the COFDM frame can use QPSK and different levels of QAM modulation The modulationsystem combines OFDM with QPSK/QAM.

2.3.5 DVB-MS

Modulation and channel coding system for the distribution of digital multi-programme Television (TV)/High

Definition Television (HDTV) by Multipoint Video Distribution Systems (MVDS) in the 40 GHz band The

System described in this ETS is based on that described in ETS 300 421 for 11/12 GHz satellite services Itallows the same consumer Integrated Receiver Decoder (IRD) to be used for either service, when used with aLow Noise Block (LNB) down-converter for the appropriate frequency band The frequency band 40,5 to 42,5GHz has been harmonized within CEPT under Recommendation T/R 52-01 The System however, is applicable

to other frequency bands above 10 GHz The System uses Quaternary Phase Shift Keying (QPSK) modulationand concatenated error protection strategy based on a convolutional code and shortened Reed-Solomon (RS)code The System is suitable for use on different MVDS transmitter bandwidths Compatibility with MovingPictures Experts Group-2 (MPEG-2) coded TV services (see ISO/IEC IS 13818-1), with a transmissionstructure synchronous with the packet multiplex, is provided Exploitation of the multiplex flexibility allows theuse of the transmission capacity for a variety of TV service configurations, including sound and data services.All service components are Time Division Multiplexed (TDM) on a single digital carrier

2.3.6 DVB-MC

Modulation and channel coding system for the distribution of digital multi-programme Television (TV)/High

Definition Television (HDTV) by Multipoint Video Distribution Systems (MVDS) below 10 GHz The System

described in this ETS is based on that described in ETS 300 429 on framing structure, channel coding andmodulation for cable systems The frequency band may/will be harmonized within CEPT The System issuitable for use on different MVDS transmitter bandwidths Compatibility with Moving Pictures Experts Group-2(MPEG-2) coded TV services (see ISO/IEC IS 13818-1), with a transmission structure synchronous with thepacket multiplex, is provided Exploitation of the multiplex flexibility allows the use of the transmission capacityfor a variety of TV service configurations, including sound and data services All service components are TimeDivision Multiplexed (TDM) on a single digital carrier

2.3.8 DVB-I (Interactive Systems)

DVB has worked in developing a set coherent practical specifications for an Interactive TV System This system

is divided in two standards:

2.3.8.1 DVB-NIP

Covers the core DVB requirements to enable interactive services supporting broadcasting to the home withnarrowband return channels The system defined in this ETS provides a generic solution for a variety of futureinteractive services, through the adoption of DSM-CC User-to-User, Download and Object Carousel protocols,

as specified in MPEG-2 The interactive services are provided on systems consisting of a high bit ratedownstream channel (up to the maximum bit rate of the Broadcast channel) from the Service Providers to

Service consumers and low bit rate interaction channels (up to 150 kbit/s) The Broadcast Service Provider andthe Interactive Service Provider need not operate from the same location The services are primarily digitalvideo broadcast enhanced with interactivity At the simplest level the Interactive channel allows the consumer toreact by voting, to order articles displayed in the broadcast programme, to select certain programme bouquets

or to choose movies in near-video-on-demand systems It is also possible to deliver text, graphics, audio andstill pictures (including e-mail) on-demand , although this may require an interactive channel with higher bitrates There are many possible network configurations covering the currently specified DVB broadcast optionsincluding satellite, terrestrial, cable, SMATV and MMDS [12] conjunction with PSTN, ISDN, cable and otherInteractive channel options The network dependent protocols are specified in ETS 300 800/801 & ETS 300

803 The implications for interactive services via these types of networks are described in a separate guidelinesdocument DTR/JTC-00DVB-44 "Guidelines for the use of the DVB network-independent protocols forinteractive services" which will also summarize the functionality of the protocols identified in this ETS

This ETS is the baseline specification for the provision of interaction channel for CATV It is noted that the DVBproject does not intend to specify a return channel solution associated to each broadcast system because theInteroperability of different delivery media to transport the return channel is desirable The solutions hereprovided for interaction channel for CATV networks are a part of a wider set of alternatives to implementinteractive services for DVB broadcasting systems

2.3.8.2 DVB-RCT

This ETS is the baseline specification for the provision of return channel based on PSTN and ISDN to DVBSystems It is noted that the DVB project does not intend to specify a return channel solution associated toeach broadcast system because the Interoperability of different delivery media to transport the return channel isdesirable Therefore the PSTN/ISDN solutions for the return channel apply to satellite, cable, SMATV,terrestrial, MMDS or any future DVB broadcasting system The solutions here provided for return channelthrough PSTN/ISDN are a part of a wider set of alternatives to implement interactive services for DVBbroadcasting systems

2.3.9 DVB-Data

DVB-Data is seen as one of the new applications for DVB Data broadcasting involves file downloading, datacarousels, and protocol tunneling for a wide range of applications including INTERNET delivery and thebroadcast of secure private data [19] This standard is in drafting status This specification is designed to beused in conjunction with the DVB-SI

2.3.10 DVB-CS (Common Scrambling)

Support for use of scrambling and Conditional Access (CA) within digital broadcasting systems [14], meant as amarketing tool to attract interested parties for applying/receiving the "CS system specification", which will bereleased to qualified applicants on receipt of a non-disclosure agreement This ETR does not include "Security"issues

The Common Scrambling Algorithm was designed to minimize the likelihood of piracy attack over a long period

of time The specification, prepared by the Conditional Access Specialists' Group, is lodged with the EuropeanTelecommunication Standards Institute (ETSI) as custodian The technical details are

distributed, and the technology licensed, by the custodian to appropriate organizations upon signature of alicense agreement

By using the Common Scrambling Algorithm system in conjunction with the standard MPEG data transport andselection mechanisms it is possible to incorporate in a DVB transmission the means to carry multiple messageswhich all enable control of the same scrambled broadcast but are generated by a number of different CAsystems This 'Simulcrypt' technique allows both the delivery of one Programme to a number of differentdecoder populations that contain different CA systems, and also for the transition between different CA systems

in any decoder population, for example to recover from piracy The 'Multicrypt' option is also available,

facilitated by the Common Interface (DVB-CI) specification proposed for standardization by CENELEC(European Committee for Electrotechnical Standardization).

2.3.11 DVB-CI (Common Interface)

The Common Interface operates at the MPEG Transport Stream level, and although specifically intended forconditional access, it may also prove useful in other applications, such as for Electronic Program Guides The

CA functions of decryption and descrambling take place in a small plug-in module, similar in size andappearance to a PCMCIA card as popularly used with lap-top PCs This CA Module may operate in conjunctionwith a smartcard through an associated smart-card reader [14] The DVB Project has agreed to an Importantprocedure, or Code of Conduct, which should help service providers gain access to markets where decryptionsystems other than their own are in use The DVB Project has also adopted a series of Recommendations onAntipiracy Legislation, which it has submitted to the European Commission and other European bodies Thesemost notably call for criminal penalties for the commercialization and use of pirated decoder devices The area

of Conditional Access has received particular attention within DVB Discussions have prove to be difficult andlengthy, but common consensus has yielded a package of measures

Figure 1 DVB-S Transmission System (Ku-band).

3.1 Transport Stream Multiplexer [6]

The PES packets are organized into a one or various Transport Packet in the Transport Stream Multiplexer (see Figure 2) A transport packet is always 188 bytes (1504 bits) long The content of each transport packet (see Figure 3) is a 4-bytes Header followed by an Adaptation Header (used to fill the 188 bytes

of the transport packet, if is necessary) and a Payload (184 Bytes without Adaptation Header).

Figure 2 Transport Stream Multiplexer

Figure 3 Transport Packet 3.2 Scrambler

The output data of the MPEG-2 Transport stream Multiplexer is randomized to ensure adequate binarytransitions to comply with ITU Radio regulations The data are randomized by a pseudo-random sequencepolynomial generator (X15 + X14+1) as shown in Figure 4, with the generator initialized to a value 00A9H((MSB)000 0000 1010 1001) every eight transport packets [1,2]

Figure 4 Pseudo-Random Sequence Generator.

All bytes of the transport stream are randomized with the exception of the sync bytes (see Figure 5) Thereceiver descrambler is provided with an initialization signal by forcing an inversion of the sync byte in the firsttransport packet in the group of eight to a value of B8HEX(is equal inverse of the sync byte (47HEX)) This invertedsync byte is followed by the first (MSB) bit of the first byte from the pseudo-random sequence generator Therandomization process continues even when the bit stream is absent or does not conform to the MPEG-2Transport stream format, to ensure that the carrier is properly modulated.

Figure 5 Randomized Transport packets.

3.3 Error Correction System

3.3.1 Outer coding (Reed Solomon)

Error Corrective Coding is needed to preserve the integrity of received data The basic concept is to addredundancy to a message in order to eliminate the need for retransmission of the data Shannon's workshowed that dramatic gains in performance could be achieved by intelligently adding redundancy to amessage This addition of redundancy to the message is called Forward Error Correction (FEC) There are

many methods of adding redundancy to a message, but one of the most efficient and popular methods is Solomon outer coding combined with convolutional inner codes.

Reed-The multiple-error-correcting capability of RS code has been used in many practical applications, includingmagnetic and optical storage systems, space and mobile communications, etc It is important to be familiarwith some of the terminology for describing Reed-Solomon codes The mathematics underlying RS codes isvery complex and it is beyond the scope of this paper A Reed-Solomon code is described as an (n,k) code,where the codewords consist of n symbols of which k are message symbols The following parameters areimportant in the characterization of the codes:

m = the number of bits per symbol

n = the code length in symbols

k = the uncoded message length in symbols

(n-k) = the number of check symbols

t = the maximum number of symbol errors that can be corrected (half of (n-k))

The outer coding and interleaving shall be performed on the input packet structure (see figure 3) Solomon RS(204,188, t=8) shortened code, derived from the original systematic RS(255,239, t = 8) code, shall

Reed-be applied to each randomised transport packet (188 byte) of figure 5 to generate an error protected packet(see figure 6) Reed-Solomon coding shall also be applied to the packet sync byte, either non-inverted (i.e

47HEX) or inverted (i.e B8HEX) [9]

Figure 6 Reed Solomon RS(204,188,t=8) error protected packet.

3.3.1.1 Implemented Algorithms

The RS code has symbol size m=8 (one byte per symbol), which means that the symbols are elements of theGalois Field GF (28) and the maximum codeword length will be 28-1=255 It has 16 symbols of CRC and thuscan correct 8 symbols of errors per codeword The codeword length is programmable by the user Theimplemented RS code has generator (Code Generator Polynomial) G(x)=(x-a0)(x-a1) (x-a15), where a (a=02HEX)

is a root of the binary primitive polynomial (Field Generator Polynomial ) P(X)= x8 + x4 + x3 + x2 + 1

3.3.1.2 RS Encoding Algorithm

Each RS codeword consists of k message bytes (Mk-1,Mk-2, ,M0) and 16 CRC bytes (C15, C14, ,C0) Thecheck polynomial C(x)=C15x15+C14x14+ +C0 is obtained as the remainder when the message polynomialM(x)=(Mk-1xk-1+Mk-2xk-2+ +M0) * x15 is divided by G(x)

3.3.1.4 RS Decoding Algorithm

It is divided into four steps:

i) Calculation of the power sum symmetric functions

ii) The Euclidean algorithm to find the error locator polynomial and error evaluator polynomial

iii) The Chain search

iv) Calculation of the error value

3.3.1.5 Encoder architecture

The polynomial division in the encoding process is implemented by a 16-stage 8-bit shift register Between anytwo consecutive shift registers, there is a 8-bit XOR for the binary finite field addition The feedback pathcontaining the quotient for each division step, is broadcasted to 16 constant finite field multipliers The messagebytes are pipeline from input, and the first k bytes of output are same as input, while last the 16 CRC bytes arefrom the data stored in the 16 shift registers after all the message bytes have been computed Figure 7 showsthe block diagram of the encoder architecture

Fig 7 Encoder Architecture 3.3.2 Forney Interleaving Coding (convolutional)

Following the conceptual scheme of figure 1, convolutional byte-wise interleaving with depth I = 12 shall beapplied to the error protected packets (see figure 6) This results in the interleaved data structure (see figure 8)

Figure 8 Interleaved Frames (Interleaving depth I=12).

The convolutional interleaving process is based on the Forney approach which is compatible with the Ramseytype III approach, with I = 12 The interleaved data bytes shall be composed of error protected packets andshall be delimited by inverted or non-inverted MPEG-2 sync bytes (preserving the periodicity of 204 bytes) Theinterleaver may be composed of I = 12 branches, cyclically connected to the input byte-stream by the inputswitch Each branch j shall be a First-in, First-out (FIFO) shift register, with depth j × M cells where M = 17 =N/I, N = 204 (see Figure 9) The cells of the FIFO shall contain 1 byte, and the input and output switches shall

be synchronized For synchronization purposes, the SYNC bytes (47HEX) and the bytes (B8HEX) shallalways be routed in the branch "0" of the interleaver (corresponding to a null delay) [9]

Figure 9 Diagram of the convolutional interleaver and de-interleaver 3.3.3 Inner Coding (convolutional)

The DVB system allow for a range of punctured convolutional codes, based on a rate 1/2 convolutional codewith constrain length K=7 (see Figure 10) This allow selection of the most appropriate level of error correctionfor a given service data rate

Figure 10 Convolutional

The system allows convolutional coding with code rate of 1/2, 2/3, 3/4, 5/6 and 7/8 as shown in the table below

Code Rates Puncturing pattern

Transmitted sequence (after parallel-to-serial conversion)

Figure 11 Punctured Convolutional Code Block Diagram (refer Table 1.).

3.4 Baseband Shaping and Modulation

Prior to modulation, the I and Q signals (mathematically represented by a succession of Dirac delta functionsspaced by the symbol duration Ts=1/Rs, with appropriate sign) are square-root raised cosine filtered The roll-off factor α is 0.35 The Baseband square-root raised cosine filter have a theoretical function defined by thefollowing expression:

for

for

where:

is the Nyquist frequency and α = 0.35 (roll-off).

Finally in the DVB-S, the coded bits are Gray mapped in the QPSK constellation (see Figure 12) with absolutemapping (no differential coding)

Figure 12 QPSK constellation 3.5 System Flexibility and Performance

3.5.1 Bit-rate capacity versus Transponder bandwidth

The available Transponder bandwidth is defined by the combined effect of the input (IMUX) and output (OMUX)filters The Figure 13 shows, as an example, the satellite amplifier characteristics of an INTELSAT VIIItransponder

Figure 13 Example of TWTA non-linear characteristic [17].

The ratio BW/Rs (Bandwidth/Symbol rate) determine the symbol rate transmitted in the satellite transponder forany particular bandwidth Decreasing BW/Rs means that the symbol rate is increased and therefore thecapacity available to transmit Programmes However, there is a BW/Rs lower limit caused by the acceptabledistortion (ISI, Inter Symbol Interference) introduced by the satellite IMUX and OMUX filter The problem ofchoosing an optimum BW/Rs value is equivalent to estimating what is acceptable as a C/N degradationcompared to the gain in symbol rate and useful bit-rate Figure 14 gives an example of the Eb/No degradation

at BER=2.0E-04, due to bandwidth limitations (IMUX and OMUX) The reference 0 dB degradation refers to thecase of a satellite transponder without bandwidth limitations (BW = ∞), and with saturated TWTA (OBO=0 dB).The inner coding rates 2/3 and 7/8, associated with QPSK modulation, have been analyzed For example, withthe adopted IMUX and OMUX filter responses, BW/RS value between 1.20 and 1.14 (see Figure 14, QPSKrate 7/8), corresponding to a ISI degradation of about 0.2 dB and 0.5 dB, respectively When a 7% margin isallowed on BW to protect against transponder instabilities, more conservative figures of BW/Rs=1.28 and 1.22are achieved