Advanced Coding and Modulation Schemes for Broadband Satellite Services Commercial Requirements DVB Document A082 July 2004
Advanced Coding and Modulation Schemes for Broadband Satellite Services Commercial Requirements Introduction The DVB-S (EN 300 421) was introduced as a standard in 1994 and DVB-DSNG (EN 301 210) in 1997. The DVB-S standard specifies QPSK modulation and is now used by most satellite operators worldwide for television broadcasting services, as well as data transmission. The DVB-DSNG standard specifies the use of 8-PSK and 16-QAM modulation for satellite news gathering and contribution services. Recently developed coding schemes, taken together with higher order modulation, promise more powerful alternatives to the coding and modulation schemes of the current DVB-S standard. Several incompatible schemes have already been implemented by a number of manufacturers. The task of this DVB Commercial Module sub-group on Broadband Satellite Services, DVB- BSS, has been to identify the commercial requirements for a successor to the current DVB-S standard that may be introduced for new services and allow for a long-term migration. We identify two new modes: DVB-S2a is the non backwards-compatible mode of the future standard DVB-S2b is the backwards-compatible mode of the future standard. It should be noted that many European broadcasters, despite being explicitly invited, have chosen not to participate in the definition of the following commercial requirements, which were mainly identified by European satellite operators, US broadcasters, and manufacturers. Objectives of a new standard The primary objective of a new broadband satellite modulation and coding specification is to enable delivery of a significantly higher data rate in a given transponder bandwidth than the current DVB-S standard. The new specification will cover transmit-end functions only, but take into account the consequent cost of receive silicon. The market will determine what features are actually implemented in receive silicon. With the large number of existing networks using DVB-S, it is vitally important that any new standard does not undermine DVB-S. Therefore, the current DVB-S standard must not be modified, and changes to other standards (e.g. SI) must not cause any existing standardised feature to become invalid. Drivers for change The most immediate requirements for change come from the USA, where the industry with over 17 million subscribers has strong demand for additional capacity. Available satellite bandwidth on the current US broadcast orbital locations is fixed. The requirement to offer new HDTV services together with the must-carry rule that obliges broadcasters to carry local and minority programming is therefore forcing broadcasters to look for more efficient methods of carrying these services within the existing transponders. One of the two main US broadcasters, Echostar, has already announced that due to commercial pressures it will adopt Page 2 of 7
a new but proprietary system in order to achieve a capacity increase of about 40% over DVB-S. The other, DIRECTV, is still studying the possibility of a scheme that is backwards compatible with their existing population of DIRECTV receivers. This may involve satellite power increases, taking advantage of the regulatory position in Region 2 (the Americas), where satellite powers are limited by co-ordination agreements. In Europe the situation is different. While the total number of subscribers (over 16 million) is comparable with the USA, the limit of available satellite bandwidth has not yet been reached, and there appears to be no prospect of significant demand for HDTV. In addition, the regulatory position for Regions 1 and 3 does not readily permit satellite power increases. These may be the main reasons, together with compatibility with current receivers, that European broadcasters appear so far not to have considered more effective coding and modulation schemes. However, satellite operators in Europe do see the need for a new standard for broadband services, mainly for Internet related unicast/multicast services and contribution links for cable headends, etc. Satellite technology is also an important driver of change. The introduction of Ka-band systems offers the opportunity to introduce new technology. Existing receive installations are not able to operate in Ka-band without modification, and fade countermeasures could make a large improvement in spectral efficiency (and hence in transmission costs) for unicast applications where a return path is present. Spotbeam technology, applicable to both Ka- and Ku-bands, will make unicast applications more cost effective, due to the small coverage area of any individual beam. Market Segments The DVB-S standard was developed primarily with unidirectional broadcast applications in mind, but has been adopted for other purposes, such as point-to-point data transmission. One of the reasons for this is the availability of inexpensive receive silicon as a result of the high volume broadcast receiver market. A similar process is anticipated for new systems, where the volume driver is expected to remain broadcast DTH applications. Clearly there is a blurring of the distinction between broadcast and non-broadcast applications, with increasing provision of entertainment services over IP networks, and increasing use of interactivity with TV. One way of segmenting the market for DVB services is shown in the diagram below, although some services may exist in more than one segment. This is not intended to imply that new transmission standards would be limited to applications mentioned here. Direct to home Contribution & distribution Non-interactive TV for home reception SMATV DSNG Cable feeds In-flight entertainment, services to trains/coaches Interactive Internet access/content delivery Interactive TV Corporate/business Internet content distribution/trunking In either case, interactive or non-interactive, this document is only concerned with the forward channel. Page 3 of 7
Broadcast DTH Services Broadcast DTH services are characterised by having a large coverage area and providing audio-visual and data services to an extensive base of similar reception systems (both antennas and receivers) with a high degree of availability. For broadcasters there are various reasons to use higher order modulation and/or advanced coding schemes. These include: Increased data throughput in a given bandwidth Increased availability through improved link margin Increased coverage area A key factor for many established broadcasters is the issue of backwards-compatibility. Large populations of DVB-S receivers in the field must continue to provide service to customers for at least several years. This is particularly important where there is a receiver subsidy, and for free to air public services. Backwards-compatible modulation systems that allow DVB-S receivers to continue operating, while providing additional capacity and services to new, advanced receivers, are seen as the only commercially viable way forward for some operators. Backwards-compatible systems however suffer from two principal disadvantages: Compatibility will cause the overall performance to fall short of that achievable by non backwards-compatible systems. Initial studies have shown that in the absence of the ability to increase satellite EIRP, backwards-compatible systems can only deliver a slight increase in overall capacity with uniform service availability. A more substantial increase in capacity can be achieved if availability of the additional data is sacrificed. With interaction and storage, such capacity might be put to good use. There will be some performance penalty in the behaviour of existing QPSK receivers. Note that some operators are reluctant to accept even a slight performance penalty, as this increases service call-outs and churn. There are expected to be applications where there is no requirement for the transmission to be received on existing DVB-S receivers. This is particularly the case for Ka-band applications, where existing Ku-band receiver installations would not operate. Of course, receivers for the new standard may also be able to receive DVB-S. The technical specification should therefore provide for two approaches: Scheme 1: A non backwards-compatible scheme (DVB-S2a), intended for use in systems requiring the highest efficiency, and not requiring that transmissions should be receivable by existing receiver populations. Scheme 2: A backwards-compatible scheme (DVB-S2b) that can be received by existing receiver populations, but provides additional capacity to enhanced receivers. This scheme should, if feasible, provide the means for migrating to DVB-S2a once all QPSKonly receivers have been replaced. Such a migration involves the provision of a high speed FEC decoder (for DVB-S2a functionality) even in early DVB-S2b receivers, although they will only be used at low speed initially. There is likely to be a cost penalty in doing this. Note: it is intended that DVB-S2a is the end point for all users. The technical specification should also take into account the following: Some US broadcasters have stated their requirement for mass production of equipment to be as early as Q3/2002. While this is an impossible target for DVB, it Page 4 of 7
indicates that pressure for new systems is already present. The DVB should therefore respond by completing its new specifications ideally by the end of 2002. A bit-rate increase of at least 30-35% over DVB-S should be achieved by DVB-S2a with unchanged link budget (C/N). The error performance of the system must be suitable for all types of services that may be carried. The long-standing definition of QEF (one uncorrected error event per hour) is insufficient as an operational target. The objective is zero errors for the great majority of hours. For DVB-S2b transmissions, there should generally be no requirement for any change to existing DVB-S receivers, including the antenna and LNB. This assumes continued use of the same transponder. (Increased transponder power can in some cases be used to overcome signal degradation.) For DVB-S2a DTH transmissions, consumer expectations require that reliable reception should be possible using receive antenna diameters comparable with current practice. Due account shall be taken of anticipated satellite system characteristics (e.g. power, bandwidth, interference) over the next ten years. While the technical specification is concerned with the transmission format, it is important that this does not impose an undue burden on receiver costs. We envisage new receiver front-end silicon, enabling multi-mode reception, including DVB-S, DVB- S2a and DVB-S2b. Such a front-end device must be economically attractive in consumer markets. Interactive Services Interactive services are defined here as services where some form of return channel is available that allows the user to have a degree of control over the data transmitted to him. In particular, the specification should be available for consideration as an alternative forward path for DVB-RCS Ku- and Ka-band systems and other data systems currently using DVB-S. In general, characteristics that are desirable exclusively for non-broadcast services should only be included in the base specification if the burden of cost to the broadcast receiver is negligible. For maximum efficiency, two-way services will need to take advantage of such techniques as adaptive modulation and adaptive coding. Such services will include: Point-to-point services (e.g. IP-backbone, and maybe DSNG) Point-to-multipoint services (e.g. virtual private network (VPN) services) Two-way mass market services (e.g. Internet access via satellite) Professional services may benefit from additional features in the toolkit to minimise overall costs. The above services are differentiated from one-way broadcast services by the following characteristics: Individual receivers may require access only to particular content within the common transport system. It is therefore not necessary to be able to decode the entire data stream at a typical user terminal. Customers may have individual quality of service targets, even for different services within the same multiplex. Receiver network volumes may not be as high as broadcast applications, but could benefit from inexpensive silicon arising from broadcast applications. Satellite capacity is generally the most expensive part of a link, always more expensive than the ground equipment. Page 5 of 7
Business users may be able to accept significantly larger receive antennas that residential users. In such cases it would be desirable to use this to allow greater user data rates to be transmitted. The technical specifications shall additionally take into account the following requirements: Service may be required as early as Q1/2003. Non-broadcast services should be receivable on antennas down to typical consumer sizes. As in the case of broadcast applications, it should be possible to manufacture low cost receivers. If the impact of advanced features such as adaptive coding and modulation that are not relevant to broadcast applications is to raise broadcast receiver costs excessively, then these features could exist in an enhanced profile, available as an option. The target decoded BER shall be at least as good as for broadcast applications. Receiver issues DVB practice is generally to define emission standards, and we anticipate that this will be the case for DVB-S2a/b. However, it is important to identify some possible characteristics of receivers in order to complete the picture. It should be noted that there is no intention to mandate anything in this section. For cases in which backwards compatibility with DVB-S receivers is required, the new receiver must necessarily be able to decode both DVB-S and DVB-S2b. It is foreseen that the DVB-S receiver population will eventually be replaced, and when this occurs, compatibility with DVB-S is no longer required. If all DVB-S2b receivers are capable of migrating to DVB- S2a, then the transmission format can be changed with no impact on receivers, other than to provide additional capacity. An additional benefit of this migration path (if technically and commercially feasible) is that it may be possible to bring current non-dvb formats within the new DVB standard. For cases where a new service does not need to be accessible by DVB-S receivers, DVB- S2a may be used from the outset. Receivers for this service may implement the ability to decode DVB-S, and even possibly DVB-S2b. Note that a receiver capable of addressing all three modes is also described in the paragraph above. Using the same type of receiver silicon for both cases should be commercially attractive. Summary of Commercial Requirements 1. General requirements 1.1. There shall be two schemes proposed: a scheme that is backwards-compatible with DVB-S (so-called DVB-S2b), and a more efficient non backwards-compatible scheme (DVB-S2a). 1.2. The technologies shall aim to optimise the use of the transponder. This includes enhanced robustness, as well as maximum data capacity. 1.3. A toolkit of system parameters shall be available to address applications across consumer to business antenna sizes, and telecom to broadcast satellite powers. 1.4. Suitable techniques already in existence shall be adopted wherever possible. 1.5. Specifications shall be completed as soon as possible, ideally by the end of 2002. Page 6 of 7
1.6. Due account shall be taken of anticipated satellite system characteristics (e.g. power, bandwidth, interference, etc.) over the next ten years. 1.7. It shall be possible to implement receiver front-end silicon at volume prices that are attractive in a consumer market. 1.8. New technical specifications shall address transmit-end functions only, but shall take account of cost implications for receive devices. 1.9. The DVB-S standard shall not be modified, nor shall changes to other specifications (e.g. SI) cause any existing feature to become invalid. 1.10. The specifications shall be transmission frequency neutral, or contain the elements allowing for an adaptation to the specifics of certain frequency ranges (e.g. C/Ku/Ka Band). 2. Broadcast DTH requirements 2.1. Backwards-compatible (DVB-S2b) systems 2.1.1. There should generally be no requirement for any change to existing DVB-S receivers, including the antenna and LNB. This assumes continued use of the same transponder. (Increased transponder power can in some cases be used to overcome signal degradation.) 2.1.2. The additional data stream shall have maximum coding compatibility with the non-backwards compatible (DVB-S2a) scheme, to simplify migration to DVB-S2a. 2.2. Non backwards-compatible (DVB-S2a) systems 2.2.1. The target for bit rate increase over equivalent DVB-S systems shall be at least 30%. 2.2.2. The error performance of the system must be suitable for all types of services that may be carried. The long-standing definition of QEF (one uncorrected error event per hour) is insufficient as an operational target. The objective is zero errors for the great majority of hours. 2.2.3. There shall be the maximum feasible commonality with the scheme used for the additional data in the backwards-compatible mode, to simplify migration to DVB-S2a. 3. Interactive systems requirements 3.1. The specification shall be available for consideration as an alternative forward path for DVB-RCS Ku- and Ka-band systems and other data systems currently using DVB-S. 3.2. The specification shall contain optional techniques for optimising the use of the transponder for unicast purposes. Wherever possible, these shall have maximum compatibility with broadcast systems. 3.3. The specification shall allow users to have individual quality of service targets, even for different services within the same multiplex. 3.4. Consideration shall be given to techniques for improving the efficiency of carriage of IP data. - o 0 o - Page 7 of 7