DVB-S2

DVB-S2 stands for Digital Video Broadcasting via Satellite version 2.

It supports quite a lot of features:

• multiple rolloff factors: 0.20 and 0.25 in addition to 0.35 used for DVB-S (saves transmission power but decreases signal quality)
• four modulation modes: QPSK, 8PSK, 16APSK and 32APSK
• forward error correction algorithms: BCH and LDPC
• content is not limited to MPEG2 Transport Stream anymore
• multiple streams
• physical layer scrambling (improves data integrity)
• adaptive modulation for optimal transmission bandwidth (used for professional applications)

Details

DVB-S2 is an extension of the DVB-S standard which is both more efficient than the existing DVB-S standard, and offers more services. It is also geared towards the transmission of HDTV content.

DVB-S2 is the second-generation specification for satellite broadcasting – developed by the DVB (Digital Video Broadcasting) Project in 2003. It benefits from more recent developments in channel coding (LDPC codes) combined with a variety of modulation formats (QPSK, 8PSK, 16APSK and 32APSK). Backwards-compatible modes are available, allowing existing DVB-S set-top-boxes to continue working during any transitional period

Four modulation modes can be selected for the transmitted payload QPSK and 8PSK are typically proposed for broadcast applications, since they are virtually constant envelope modulations and can be used in non-linear satellite transponders driven near saturation. The 16APSK and 32APSK modes, mainly targeted at professional applications, can also be used for broadcasting, but these require a higher level of available C/N and the adoption of advanced pre-distortion methods in the up-link station to minimize the effect of transponder non-linearity. Whilst these modes are not as power-efficient as the other modes, the spectrum efficiency is much greater.

For broadcasters there are various reasons to use higher order modulation and/or advanced coding schemes, including:

• 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 subsidy. 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 disadvantages:

• Compatibility will cause the overall performance to fall short of that achievable by non backwards-compatible systems.

• 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.

The DVB-S2 system may be used in "single carrier per transponder" or in "multi-carriers per transponder" (FDM) configurations. In single carrier per transponder configurations, the transmission symbol rate Rs can be matched to given transponder bandwidth BW (at -3 dB), to achieve the maximum transmission capacity compatible with the acceptable signal degradation due to transponder bandwidth limitations.

Backwards-compatible modes The large number of DVB-S receivers already installed makes it very difficult for many established broadcasters to think of an abrupt change of technology in favour of DVB-S2 – especially where there is a receiver subsidy and for free-to-air public services. In such scenarios, backwards-compatibility may be required in the migration period, allowing legacy DVB-S receivers to continue operating, while providing additional capacity and services to new, advanced receivers. At the end of the migration process, when the complete receiver population has migrated to DVB-S2, the transmitted signal could be modified to the non-backward compatible mode, thus exploiting the full potential of DVB-S2. Optional backwards-compatible (BC) modes have therefore been defined in DVB-S2, intended to send two Transport Streams on a single satellite channel. The first (High Priority, HP) stream is compatible with DVB-S receivers (according to EN 300 421 [3]) as well as with DVB-S2 receivers, while the second (Low Priority, LP) stream is compatible with DVB-S2 receivers only.

Backwards compatibility can be implemented by hierarchical modulation [4], where the two HP and LP Transport Streams are synchronously combined at modulation symbol level on a non-uniform 8PSK constellation. The LP DVB-S2-compliant signal is BCH and LDPC encoded, with LDPC code rates 1/4, 1/3, 1/2 or 3/5. Then the hierarchical mapper generates the non-uniform 8PSK constellation

In fact, with a large number of DVB-S receivers already installed, backwards compatibility may be required for a period of time, where old receivers continue to receive the same capacity as before, while the new DVB-S2 receivers could receive additional capacity broadcasts. When the complete receiver population has migrated to DVB-S2, the transmitted signal can be modified to a non-backward compatible mode, thus exploiting the full potential of DVB-S2.

Optional backwards-compatible (BC) modes have therefore been defined in DVB-S2, intended to send, on a single satellite channel, two Transport Streams, the first (High Priority, HP) being compatible with DVB-S receivers (according to EN 300 421), as well as with DVB-S2 receivers, the second (Low Priority, LP) being compatible with DVB-S2 receivers only. Backwards compatibility can be implemented according to two approaches

• layered modulations, where a DVB-S2 and a DVB-S signals are asynchronously combined on the radio-frequency channel (therefore this operational mode does not require any specific tool in the DVB-S2 specification), the DVB-S signal being transmitted at significantly higher power level than DVB-S2. Since the resulting signal shows large envelope variations, it must be transmitted on a quasi-linear transponder, far from saturation. As an alternative, to better exploit the satellite power resources, HP and LP signals can be independently transmitted on the up-link, and amplified each by an independent satellite amplifier (HPA), driven near saturation; the resulting signals are then combined on the down-link channel. This requires the design and launch of new generation satellites.

• hierarchical modulation, where the two HP and LP Transport Streams are synchronously combined at modulation symbol level on a non-uniform 8PSK constellation (note that hierarchical modes are also used in EN 300 744 [40]). Since the resulting signal has quasi-constant envelope, it can be transmitted on a single transponder driven near saturation. This solution is included in the DVB-S2 standard as an option.

The DVB-S2 system may also deliver broadcasting services over multiple Transport Streams, providing differentiated error protection per multiplex (VCM). A typical application is broadcasting of a highly protected multiplex for SDTV, and of a less protected multiplex for HDTV. Assuming we transmit a symbol rate of 27.5 Mbaud and use 8PSK 3/4 and QPSK 2/3 modulation, 40 Mbit/s could be available for two HDTV programmes and 12 Mbit/s for two to three SDTV programmes, with a difference in C/N requirements of around 5 dB.

The DVB-S2 system may deliver broadcasting services over multiple Transport Streams, providing differentiated error protection per multiplex (VCM mode)

DVB-S.2 is compatible with moving pictures experts group (MPEG-2 and MPEG-4) coded TV services, with a Transport Stream packet multiplex. Multiplex flexibility allows the use 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.

DVB-S2 does not specify symbol rate range, however implementations by different vendors are planned to cover the range from 100 ksymbol/s to 60 Msymbol/s, resulting in maximum transmission rates as high as 300 Mbit/s.

ISI: Input Stream Identifier, second byte of the BBHEADER field when for multiple input streams. It provides a way to separate different BBFRAMEs within a single multiplex, defining logical channels for BBFRAMEs.

Physical Layer and Pilot Structure

Frame synchronization is needed to indicate the start of each FEC block for the decoder. It also provides the necessary information for the receiver to apply the appropriate demodulator and decoder to demodulate and decode the transmitted information. Given that some overhead is necessary for frame synchronization, it is also designed such that it can be used to reduce initial frequency and phase uncertainty of the modulated signal. The frame synchronization is designed to provide reliable operation in the worst case Signal to Noise ratio with minimum overhead. It is also used to minimize the demodulator implementation loss in the presence of consumer quality low-noise-block (LNB) phase noise. In fact, phase noise is particularly detrimental to demodulator performance for higher-order modulation such as 8PSK, 16APSK, and 32APSK. To preserve the near Shannon limit performance of the DVB-S2 FEC, pilot symbols may be added to assist the demodulator to minimize probability of cycle-slips and to provide more accurate phase estimates. These pilot symbols are also designed to use a minimum overhead of the overall bandwidth, and can be turned on or off as desired. The frame synchronization structure and the pilot structure are described in this annex. The frame and carrier synchronization algorithms that make use of this framing structure are described in annex C.

Physical Layer Frame Synchronization

Each LDPC coded block is preceded by the Start of Frame (SOF) and the Physical Layer Signalling (PLS) code (PLSCODE). SOF is a known 26-symbol pattern. PLSCODE is a 64-bit linear binary code, which conveys 7 bits of information with a minimum distance 32, i.e. a [64, 7, 32] code. In total, SOF and PLSCODE occupy one slot (90 symbols).

From Appendix 4 to TM 2745, DVB-S2-001 rev.2

5. The new standard[s] will not undermine DVB-S, DVB-DSNG and DVB-RCS; therefore it will not modify existing standards (DVB-S, DVB-DSNG) or make any existing standardised feature to become invalid; the group will cooperate with AHG GBS in case extensions to DVB-SI come out to be necessary

6. The new standard for broadcast applications will consist in two modes: a non backwardscompatible (NBC) mode and a backwards-compatible (BC) mode, the first being incompatible with current DVB-S receivers, the second delivering a waveform partially decodable by current DVB-S receivers;

7. The new standard applications will support dynamic adaptation of channel coding and modulation if technically feasible and commercially viable.

Hardware

currently none supported.

There are couple of DVB-S2 based cards and a few frontends too The frontend chips that are available are

1. STB0899 + STB6100
2. Broadcom 4500
3. Conexant

The cards that do have them are:

KNC1 1	STB0899 + STB6100 + SAA7146 (PCI32, supports CI)
Twinhan (VP-1041) 2	STB0899 + STB6100 + Mantis
TT 3	STB0899 + STB6100 + SAA7146 (PCI32, not sure about CI support)
Genpix 8PSK module 4	Broadcom 4500 (USB) Couldn't see any S2 specifics in this one, so don't know whether it is S2 compliant
Micronas 5	STB0899 + STB6100 + Micronas PCI Bridge (Dual DVB-S2, PCIe, no CI)

Only the genpix 8PSK module can be bought for now and they have a driver too (similar one to the dst module). The rest of the devices will be available soon in the market.

haven't heard much about the Conexant chip.

PS: The Linux DVB-S2 API itself is not yet ready. Work is in progress, Cards ¹ and ³ will be the first supported cards, ² and ⁵ needs some more work at the relevant PCI Bridge. Regarding ⁴ there are patches floating around.