Hi All,
I felt that many have misconceptions about dvbs2, so i thought i will do
a small writeup.
NOTE: This writeup i have composed from many specifications, datasheets
and articles
Conclusion: In the end all providers will switch over to dvbs2
completely. No provider will stick to dvbs in the long run. This helps
them to reduce operational costs, as well provide better services.
Regards,
Manu
--
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
From DVB.org
DVB-S2 provides:
• 30% greater efficiency than DVB-S;
• an increased range of applications by combining the functionality of
DVB-S (for direct-to-home applications), and DVB-DSNG (for professional
applications);
• techniques such as adaptive coding to maximise the usage of value
satellite transponder resources.
DVB-S2 Application Areas
In order to cope with an expanded range of applications typical of DVB
satellite channel coding and modulation, DVB-S2 is designed to be used
in the following application areas:
Broadcast Services (BS)
BS is covered today with DVB-S, but with the added flexibility of VCM
(Variable Coding and Modulation) enabling different levels of protection
for each service (e.g. robust SDTV, with less-robust HDTV). There are
also BC-BS (backwards compatible broadcast services) for added
interoperability with DVB-S decoders, and a more optimised NBC-BS
(non-backwards compatible).
Interactive Services (IS)
IS is designed to be used with existing DVB return channel standards
(e.g. RC-PSTN, RCS, etc.), DVB-S2 can operate in CCM (constant coding &
modulation) and ACM (Adaptive Coding and Modulation) modes. ACM enables
each receiving station to control the protection around the traffic
addressed to it.
Digital TV Contribution and Satellite News Gathering (DTVC/DSNG)
DTVC/DSNG builds on the DVB-DSNG standard, facilitating point-to-point,
or point-to-multipoint communications of single or multiple MPEG
transport streams using either CCM, or ACM modes.
Other Professional Applications (PS)
These include for example data content distribution/trunking: this mode
is generallyreserved for professional point-to-point and
point-to-multipoint applications using the CCM, VCM or ACM techniques
described above.
Technical Characteristics
With increased flexibility requirements, and a wish to design a system
which on average would yield 30% performance gains over DVB-S, DVB-S2
has the following characteristics:
Modulation Modes
There are 4 modulation modes: QPSK, 8PSK for broadcast applications
through non-linear satellite transponders driven near to saturation.
16APSK and 32APSK are more geared towards professional applications
requiring semi-linear transponders. The latter schemes trade-off power
efficiency for much greater throughput.
Bandwidth and Roll-off:
For tighter bandwith shaping, DVB-S2 adds roll-off factors of
“alpha”=0.25 and “alpha”=0.20 to the DVB-S traditional roll-off of
“alpha”=0.35
Forward Error Correction
DVB-S2 uses a powerful FEC system based on concatenation of BCH
(Bose-Chaudhuri-Hocquenghem) with LDPC (Low Density Parity Check) inner
coding. The result is performance which is at times only 0.7dB from the
Shannon limit. The choice of FEC parameters depends on the system
requirements. With VCM and ACM, the code rates can be changed
dynamically, on a frame by frame basis.
Typical Applications
With the advent of new source coding techniques, e.g. Microsoft Windows
Media 9, MPEG-4 Part 10 / AVC, DVB-S2 provides the ideal platform for
the delivery of advanced video and audio to consumers. DVB-S2 is thus
particularly well suited to the delivery of HDTV.
1080p
Refers to one or two 1920x1080 high definition digital TV formats (HDTV)
that have 1,080 lines of progressive scan resolution (the "p"). Also
called "true HDTV" and "ultra HD," the 1080p progressive formats provide
the maximum lines of resolution and best image quality in the HDTV standard.
1080i
Refers to the 1920x1080 high definition digital TV format (HDTV) that
has 1,080 lines of interlaced resolution (the "i"). The 1080-line
formats provide the maximum lines of resolution in the HDTV standard;
however, progressive scan 1080p provides a better image than interlaced
1080i
720p
Refers to any or all three 1280x720 high definition digital TV formats
(HDTV) that have 720 lines of progressive scan resolution (the "p"). The
720p formats provide the minimum lines of resolution in the HDTV standard.
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