TELE-audiovision - Weltweit größte Digital TV Fachzeitschrift - page 160

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TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine
— 1 -12/2013
FEATURE
New DVB Standards
The DVB-S2 standard was first pub-
lished in 2005. While the performance
it offers may still impress us nowadays,
the advances in technology and grow-
ing demand for higher and higher data
rates make the satellite industry search
for even more efficient ways to transfer
huge amount of data most notably the
high demands generated by Ultra High
Definition TV (UHDTV) and high speed
IP services over satellite.
Recently in TELE-audiovision 07-
08/2013 we have published a feature
articles about the new UHDTV standard
and the video compression standard
HEVC, also known as H.265 – a suc-
cessor of MPEG-4 (H.264). These new
solutions require new hardware and
software. It makes a good moment to
improve also the modulation, coding
and error correction standard as de-
fined by the DVB-S2 standard.
As the proposed improvements have
not yet been blessed by the DVB or-
ganization we will call them the pro-
posed extensions to DVB-S2. However,
real hardware devices have been built
and various test have been performed
to prove these new concepts. These
new DVB-S2 extensions are labelled as
DVB-S2EB1, DVB-Sx or even DVB-S3
although such a standard does not yet
officially exist in this moment.
The proposed extensions can give a
20% increase in data rate compared
to DVB-S2 in DTH (Direct-To-Home)
broadcasts. For professional services,
like VSAT communication, the gain can
be as high as 64%.
So, what exactly are these exten-
sions? Here is our list:
- reduction of the roll-off factors and
the side lobes of digitally modulated
carriers
- use of wide bandwidth transponders
- additional modulation: 64 APSK
- more modulation and coding (MOD-
COM) schemes and forward error cor-
rection (FEC) choices and non-linear
MODCOMs
Not all of the extensions are easy to
What‘s coming next
after DVB-S2?
Jacek Pawlowski
Figure 1. With smaller roll-offs, transponders can be squeezed closer to one another.
comprehend but in this features article
we try to help you to get a general idea
on most of them.
The roll-off factor describes the
shape of the transponder spectrum as
seen on a spectrum analyzer. Its value
tells you how close to an ideal rectan-
gular the spectrum is. The smaller it
is the more steep are the slopes of a
transponder spectrum. DVB-S requires
a roll-off of 35%, DVB-S2 of 20% and
25% while the the proposed S2 exten-
sions aim at 15%, 10% and 5%. It is
easy to understand that with smaller
roll-offs one can position transponders
closer to one another in the frequency
domain and gain a free space for ad-
ditional ones in the same Ku-Band or
C-Band.
However, not only (relatively) big roll-
offs prevent closer location of the DVB-
S2 transponders. So called side lobes
are normally present on both sides of
the useful signal. These are unwanted
artifacts after modulation. With today’s
technology it is possible to practically
get rid of them thanks to improved
filtering. Once they are removed, the
center frequencies of the neighboring
transponders can be set closer to one
another.
If you take a look at Figure 2, you can
come to the conclusion that even af-
ter removing side lobes and improving
roll-offs, there is still some spectrum
wasted between the transponders. And
that’s why wideband transponders are
the next trick in improving efficiency.
Their throughput is increased to 72 Ms/
sec. When compared with the most
popular 27.5 Ms/sec transponders, the
wideband ones are three times wider in
spectral view.
Every new DVB standard introduces a
new modulation schemes. DVB-S2 end-
ed up with 32 APSK. The proposed ex-
tensions call for 64 APSK. In this modu-
lation, every symbol is made up of 6
bits. Of course, the higher the order of
modulation, the smaller the differences
in amplitude and phase between similar
symbols. We can send more data in the
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