Ultra HD - TELE-satellite International Magazine

FEATURE
■
DVB at its best
Ultimate
Spectral
Efficiency
DVB-C2 is
around the
corner
Jacek Pawlowski
The old DVB-C standard has been in use since 1994. The
time for a second generation standard came in 2006
when European cable operators expressed their big
concerns about the limited bandwidth they have for the
distribution of their services. The major goal was the more
efficient use of the available bandwidth in cable networks.
The DVB-C2 standard was developed by experts from
20 different companies and scientists from various
universities. Kabel Deutschland, one of Europe’s major
cable operators, provided the chairmanship for this effort.
The standard was finalized last year and supplements the
DVB-S2 and DVB-T2 standards developed earlier.
The need for better utilization of coaxial
cable bandwidth is understandable. While
in satellite TV it’s enough to turn the dish
to another satellite and again have a frequency
spectrum of 1 or 2 GHz, this can’t
be done with cable TV unless you install
additional cables and nobody wants that.
The growing competition from IPTV as
well as the growing popularity of HD
channels has forced cable operators to
take action.
DVB-C has extremely efficient modulation
and error correction schemes – far
better than DVB-S and DVB-T. Improving
it significantly in DVB-C2 was not a walk
Table 1. Possible combinations of LDPC coding rates and QAM constellations.
10 TELE-satellite — Global Digital TV Magazine — 08-09/2010 — www.TELE-satellite.com
in the park. Yet, they succeeded! In the
worst case, DVB-C2 offers a 30% gain
in spectral efficiency over DVB-C. This
figure grows to 80% if you upgrade from
DVB-C based on 64-QAM. But even a 30%
increase allows a provider to transmit 60
Mb/s through a 6 MHz channel (the maximum
for 256-QAM in DVB-C is only 38.47
Mb/s).
As a reminder: one HD channel requires
about 20 Mb/s. The best quality SD channels
need 4-5 Mb/s while many of them
typically use only 2-3 Mb/s. Speaking of
channel bandwidths, there are 6 and 8
MHz channels in coaxial cable TV channel
plans. However, DVB-C2 permits the creation
of wider channels with much higher
bandwidths. Such a new wide channel
does not have to be necessarily a multiple
of 6 or 8 MHz. The bonus is that we do
not waste bandwidth for channel separation
and the spectral efficiency goes even
higher.
As you might expect, the technical
solutions of DVB-C2 have quite a bit in
common with DVB-S2 and DVB-T2. Forward
error correction (FEC) is composed
of Bose-Chaudhuri-Hocquenghem (BCH)
outer code and a Low Density Parity
Check (LDPC) inner code. The role of BCH
is to eliminate the error floor which may
be produced by the LDPC decoder in the
receiver. It adds a mere 1% redundancy.
LDPC is for correcting transmission errors
and has code rates ranging from 2/3 to
9/10.
The modulation also had to change.
Multi-carrier OFDM (known from terrestrial
transmission) rather than single carriers
QAM has been adopted for DVB-C2.
The particular carrier in OFDM is modulated
with QAM: the choices range from
16-QAM up to 4096-QAM (4k-QAM). Some
sources say that it may even be extended
to 64k-QAM in future revisions. Not all
LDPC code rates are allowed for every
QAM constellation. Table 1 lists the possible
combinations.
The new error correction algorithms
ensure much better noise performance
over classical DVB-C. For example, mathematical
simulations show that we can
expect an improvement of 7 dB in signalto-noise
ratio after switching from DVB-C
to DVB-C2 (same QAM constellation and
FEC)! DVB-C2 is very close to the Shannon
limit which means it transmits almost