Chapter 8 • Transmission features
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multiplexed DVB streams, the operator can take advantage of
single carrier mode across the transponder by operating the
carrier in saturation.
Figure 11 illustrates how two DVB streams
can be multiplexed together.
DVB-S2 spectral efficiency
The most recent enhancement to the DVB standards is DVB-S2,
which introduces several important new features that, together,
provide significant spectral efficiencies over DVB-S and
proprietary, non-DVB, channel formats. DVB-S2 provides for
8PSK and QPSK by using a powerful FEC system based on the
concatenation of Bose, Ray-Chaudhuri, Hocquenghem (BCH)
codes with LDPC (low-density parity checking) inner coding.
The result of the BCH/LDPC coding is only 0.7 dB from the
Shannon limit. This is a significantly better performance than any
proprietary turbocode—most of which operate roughly 2 dB from
the Shannon limit.
The bottom line for operators is that the DVB-S2 can provide up
to 2.25 bits per Hz or more (depending on the link budget),
resulting in better bandwidth economics.
DVB-S2 outbound adaptive
coding and modulation
A powerful feature of DVB-S2 is adaptive coding and modulation
(ACM), an addition specifically for interactive services such as
broadband IP over satellite.
ACM allows an operator to vary the modulation and coding of the
outbound channel on a per-satellite router basis. This feature can
be applied in two ways—first to optimize the link budget of the
outbound channel, and second to make dynamic adjustments to
compensate for atmospheric attenuation of the outbound channel.
In the first application of ACM—optimizing the link budget—an
operator can predefine the outbound coding/modulation
combinations for each satellite router based on the satellite
footprint or EiRP contour. As shown in
Figure 12, the satellite
routers that are at beam edge can be configured for the most
robust coding/modulation combination (QPSK rate ½), while the
Figure 11: Multiplexing DVB Streams
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