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S7200 TV Signal Analyzer Operation Manual
Figure 6-36 DVB-C2 1024QAM Constellation
A DVB-C2 frame begins with preamble symbols which are repeated every
7.61MHz and have a width of 3408 carriers each. These are followed by the data
symbols, a total 448 data symbols. The preamble symbols are used both for time
and frequency synchronization and for signaling of the Layer-1 (L1) parameters.
The preamble symbols are arranged with respect to frequency in such a way that
a receiver with a receiver bandwidth of 7.61Mhz will get all the data necessary for
finding the Layer-1 parameters.
There are actually no longer any channels in DVB-C2 but only two channel rasters
of either 6 or 8 MHz. Channels can be bundled together to form a channel with a
total width of approx. 450MHz. There are no longer any gaps between the orginal
channels. The lack of any more gaps (guard bands) then enables the frequency
spectrum to be used more effectively and allows a higher data rate overall.
However, there are very frequently also disturbed frequency bands in the cable
or frequency bands which could interfere with other radio services. These can be
notched out in DVB-C2 by simply switching off certain OFDM carriers. This is called
nothing which produces gaps in the frequency spectrum.
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