Digital Signal Processing Reference
In-Depth Information
in the receiver which thus becomes simpler. However, this is at the cost of
twice the bit error ratio in comparison with coherent coding.
Thus, the data carriers can be coded as follows:
•
Coherent
•
Differentially coded
The edge carriers, that is to say the top and bottom carriers, are not used
in most cases and are set to zero and do not carry any information at all.
They are called zero-information carriers and there are two basic reasons
for the existence of these unused zero-information carriers:
•
Preventing adjacent channel crosstalk by facilitating the filtering of
the shoulders of the COFDM spectrum
•
Adapting the bit capacity per symbol to the input data structure
Fig. 19.20.
Real DVB-T COFDM spectrum with shoulders
A COFDM spectrum (Fig. 19.20.) has so-called 'shoulders' which are
simply the result of the sin(x)/x-shaped tails of each individual carrier.
These shoulders cause interference in the adjacent channels and it is, there-
fore, necessary to improve the so-called shoulder attenuation by applying
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