Coded Orthogonal Frequency Division Multiplex (COFDM) - Digital Video and Audio Broadcasting Technology - page 331

suitable filtering measures. These filtering measures, in turn, are made eas-
ier by simply not using the edge carriers because the filters do not need to
be so steep in this case.
Following an integral multiple of symbols, it is also often necessary to
join up with the input data structure which is also often structured in
blocks. A symbol can carry a certain number of bits due to the data carriers
present in the symbol. The data structure of the input data stream can also
supply a certain number of bits per block. The number of payload carriers
in the symbol is then selected to be only such that the calculation comes
out exactly after a certain number of complete data blocks and symbols.
Because IFFT is used, however, it is necessary to select a power of two as
the number of carriers which, after subtracting all data and pilot carriers,
still leaves carriers, namely the zero-information carriers.
There are then also the following pilot carriers:

• Pilot carriers with a fixed position in the spectrum

• Pilot carriers with a variable position in the spectrum

Pilot carriers with a fixed position in the spectrum are used for auto-
matic frequency control (AFC) in the receiver, i.e. to lock it to the trans-
mitted frequency. These pilot carriers are usually cosinusoidal signals and
are thus located on the real axis at fixed amplitude positions. There is usu-
ally a number of such fixed pilots in the spectrum. If the receive frequency
is not tied to the transmit frequency, all constellation diagrams will rotate
also within one symbol. At the receiving end, these fixed pilots within a
symbol are simply missed out and the receive frequency is corrected in
such a way that the phase difference from one fixed pilot to the next within
a symbol becomes zero.
The pilots with variable position in the spectrum are used as measuring
signal for channel estimation and channel correction at the receiving end in
the case of coherent modulation. One could say they represent a sweep
signal for the channel estimation in order to be able to measure the chan-
nel.
Special data carriers with supplementary information are very often
used as a fast information channel from transmitter to receiver in order to
inform the receiver of changes made in the type of modulation, e.g. switch-
ing from QPSK to 64QAM. In this way, frequently all current transmission
parameters are transmitted from transmitter to receiver, e.g. in DVB-T. It
is then only necessary to set the approximate receiving frequency at the re-
ceiver.