Terrestrial Transmission of Digital Television Signals (DVB-T) - Digital Video and Audio Broadcasting Technology - page 345

64QAM with Α = 1, 2, 4 and 16QAM with Α = 1, 2, 4. The information
about the presence or absence of hierarchical modulation and the Α factor
and the code rates for LP and HP are transmitted in the TPS carriers. This
information is evaluated in the receiver which automatically adjusts its de-
mapper accordingly. The decision to demodulate HP or LP in the receiver
can be made automatically in dependence on the current conditions of re-
ception (channel bit error rate) or left to the user to select manually. Hier-
archical modulation is provided as an option in modern DVB-T chipsets
and set-top boxes since, in practice, no additional hardware is required. In
many DVB-T receivers, however, no software is provided for this option
since it is currently not used in any country. At the beginning of 2002, hi-
erarchical modulation was tested in field tests in Australia but it is cur-
rently not used there, either.

20.4 DVBT System Parameters of the 8/7/6 MHz Channel

In the following paragraphs, the system parameters of DVB-T will be de-
rived and explained in detail. These parameters are:

• IFFT sampling frequencies

• DVB-T signal bandwidths

• Spectrum occupied by the 8/7 and 6 MHz DVB-T channel

• Data rates

• Signal levels of the individual carriers

The basic system parameter in DVB-T is the IFFT sampling frequency
of the 8-MHz channel which is defined as:

f sample IFFT 8MHz = 64/7 MHz = 9.142857143 MHz.

From this basic parameter, all other system parameters can be derived,
i.e. those of the 8/7 and 6 MHz channel. The IFFT sampling frequency is
the sampling rate of the COFDM symbol or, respectively, the bandwidth
within which all 2K (= 2048) and 8K (= 8192) subcarriers can be accom-
modated. However, many of these 2048 or 8192 subcarriers are set to zero
and the bandwidth of the DVB-T signal must be narrower than that of the
actual 8, 7 or 6 MHz wide channel. As will be seen, the signal bandwidth
of the 8 MHz channel is only about 7.6 MHz and there is thus a space of
approx. 200 kHz between the top and the bottom of this channel and its ad-
jacent channels.