Terrestrial Transmission of Digital Television Signals (DVB-T) - Digital Video and Audio Broadcasting Technology - page 368

Example: At a transmitting frequency of 500 MHz and a speed of 200
km/h, the Doppler shift is 94 Hz.
If, however, multiple echoes are added (Fig. 20.21.), the COFDM spec-
trum becomes smeared. This smearing is due to the fact that the mobile re-
ceiver is both moving towards signal paths and moving away from other
sources. I.e. there are now spectral COFDM combs which are shifting up-
ward and downward. Due to its subcarrier spacing, which is narrower by a
factor of 4, the 8K mode is much more sensitive to such smearing in the
frequency domain than the 2K mode. The 2K mode is thus the better
choice for mobile reception, although DVB-T was originally not intended
for mobile reception.
Considering then the behavior of the DVB-T receiver in the presence of
noise. More or less noise in the DVB-T channel leads to more or fewer bit
errors during the reception. The Viterbi decoder can correct more or fewer
of these bit errors depending on the code rate selected in the convolutional
encoder. In principle, the same rules apply to DVB-T as do for a single
carrier method (DVB-C or DVB-S), i.e. the same “waterfall” curves of bit
error ratio vs. signal/noise ratio apply. The only caution is advised with re-
spect to the signal/noise ratio which is also often called carrier/noise ratio.
The two differ slightly in DVB-T, the reason being the power in the pilot
carriers and auxiliary carriers (continual and scattered pilots and TPS car-
riers). To determine the bit error rate in DVB-T, only the power in the ac-
tual payload carriers can be used as the signal power. In DVB-T, the dif-
ference between the overall carrier power and the power in the pure
payload carriers is 0.857 dB in the 2K mode and 0.854 dB in the 8K mode
but the noise bandwidth of the pure payload carriers is reduced with re-
spect to the overall signal.
The reduced noise bandwidth of the payload carriers is:
10 log(1512/1705) = -0.522 dB; in 2K mode
and
10 log(6048/6917) = -0.520 dB; in 8K mode.
Thus, the difference between C/N and S/N in DVB-T is:
C/N - S/N = -0.522 dB -(-0.857 dB) = 0.34 dB; in 2K mode, and
C/N - S/N = -0.52 dB -(-0.854 dB) = 0.33 dB; in 8K mode.