Measuring DVB-T Signals - Digital Video and Audio Broadcasting Technology - page 386

the most important one of these being the analysis of the DVB-T constella-
tion pattern. Although extensive knowledge in the field of DVB-C constel-
lation analysis has been gathered since the 90s, simply copying it across
into the DVB-T world is not sufficient. This chapter deals mainly with the
special features of DVB-T constellation analysis, points out problems and
provides assistance in interpreting the results of the measurements.
In comparison with DVB-C constellation analysis, DVB-T constellation
analysis is not simply a constellation analysis on many thousands of sub-
carriers and many things do not lend themselves to being simply copied
across.
Fig. 21.1. shows the constellation diagram of a 64QAM modulation in
DVB-T. The positions of the scattered pilots and of the continual pilots (on
the left and on the right outside the 64QAM constellation diagram on the I
axis) and of the TPS carriers (constellation points inside the constellation
diagram, also on the I axis) can be easily seen. The scattered pilots are
used for channel estimation and correction and thus represent a checkpoint
in the constellation diagram which is always corrected to the same posi-
tion. The transmission parameter signalling carriers (TPS) serve as a fast
information channel from the transmitter to the receiver. Apart from noise,
there are no further influences acting on the constellation diagram shown
(Fig. 21.1.).
Fig. 21.1. Constellation diagram in 64QAM DVB-T