Terrestrial Transmission of Digital Television Signals (DVB-T) - Digital Video and Audio Broadcasting Technology - page 360

Knowing about the operation of channel estimation and correction is of
importance to understanding test problems in DVB-T. From the channel
estimation data, it is possible to deduce both a large amount of test infor-
mation in the DVB-T test receiver (channel transfer function, impulse re-
sponse etc) and problems in the DVB-T modulator (I/Q modulator, center
carrier).

Symb./
bit de
inter
leaver

Conv.
de
inter
leaver

Reed
Solom.
dec.

Energy
disp.
rem.

Base
band
interf.

Sync.
inv.
rem.

FEC
data in
from
de
mapper

TS
out

Viterbi
dec.

Code rate
1/2...(3/4)...7/8

From TPS decoder

Fig. 20.13. Block diagram of the DVB-T receiver (part 2), channel decoding

In parallel with the channel correction, the TPS carriers are decoded in
the uncorrected channel. The transmission parameter signalling carriers do
not require channel correction since they are differentially encoded, the
modulation of the TPS carriers being DBPSK (differential bi-phase shift
keying). Each symbol contains a large number of TPS carriers and each
carrier carries the same information. The respective bit to be decoded is
determined by differential decoding with respect to the previous symbol
and by majority voting within a symbol. In addition, the TPS information
is error-protected. Therefore, the TPS information can be evaluated cor-
rectly for the DVB-T transmission before the threshold to the “fall off the
cliff” is reached. The TPS information is needed by the de-mapper follow-
ing the channel correction, and also by the channel decoder. The TPS car-
riers make it possible to derive the currently selected type of modulation
(QPSK, 16QAM or 64QAM) and the information about the presence of hi-
erarchical modulation. The de-mapper is then correspondingly set to the
correct type of modulation, i.e. the correct de-mapping table is loaded. If
hierarchical modulation is provided, a decision about which path (high pri-
ority (HP) or low priority (LP)) is to be decoded must be made in depend-
ence on the channel bit error ratio, either manually or automatically. Fol-
lowing the demapper, the data stream is available again and is provided for
channel decoding.
Apart from the symbol and bit de-interleaver, the channel decoder (Fig.
20.13.) is configured exactly the same as that for the DVB-S satellite TV