Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 466

In the DAB modulator, the COFDM is carried out (Fig. 26.5.). The data
stream is first provided with error protection and then COFDM-modulated.
After the modulator, the RF signal power is amplified and then radiated via
the antenna.
In DAB, all subchannels are error-protected individually and to different
degrees. Up to 64 subchannels are possible. The FEC is provided in the
DAB modulator. In many block diagrams, FEC is often described in con-
junction with the DAB multiplexer which, although it is not wrong in prin-
ciple, does not correspond to reality. The DAB multiplexer forms the ETI
data signal in which the subchannels are transmitted synchronously and
unprotected.
The ETI, however, carries the information about how much protection is
to be provided for the individual channels. The ETI data stream is then
split up in the DAB modulator and each subchannel is then error-protected
to a different degree in accordance with the signalling in the ETI. The sub-
channels provided with FEC are then supplied to the COFDM modulator.

S C1

Convolutional
coding

Time
interleaving

Scrambling

SC2

Convolutional
coding

Time
interleaving

Scrambling

…

SCn

Convolutional
coding

Time
interleaving

Scrambling

SC = subchannel (up to 64)

Fig. 26.6. Forward error correction (FEC) in DAB

The error protection in DAB (Fig. 16.6.) is composed of scrambling fol-
lowed by convolutional coding. In addition, the DAB signal is then sub-
jected to long time interleaving, i.e. the data are interleaved over time so
that they are more resistant to block errors during the transmission. Each
subchannel can be error-protected to a different degree (unequal forward
error correction). The data from all subchannels are then supplied to the
COFDM modulator which first carries out frequency interleaving and then
modulates them onto a large number of COFDM subcarriers.