Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 472

about 1/4 of the symbol length. The total length of a symbol is thus be-
tween about 156 Μs and 1.246 ms. Symbol No. 0 is the so-called null sym-
bol. During this time, the RF carrier is completely gated off. The null sym-
bol starts the DAB frame and is followed by the time frequency phase
reference (TFPR) used for frequency and phase synchronisation in the re-
ceiver. It does not contain any data.

Data symbols (DQPSK) (2.4 Mbit/s)

76 1 2 3 73 74 75 76

(152)

Guard Symbol

FIC (96 kbit/s) MSC (2.304 Mbit/s)

FIC = Fast Information Channel
MSC = Main Service Channel

Frame

(24, 48, 96 ms)

Fig. 26.11. DAB frame

All COFDM carriers are set to defined amplitude and phase values in
the phase reference symbol. The actual data transmission starts with the
second symbol. In contrast to DVB, the data stream in DAB is completely
synchronous with the COFDM frame. In the first symbols of the DAB
frame, the Fast Information Channel (FIC) is transmitted, the length of
which is dependent on the DAB mode. The data rate of the FIC is
96 kbit/s. In the FIC, important information for the DAB receiver is trans-
mitted. Following the FIC, the transmission of the Main Service Channel
(MSC) starts in which the actual payload data are found. The data rate of
the MSC is a constant 2.304 Mbit/s and is mode-independent. Both FIC
and MSC additionally contain FEC gated in by the DAB COFDM modula-
tor. The FEC in DAB is very flexible and can be configured differently for
the various subchannels, resulting in net data rates of (0.8) 1.2 to 1.73
Mbit/s for the actual payload (audio and data). The type of modulation
used in DAB is differential QPSK. The aggregate gross data rate of FIC
and MSC is 2.4 Mbit/s. The length of a DAB frame is between 24 and
96 ms (mode-dependently).