Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 476

In the right-hand drawing of Fig. 26.15., the state transitions of a /4-
shift DQPSK are shown with phase shifts of +/-45 degrees and +/-135 de-
grees. The carrier never dwells on a constant phase, neither are there any
180-degree phase shifts.
The zero symbol is the very first symbol of a DAB frame, called symbol
no. 0 in numerical order. During this time, the amplitude of the COFDM
signal is zero. The length of a null symbol corresponds approximately to
the length of a normal symbol plus guard interval. In reality, however, it is
slightly longer because it is used for adjusting the DAB frame length to
exactly 14, 48 or 96 ms to match the frame length of the MPEG-1 or -2
audio layer II. The null symbol marks the beginning of a DAB COFDM
frame. It is the first symbol of this frame and can be easily recognized
since all carriers are zeroed during this time. It is thus used for roughly
synchronizing the receiver timing. During the zero symbol, a transmitter
ID, a so-called TII (transmitter identification information), can also be
transmitted. In the case of a TII, certain carrier pairs in the null symbol are
set and can be used for signalling the transmitter ID.

ETI from ensemble MUX

FIC

SC1 SC2 SC3 SC4 … SCn

Header

MST (Main Stream Data)

1

+FEC +FEC

SC1 SC2 SC3 SC4 … SCn

FIC (96 kbit/s)

MSC (2.304 Mbit/s)

SC = subchannel
net data rate
= n * 8 kbit/s

FIC = Fast Information Channel
MSC = Main Service Channel

Frame

(24, 48, 96 ms)

Fig. 26.16. DAB frame

The frame lengths, the symbol lengths and thus also the zero symbol
lengths depend on the DAB mode and are listed in Table 26.2.
The phase reference symbol or TFPR (Time Frequency Phase Refer-
ence) symbol is the symbol following directly after the null symbol.
Within this symbol, all carriers are set to certain fixed phase positions ac-