Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 473

In the further description, the details of the COFDM implementation in
DAB will be discussed in greater detail. In DAB, a COFDM frame starts
with a null symbol. All carriers are simply set to zero in this symbol.
However, Fig. 26.12. only shows a single carrier over a number of sym-
bols. The first symbol shown at the left-hand edge of the picture is the null
symbol where the vector has the amplitude zero. This is followed by the
phase reference symbol to which the phase of the first data symbol (sym-
bol no. 2) i referred. The difference between the phase reference symbol
and symbol no. 2 and, continuing from there, the difference between two
adjacent symbols provides the coded bits. I.e., the information is contained
in the phase change.

90 o +180 o +90 o +180 o 0 o

90 o

Null
symbol

Phase
ref.
symbol

Symbol
2

Symbol
3

Symbol
4

Symbol
5

Symbol
6

Symbol
7

Fig. 26.12. DQPSK sequence with null symbol and phase reference symbol

The principle shown in Fig. 26.12. does still not correspond to the pre-
cise reality in DAB which, however, we are approaching step by step.
Fig. 26.12. shows the mapping and the state transitions in the case of
simple QPSK or simple DQPSK. It can be seen clearly that phase shifts of
+/-90 degrees and +/-180 degrees are possible. In the case of +/-180 degree
phase shifts, however, the voltage curve passes through zero which leads
to the envelope curve being pinched in. In single carrier methods it is
usual, therefore, to carry out so-called /4-shift DQPSK instead of
DQPSK, thus avoiding this problem. In this type of modulation, the carrier
phase is shifted by 45 degrees from phase to phase, i.e. by /4. The re-
ceiver is informed about this and cancels out this process. An example of
/4-shift DQPSK is the TETRA mobile radio standard. In DAB, too, this