Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 471

In reality, however, DAB does not use DQPSK but /4-shift DQPSK,
which will be discussed in detail later. Many references wrongly mention
only DQPSK in DAB. If the DAB standard is analyzed in detail, however,
and especially the COFDM frame structure, this special type of DQPSK is
encountered automatically via the phase reference symbol (TFPR).
Fig. 26.10. Real DAB spectrum after the mask filter
COFDM signals are generated with the aid of an Inverse Fast Fourier
Transform (IFFT) (s. COFDM chapter) which requires a number of carri-
ers corresponding to a power of two. In the case of DAB, either a 2048-
point IFFT, a 512-point IFFT, a 256-point IFFT or a 1024-point IFFT is
performed. The cumulative IFFT bandwidth of all these carriers is greater
than the channel bandwidth but the edge carriers are not used and are set to
zero (guard band), making the actual bandwidth of DAB 1.536 MHz. The
channel bandwidth is 1.75 MHz. The subcarrier spacing is 1, 4, 8 or 2 kHz
depending on DAB mode (Mode I, II, II or IV) (see Fig. 26.8. and 26.9.).
Fig. 26.10. shows a real DAB spectrum as it would be measured with a
spectrum analyzer at the transmitter output after the mask filter. The width
of the spectrum is 1.536 MHz. There are also signal components which ex-
tend into the adjacent channels, the relevant terms being shoulders and
shoulder attenuation. The shoulders are lowered by using mask filters.
In DAB, a COFDM frame (Fig. 26.11.) consists of 77 COFDM sym-
bols. The length of a COFDM symbol depends on the DAB mode and is
between 125 Μs and 1 ms, to which the guard interval is added which is