Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 470

where echoes due to multipath reception can "wear themselves down".
This prevents intersymbol interference as long as a maximum echo interval
is not exceeded.

IFFT
bandwidth

Channel
bandwidth

Central carrier
= 0 (set ot zero) Subcarrier

spacing
1 / 4 / 8 / 2 kHz

Signal
bandwidth
1.536 MHz

Carrier # k/2
768/192/96/
384

Carrier # +k/2
768/192/96/384

No. of carrier: 1535 /
383 / 191 / 767

Fig. 26.9. DAB spectrum

Instead of one carrier, COFDM involves hundreds to thousands of sub-
carriers in one channel (Fig. 26.8.). The carriers are equidistant from one
another. All carriers in DAB are /4-shift DQPSK (Differential Quadrature
Phase Shift Keying) modulated. The bandwidth of a DAB signal is 1.536
MHz, the channel bandwidth available, e.g. in VHF Band 12 (223 ... 230
MHz) is 1.75 MHz which corresponds to exactly 1/4 of a 7 MHz channel.
Firstly, however, let us turn to the principle of differential QPSK: The
vector can take up four positions, which are 45, 135, 225 and 315 degrees.
However, the vector is not mapped in absolute values but differentially.
I.e., the information is contained in the difference between one symbol and
the next. The advantage of this type of modulation lies in the fact that no
channel correction is necessary. It is also irrelevant how the receiver is lo-
cked in phase, the decoding will always operate correctly. There is also a
disadvantage, however: the arrangement requires a signal to noise ratio
which is better by about 3 dB than in the case of absolute mapping (coher-
ent modulation) since in the case of an errored symbol, the difference with
respect to the preceding symbol and the following symbol is false and will
lead to bit errors. Any interference event will then cause 2 bit errors.