Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 493

and must operate completely synchronously for this reason. Signals from
adjacent transmitters look to a DAB receiver as if they were simply ech-
oes.
The condition which can be met most simply is the frequency synchro-
nisation because frequency accuracy and stability already had to meet high
requirements in analog terrestrial radio. In DAB, the RF of the transmitter
is tied to the best possible reference. Since the signal of the GPS (Global
Positioning System) satellites is available throughout the world, it is used
as reference for synchronizing the transmitting frequency of a DAB single-
frequency network.
The GPS satellites radiate a 1pps signal to which a 10 MHz oscillator is
tied in professional GPS receivers which is used as reference signal for the
DAB transmitters.
However, there is also a strict requirement with regard to the maximum
transmitter spacing. The maximum possible transmitter spacing is a result
of the length of the guard interval and the velocity of light and the associ-
ated propagation time. Inter-symbol interference can only be avoided if in
multi-path reception no path has a longer propagation time than the guard
interval length. The question about what would happen if a signal of a
more remote transmitter violating the guard interval is received can be eas-
ily answered. Inter-symbol interference is produced which becomes no-
ticeable as disturbing noise in the receiver. Signals from more remote
transmitters must simply be attenuated sufficiently well. The threshold for
virtually error-free operation is set by the same conditions as in the case of
pure noise. It is of particular importance therefore that a single-frequency
network has the correct levels. It is not the maximum transmitting power
which is required at every site but the correct one. Network planning re-
quires topographical information.
With the velocity of light of C=299792458 m/s, a signal delay of
3.336 µs per kilometer transmitter distance is obtained.
The maximum distances between adjacent transmitters possible with
DAB in a single-frequency network are shown in Table 26.9.

Table 26.9. SFN parameters in DAB

Mode I Mode IV Mode II Mode III

Symbol dura-
tion

1 ms 500 Μs 250 Μs 125 Μs

Guard interval 246 ms

123 Μs 62 Μs 31 Μs

Symbol+guard

1246 Μs 623 Μs 312 Μs 15 6Μs

Max. transmit-
ter distance

73.7 km 36.8 km 18.4 km 9.2 km