Final Thesis

3.3.5 Single Frequency Network (SFN) 
 
The concept Single Frequency Network (SFN) is a network that consists in a several transmitters that are 
working at the same frequency. Due to the CODFM modulation that is used in DVB-T system, is needed a 
careful synchronization of transmitters, and a non destructive interference between signals received form 
different transmitters. All the receivers have the same clock status as all due to the synchronization by GPS. 
This one is reached by introducing the timing information at head-end of the network and by providing an 
automatic alignment system is each transmitter.
The SFN is used in several European countries and introduces several benefits as improved coverage and 
better use of the available spectrum. [9] [14] [17] 

a) Conventional network:
 
b) SFN network:
1 program, several frequencies 
 
 
1 program, 1 frequency 
Fig. 9 Radiowaves planning 

---

TAMK University of Applied Sciences 
Escola Politécnica Superior de Castelldefels 
Marcos Tejedor Alonso 
26 (44) 
3.3.6 Guard interval 
To minimize inter-symbol interference problems (ISI) caused by the echoes are introduced an additional 
guard interval at the beginning of each OFDM symbol. This means that each symbol is transmitted during a 
period longer than its useful. At the reception, the period to recover the signal, will start a time later when 
there will be a change of symbol in the main received signal (due to the interval guard). The period will be 
extended during the time that the signal is useful. The ISI occurs during this guard interval and out of the 
useful time in reception and for this reason the signal is protected against natural echoes (distant rebounds) 
and artificial echoes due to the transmission of the same signal from two distant transmitters as in Single 
Frequency Network (SFN) happen. 
The addition of the guard interval reduces the transmission capacity depending on its length. In the DVB-T 
there are several options range on the length of the Fourier Transform (FFT) chosen. The 
Table 3 shows the normalized values for DVB-T.