SFN - Single Frequency Network
A Single Frequency Network (SFN) is a network of transmitting stations that use the same frequency to transmit the same information. A Single Frequency Network is a means to extend the coverage area without the use of additional frequencies.
An SFN is particularly interesting for broadcasting. Both T-DAB (digital radio via terrestrial transmitters) and DVB-T (digital television via terrestrial transmitters) have the possibility for a single frequency network. An SFN can be used with other radio communication systems, such as wireless local area networks as well.
The SFN is based on the use of Coded Orthogonal Frequency Division Multiplexing (COFDM). COFDM has the advantage that it is very robust against reception of a signal together with echoes of the same signal (multipath reception). This robustness against multipath reception is obtained through the use of a 'guard interval'. This is a proportion of the time there is no data transmitted between the symbols. This guard interval reduces the transmission capacity.
This multipath immunity can be used to build a SFN with an overlapping network of transmitter stations which use the same frequency. In the areas of overlap, the weaker of the two signals is considered as an echo due to multipath reception. However, the echo has to fall within the guard interval and the stations must be synchronized. Hence, if two stations are far apart, the time delay between the two signals can be large and the system will need a large guard interval.
This extension of the coverage area does not come for free. Drawbacks of a SFN are: