SFN

Using COFDM enables a Single Frequency Network (SFN) to be created where many transmitters are broadcasting on the same frequency and it does not matter if the individual transmitter coverage area overlaps. This is not possible with analogue transmissions as in areas that receive signals from different transmitters broadcasting on the same frequency there is considerable mutual interference.

For SFN to work each transmitter must be broadcasting exactly the same data, on exactly the same frequency and all transmitters must broadcast at the same time. COFDM is immune to multi-path reception due to the ‘cyclic prefix’ or guard interval which duplicates part of the data being broadcast (see Cyclic Prefix page).

SFN will have a profound effect on the medium wave bandplan as in the UK and Europe there are many ‘synchronised networks’ where several frequencies are needed so that a radio station can have national or regional coverage.

Here are just two examples of AM synchronised networks, but there are many more.

In the UK commercial rock music station Virgin Radio has 13 transmitters on 1215 kHz ranging from a few hundred watts to a powerful 200 kW (five at 100 kW) that provide the main coverage. To achieve national coverage requires additional transmitters to cover areas where reception of the main frequency is poor or non-existent.

1197 kHz - ten transmitters (200 watts to 2 kW).
1233 kHz - five transmitters (100 watts to 500 watts).
1242 kHz - four transmitters (500 watts to 2 kW).
1260 kHz - two transmitters (500 watts and 1 kW).

Another example of a synchronised network is commercial radio station talkSPORT (the station’s name describes the format).
To get national coverage talkSPORT requires -

1053 kHz - twelve transmitters (including 500 kW Droitwich).
1071 kHz - two 1 kW transmitters.
1089 kHz - seven transmitters (including two 400 kW).
1107 kHz - six transmitters (500 watt to 2 kW).

Consequently these two stations require a total of nine frequencies and 61 AM transmitters to provide national coverage and in some areas reception is possible on more than one frequency. This makes any analogue synchronised network wasteful in terms of spectrum usage and the number of transmitters needed.

DRM only requires one frequency and as many transmitters as needed to achieve any desired broadcast area. This makes DRM much more efficient in terms of radio spectrum use. Unfortunately Virgin Radio and talkSPORT also broadcast on the ‘Digital 1 Network’ national DAB multiplex, so there is little incentive for them to switch from AM to DRM as they are already ‘digital’.