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A single-frequency network or SFN is a type of radio network that operates several transmitters on a single frequency. To avoid interference, each station is usually run synchronously with the others, using GPS or a signal from the main station or network as a reference clock. Both FM and AM radio stations can operate in this manner, as can other non-broadcast operations, but television has proven to be more difficult. When this technology is used in a simplified form, a secondary transmitter may be known as a booster or on-channel repeater. Multiple transmitters operating at similar powers is referred to as distributed transmission (DTx).
Synchronization of multiple signals can prove to be very difficult, particularly in systems that require high bandwidth. Most attempts at repeating analog television on the same channel results in ghosting, since the repeater creates a second path of information (multipath). However, the conversion to digital television will allow SFNs to be used reliably for carrying moving images. This will be easiest in systems that use COFDM as their modulation method. COFDM uses a large number of very low-bandwidth signals, so it is fairly easy to synchronize multiple transmitters. DVB-T (used in Europe and many other areas) and ISDB-T (used in Japan and soon Brazil) both use COFDM and are well-suited to SFN operation. COFDM is also widely used in digital radio systems, including DAB, HD Radio, and T-DMB.
The 8VSB modulation system used in the North America for digital TV allows the use of single-frequency networks, specified in ATSC standard A/110. The system was not designed with on-channel repeaters in mind, but because the system is relatively good at ghost cancellation, it is possible if very carefully designed. Early ATSC tuners were not very good at handling multipath, but later systems have seen significant improvements.
An alternative technical solution to the SFN is the multi-frequency network (MFN). Through the use of virtual channel numbering, an MFN can appear as an SFN to the viewer in ATSC.
See also
"Single-frequency network"