Single-frequency network
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A single-frequency network or SFN is a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel.
Analogue FM and AM radio broadcast networks as well as digital broadcast networks can operate in this manner. Analog television transmission has proven to be more difficult, since the SFN results in ghosting due to echoes of the same signal.
A simplified form of SFN can be achieved by a low power co-channel repeater, booster or broadcast translator, which is utilized as gap filler transmitter.
The aim of SFNs is efficient utilization of the radio spectrum, allowing a higher number of radio and TV programs in comparison to traditional multi-frequency network (MFN) transmission. An SFN may also increase the coverage area and decrease the outage probability in comparison to an MFN, since the total received signal strength may increase to positions midway between the transmitters.
SFN schemes are somewhat analogous to what in non-broadcast wireless communication, for example cellular networks and wireless computer networks, is called transmitter macrodiversity, CDMA soft handoff and Dynamic Single Frequency Networks (DSFN).
SFN transmission can be considered as a severe form of multipath propagation. The radio receiver receives several echoes of the same signal, and the constructive or destructive interference among these echoes (also known as self-interference) may result in fading. This is problematic especially in wideband communication and high-data rate digital communications, since the fading in that case is frequency-selective (as opposed to flat fading), and since the time spreading of the echoes may result in intersymbol interference (ISI). Fading and ISI can be avoided by means of diversity schemes and equalization filters.
In wideband digital broadcasting, self-interference cancellation is facilitated by the OFDM or COFDM modulation method. OFDM uses a large number of slow low-bandwidth modulators instead of one fast wide-band modulator. Each modulator has its own frequency sub-channel and sub-carrier frequency. Since each modulator is very slow, we can afford to insert a guard interval between the symbols, and thus eliminate the (ISI). Although the fading is frequency-selective over the whole frequency channel, it can be considered as flat within the narrowband sub-channel. Thus, advanced equalization filters can be avoided. A forward error correction code (FEC) can counteract that a certain portion of the sub-carriers are exposed to too much fading to be correctly demodulated.
OFDM is utilized in the terrestrial digital TV broadcasting systems DVB-T (used in Europe and many other areas) and ISDB-T (used in Japan and soon Brazil). OFDM is also widely used in digital radio systems, including DAB, HD Radio, and T-DMB. Therefore these systems are well-suited to SFN operation.
The 8VSB modulation method used in North America for digital TV, specified in ATSC standard A/110, may perhaps also allow the use of SFN transmission. The system was not designed with on-channel repeaters in mind, but because the system is relatively good at ghost cancellation, it may be possible if very carefully configured. Early ATSC tuners were not very good at handling multipath propagation, but later systems have seen significant improvements. However, no trial of this technology has yet succeeded.
Through the use of virtual channel numbering, a multi-frequency network (MFN) can appear as an SFN to the viewer in ATSC.
Alternatives to using OFDM modulation in SFN self-interference cancellation would be:
- CDMA Rake receivers.
- MIMO channels (i.e. phased array antenna).
- Single-carrier modulation in combination by guard intervals and frequency domain equalization.
In a Single Frequency Network, the transmitters and receivers are usually synchronized with the others, using GPS or a signal from the main station or network as a reference clock. For example, the DVB forum specifies (TS 101 191) the use of a special marker, the Mega-frame Initialization Packet (MIP) that is inserted in the bit stream at a central ditribution point, and signals the SFN transmitters the absolute time (as read from a GPS receiver) at which this point in the data stream needs to be broadcast over the air.
