Editing Single-frequency network

{{short description|Broadcast network}}
{{other uses|SFN (disambiguation)}}
A '''single-frequency network''' or '''SFN''' is a [[broadcast network]] where several [[transmitter]]s simultaneously send the same signal over the same [[frequency]] channel.

[[File:SFN model.svg|thumb|upright|'''Single Frequency Network model'''<br />Top:Multi Frequency Network<br />Bottom:Single Frequency Network]]
Analog [[amplitude modulation|AM]] and [[Frequency modulation|FM]] radio broadcast networks as well as digital broadcast networks can operate in this manner. SFNs are not generally compatible with [[analog television]] transmission, since the SFN results in [[ghosting (television)|ghosting]] due to echoes of the same signal.

A simplified form of SFN can be achieved by a low power co-channel [[Repeater#Radio repeater|repeater]], booster or [[broadcast translator]], which is utilized as a 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-[[Broadcasting|broadcast]] wireless communication, for example [[cellular network]]s and wireless computer networks, is called transmitter [[macrodiversity]], [[CDMA]] [[soft handoff]] and [[Dynamic Single Frequency Network]]s (DSFN).

SFN transmission can be considered as creating a severe form of [[multipath propagation]]. The radio receiver receives several echoes of the same signal, and the constructive or destructive [[Interference (wave propagation)|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 scheme]]s and [[Equalization (communications)|equalization filter]]s.

Transmitters, which are part of a SFN, should not be used for navigation via [[direction finding]] as the direction of signal minima or signal maxima can differ from the direction to the transmitter.

== OFDM and COFDM ==
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 [[modulator]]s 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, one 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 fading|flat]] within the narrowband sub-channel. Thus, advanced equalization filters can be avoided. A [[forward error correction]] code (FEC) can counteract some of the sub-carriers being exposed to too much fading to be correctly demodulated.

OFDM is utilized in the terrestrial [[digital TV]] broadcasting system  [[DVB-T]] (used in [[Europe]] and other regions), [[ISDB-T]] (used in [[Japan]], [[Brazil]], and the [[Philippines]]) and in [[ATSC 3.0]]. OFDM is also widely used in [[digital radio]] systems, including [[Digital audio broadcasting|DAB]], [[HD Radio]], and [[T-DMB]]. Therefore, these systems are well-suited to SFN operation.

== DVB-T SFN ==

In DVB-T a SFN functionality is described as a system in the implementation guide.<ref name=ETSITR101190>ETSI TR 101 190: [http://www.etsi.org/deliver/etsi_tr/101100_101199/101190/01.03.01_60/tr_101190v010301p.pdf ''Digital Video Broadcasting (DVB); Implementation guidelines for DVB terrestrial services; Transmission aspects'']</ref> It allows for re-transmitters, gap-filler transmitters (essentially a low-power synchronous transmitter) and use of SFN between main transmitter towers.

The DVB-T SFN uses the fact that the guard interval of the COFDM signal allows for various length of path echoes to occur is not different from that of multiple transmitters transmitting the same signal onto the same frequency. The critical parameters is that it needs to occur about in the same time and at the same frequency. The versatility of time-transfer systems such as [[GPS]] receivers (here assumed to provide PPS and 10&nbsp;MHz signals) as well as other similar systems allows for phase and frequency coordination among the transmitters. The guard interval allows for a timing budget, of which several microseconds may be allocated to time errors of the time-transfer system used.<ref name=ETSITR101190/> A GPS receiver worst-case scenario is able to provide +/- 1 μs time, well within the system needs of DVB-T SFN in typical configuration.

In order to achieve the same transmission time on all transmitters, the transmission delay in the network providing the transport to the transmitters needs to be considered. Since the delay from the originating site to the transmitter varies, a system is needed to add delay on the output side such that the signal reaches the transmitters at the same time. This is achieved by the use of special information inserted into the data stream called the Mega-frame Initialization Packet (MIP) which is inserted using a special marker in the MPEG-2 Transport Stream forming a mega-frame. The MIP is time-stamped in the SFN adapter, as measured relative the PPS signal and counted in 100 ns steps (period time of 10&nbsp;MHz) with the maximum delay (programmed into the SFN adapter) alongside. The SYNC adapter measures the MIP packet against its local variant of PPS using the 10&nbsp;MHz to measure the actual network delay and then withholding the packets until the maximum delay is achieved. The details is to be found in ETSI TR 101 190 <ref name=ETSITR101190/> and mega-frame details in ETSI TS 101 191.<ref name=ETSITS101191>ETSI TS 101 191: [http://www.etsi.org/deliver/etsi_ts/101100_101199/101191/01.04.01_60/ts_101191v010401p.pdf ''Digital Video Broadcasting (DVB); DVB mega-frame for Single Frequency Network (SFN) synchronization'']</ref>

It should be understood that the resolution of the mega-frame format is being in steps of 100 ns, whereas the accuracy needs can be in the range of 1-5 μs. The resolution is sufficient for the needed accuracy. There is no strict need for an accuracy limit as this is a network planning aspect, in which the guard-interval is being separated into system time error and path time-error. A 100 ns step represents a 30 m difference, while 1 μs represents a 300 m difference. These distances needs to be compared with the worst-case distance between transmitter towers and reflections. Also, the time accuracy relates to nearby towers in a SFN domain, since a receiver is not expected to see the signal from transmission towers being geographically far apart, so there is no accuracy requirements between these towers.

So called GPS-free solutions exist, which essentially replace GPS as the timing distribution system. Such system may provide benefit in integration with transmission system for the MPEG-2 Transport Stream. It does not change any other aspect of the SFN system as the basic requirements can be met.

==ATSC and 8VSB==
{{main article|Distributed transmission system}}
While not designed with on-channel repeaters in mind, the [[8VSB]] modulation method used in [[North America]] for digital TV is relatively good at [[phase cancellation|ghost cancellation]].  Early experiments at [[WPSU-TV]] led to an [[ATSC standard]] for SFNs, A/110.  ATSC SFNs have seen widest use in mountainous areas like [[Puerto Rico]] and [[Southern California]], but are also in use or planned in gentler terrain.<ref>{{cite web |url=http://www.rabbitears.info/oddsandends.php?request=drlist&class=DD |title = RabbitEars.Info}}</ref>

Early [[ATSC tuner]]s were not very good at handling multipath propagation, but later systems have seen significant improvements.<ref>{{cite web
|url=https://www.fcc.gov/oet/info/documents/reports/TR-05-1017-ATSC-reception-testing.pdf |title=Tests of ATSC 8-VSB Reception Performance of Consumer Digital Television Receivers Available in 2005|date=2 November 2005 |website=FCC|access-date=17 July 2023}}</ref>

Through the use of [[virtual channel]] numbering, a multi-frequency network (MFN) can appear as an SFN to the viewer in ATSC.

==Alternative modulations==

Alternatives to using OFDM modulation in SFN self-interference cancellation would be:
* [[CDMA]] [[rake receiver]]s.
* [[MIMO]] channels (i.e. [[phased array]] antenna)
* [[Single-carrier frequency-domain-equalization]] (SC-FDE), i.e. single-carrier modulation combined with guard intervals and [[fast Fourier transform|FFT]]-based frequency domain equalization, or its multi-user version [[Single-carrier FDMA]] (SC-FDMA).

== See also ==
*[[Distributed transmission system]]
*[[Broadcast translator]]
*[[Cooperative diversity]]
*[[Macro-diversity]]
*[[Multicast-Broadcast Single Frequency Network]]
*[[Digital Video Broadcasting]], [[ISDB-T]], [[ATSC]]
*[[OFDM]], [[guard interval]]
*[[Quasi-synchronous transmission]]

==References==
{{Reflist}}

==External links==
*[http://ftp.enensys.com/documents/whitePapers/ENENSYS%20Technologies%20-%20Single_frequency_network%20Overview.pdf Technical overview of Single Frequency Network]
*for an example of field measured benefits of SFN in mobile cellular urban environments and cell topologies, see Christian Le Floc’h, Regis Duval "SFN over DVB-SH manifestations at full network level (S-UMTS band radio propagation performances evaluation)", March 20, 2009, at open access website [https://sites.google.com/site/dvbsh4real/articles-1 Google Sites: Sign-in]

[[Category:Digital television]]
[[Category:Broadcast engineering]]
[[Category:Radio resource management]]