Single-frequency network
Encyclopedia
A single-frequency network or SFN is a broadcast network
where several transmitter
s simultaneously send the same signal over the same frequency
channel.
and AM
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
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 network
s 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 scheme
s and equalization filters.
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 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 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 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. A GPS receiver worst case scenario is able to provide +/- 1 us 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 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 using the use of a 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 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 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 and mega-frame details in.
It should be understood that the resolution of the mega-frame format is being in steps of 100 ns, where as the accuracy needs can be in the range of 1-5 us. 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 us 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 near-by 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 exists, which essentially replaces 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 changes any other aspect of the SFN system as the basic requirements can be met.
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 tuner
s 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.
Broadcast network
A broadcast network is an organization, such as a corporation or other voluntary association, that provides live television or recorded content, such as movies, newscasts, sports, Public affairs programming, and other television programs for broadcast over a group of radio stations or television...
where several transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
s simultaneously send the same signal over the same frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
channel.
Overview
Analogue FMFrequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
and AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
radio broadcast networks as well as digital broadcast networks can operate in this manner. SFNs are not generally compatible with analog television
Analog television
Analog television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal...
transmission, since the SFN results in ghosting
Ghosting (television)
In television, a ghost is a replica of the transmitted image, offset in position, that is super-imposed on top of the main image on an analogue broadcast.-Common causes:Common causes of ghosts are:...
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
Radio spectrum
Radio spectrum refers to the part of the electromagnetic spectrum corresponding to radio frequencies – that is, frequencies lower than around 300 GHz ....
, allowing a higher number of radio and TV programs in comparison to traditional multi-frequency network
Multi-frequency network
-Introduction:Data networks, such as wireless communication networks, have to trade off between services customized for a single terminal and services provided to a large number of terminals. For example, the distribution of multimedia content to a large number of resource limited portable devices ...
(MFN
MFN
MFN may refer to:* Most favoured nation* Multi-frequency network* Metromedia Fiber Network, Inc., now known as AboveNet* Milford Sound Airport, New Zealand* Music for Nations, a British record label...
) 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
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
wireless communication, for example cellular network
Cellular network
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide radio coverage over a wide geographic area...
s and wireless computer networks, is called transmitter macrodiversity
Macrodiversity
In the field of wireless communication, Macrodiversity is a kind of space diversity scheme using several receiver antennas and/or transmitter antennas for transferring the same signal...
, 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
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
. This is problematic especially in wideband
Wideband
In communications, wideband is a relative term used to describe a wide range of frequencies in a spectrum. A system is typically described as wideband if the message bandwidth significantly exceeds the channel's coherence bandwidth....
communication and high-data rate digital communications, since the fading in that case is frequency-selective (as opposed to flat fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
), and since the time spreading of the echoes may result in intersymbol interference
Intersymbol interference
In telecommunication, intersymbol interference is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable...
(ISI). Fading and ISI can be avoided by means of diversity scheme
Diversity scheme
In telecommunications, a diversity scheme refers to a method for improving the reliability of a message signal by using two or more communication channels with different characteristics. Diversity plays an important role in combatting fading and co-channel interference and avoiding error bursts...
s and equalization filters.
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 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 intervalGuard interval
In telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another. These transmissions may belong to different users or to the same user ....
between the symbols, and thus eliminate the ISI
Intersymbol interference
In telecommunication, intersymbol interference is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable...
. Although the fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
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
Forward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
code (FEC
FEC
FEC is an acronym which may refer to:*Foreign exchange certificate, a type of currency*Forward error correction, a system of error control for data transmission*Family entertainment center, a small amusement park or indoor equivalent intended for family fun...
) 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
DVB-T
DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998...
(used in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and many other areas) and ISDB-T (used in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
and Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
). OFDM is also widely used in digital radio
Digital radio
Digital radio has several meanings:1. Today the most common meaning is digital radio broadcasting technologies, such as the digital audio broadcasting system, also known as Eureka 147. In these systems, the analog audio signal is digitized into zeros and ones, compressed using formats such as...
systems, including DAB
Digital audio broadcasting
Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....
, HD Radio
HD Radio
HD Radio, which originally stood for "Hybrid Digital", is the trademark for iBiquity's in-band on-channel digital radio technology used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals...
, 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 . 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 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. A GPS receiver worst case scenario is able to provide +/- 1 us 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 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 using the use of a 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 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 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 and mega-frame details in.
It should be understood that the resolution of the mega-frame format is being in steps of 100 ns, where as the accuracy needs can be in the range of 1-5 us. 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 us 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 near-by 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 exists, which essentially replaces 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 changes any other aspect of the SFN system as the basic requirements can be met.
ATSC and 8VSB
The 8VSB8VSB
8VSB is the modulation method used for broadcast in the ATSC digital television standard. ATSC and 8VSB modulation is used primarily in North America; in contrast, the DVB-T standard uses COFDM....
modulation method used in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
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 tuner
ATSC tuner
An ATSC tuner, often called an ATSC receiver or HDTV tuner is a type of television tuner that allows reception of digital television television channels transmitted by television stations in North America, parts of Central America and South Korea that use ATSC standards...
s 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
Virtual channel
In telecommunications, a logical channel number , also known as virtual channel, is a channel designation which differs from that of the actual radio channel on which the signal travels....
numbering, a multi-frequency network
Multi-frequency network
-Introduction:Data networks, such as wireless communication networks, have to trade off between services customized for a single terminal and services provided to a large number of terminals. For example, the distribution of multimedia content to a large number of resource limited portable devices ...
(MFN
MFN
MFN may refer to:* Most favoured nation* Multi-frequency network* Metromedia Fiber Network, Inc., now known as AboveNet* Milford Sound Airport, New Zealand* Music for Nations, a British record label...
) 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 receiverRake receiverA rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several "sub-receivers" called fingers, that is, several correlators each assigned to a different multipath component...
s. - MIMOMIMOIn radio, multiple-input and multiple-output, or MIMO , is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology...
channels (i.e. phased arrayPhased arrayIn wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
antenna) - Single-carrier frequency-domain-equalization (SC-FDE), i.e. single-carrier modulation combined with guard intervals and fftFast Fourier transformA fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform and its inverse. "The FFT has been called the most important numerical algorithm of our lifetime ." There are many distinct FFT algorithms involving a wide range of mathematics, from simple...
based frequency domain equalization, or its multi-user version Single-carrier FDMASingle-carrier FDMASingle-carrier FDMA is a frequency-division multiple access scheme. Like other multiple access schemes , it deals with the assignment of multiple users to a shared communication resource...
(SC-FDMA).
See also
- Broadcast translator
- Cooperative diversityCooperative diversityCooperative diversity is a cooperative multiple antenna technique for improving or maximising total network channel capacities for any given set of bandwidths which exploits user diversity by decoding the combined signal of the relayed signal and the direct signal in wireless multihop networks...
- Macro-diversity
- Digital Video Broadcasting, ISDB-T, ATSCATSCATSC standards are a set of standards developed by the Advanced Television Systems Committee for digital television transmission over terrestrial, cable, and satellite networks....
- OFDM, guard intervalGuard intervalIn telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another. These transmissions may belong to different users or to the same user ....
External links
- 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 20th, 2009, at open access website https://sites.google.com/site/dvbsh4real/articles-1