Single Frequency Network: An Overview
Around the world, SFNs help broadcasters and network operators fill in coverage areas that are challenging to reach, whether due to terrain, distance or other interference-related issues.
Single Frequency Networks (SFNs) are generally identified as a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel. It is not considered suitable for FM radio. As the over-the-air-broadcast industry seeks to establish its place in the future of television distribution, the number of stations replanning the FM-band with the use of SFN continues to rise.
It is not difficult to figure out that SFN is more demanding from a technological point of view. However, it is considered to be an interesting broadcasting advancement. It offers the availability for both T-DAB (digital radio via terrestrial transmitters) and DVB-T (digital television via terrestrial transmitters), and it can also be used in communication systems.
Single Frequency Network (SFN) Planning
Planning for a Single Frequency Network (SFN) requires careful consideration. With a well-designed SFN, increased signal strength and reduced interference at the receiver can be expected. It also offers the same benefits to vehicles in the coverage contour, and portable devices such as mobile phones in buildings. SFN is also much more desired for DVB-H signals, especially if the SFN is built with a dense grid of small power transmitters.
As part of the effective Single Frequency Network (SFN) planning, SFNs cannot be used over arbitrarily large areas because of self-interference even without the constraints imposed by editorial requirements.
SFN network planning offers increased spectrum efficiency when planning a broadcast network. It progressively adds or changes the network to meet new needs or coverage requirements. As part of its coverage, the following parameters should be highly considered and balanced:
- Transmitter Power – TPO
- Due to lower antenna gain, SFN can be susceptible to multipath if not designed correctly
- Antenna Polarization
- Transmission Line – Line Losses
- Tower Height – HAAT & HAGL
- Modulation Parameters – Data Rate & Guard interval
Applications of Single Frequency Network (SFN)
SFN is based with Coded Orthogonal Frequency Division Multiplexing (COFDM). This powerful reception could work against signal interruptions with densification of the main signal. It also offers the following applications:
- It can be used in small and large areas.
- The extent of the area is limited by internal network interference.
- It can be used with all reception modes.
- Most applications are in relation to indoor and mobile reception.
- It can be used in a mixed configuration with MFNs, e.g.
- Main transmitters in MFN and additional fill-in transmitters in SFN mode
- Main transmitter supplemented by lower power transmitters in towns to improve indoor reception.
Benefits & Limitations
Single Frequency Networks (SFNs) offer many advantages including better coverage, less interference, less power, and higher reliability. It also offers the following advantages:
- Greater efficiency wherein a broadcaster’s available spectrum can be packed with more radio and TV channels than possible with a multi-
- frequency topology.
- Better coverage, less interference, less power, and higher reliability.
- Overcomes difficult geographical conditions that interfere with signal delivery.
- No need to re-tune when traveling through the area (mobile reception).
- More homogeneous field strength distribution for portable and mobile reception (DVB-H).
The drawbacks of an SFN are:
- feed control is required;
- network splitting is not possible.
- synchronization of the transmitting stations is required.
- the capacity can be reduced due to guard intervals.
- programming for local variations within the contour is not available.
With SFNs improving the efficiency of spectrum usage considerably, the SFN-feature gets an important advantage of OFDM systems over analog and single-carrier digital systems. As long as stations fulfill the requirements for the network coverage and observe all the important considerations, SFN usage is suitable.
Distributed Transmission System (DTS)
A Distributed Transmission System uses many strategically placed low-power broadcasting stations. It is a form of single-frequency network in which a single broadcast signal is fed via microwave, landline, or communications satellite to multiple synchronized terrestrial radio transmitter sites. A GPS signal enables synchronous operation.
A typical distributed transmission system includes a distributed transmitter adapter (DTxA) and distributed slave transmitters. New mechanism standards were defined for synchronizing transmitters emitting 8-VSB signals in accordance with the ATSC DTV Standard (A/53C) in order to support the establishment of single frequency network (SFN) transmission systems utilizing distributed transmission.
As part of the DTS planning, the synchronization of trellis coders and sync segments in slave transmitters is crucial. Now taking a different approach, it adopts a specific bright-line technical standard for determining the placement of DTS transmitters without causing more spillover than necessary to improve a station’s coverage.
For more information on SFN transmission networks, contact us at Smith & Fisher.