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DTT single-frequency operation gathers speed in Europe

Finnish DTV operator DNA has become one of the first in the world to set up a DVB-T2 terrestrial infrastructure using a single-frequency network (SFN) configuration. Several other European operators also are working on SFN deployment, including Portugal Telecom, which has been implementing SFN across a national network comprising more than 100 transmitters. The European Union (EU) has been encouraging SFN deployment despite resistance from some member states, holding workshops on the subject, while the European Broadcast Union (EBU) has published guidelines for implementation.

SFN brings huge gains in spectral efficiency, because without it, neighboring transmitters have to transmit each channel at different frequencies to avoid interference. In practice the savings can approach an order of magnitude. Up to nine or 10 separate frequencies can be required for each channel without SFN; otherwise, quite distant transmitters can interfere with each other sufficiently to cause reception issues. This combined with the savings achieved by the analog switch-off can increase transmission capacity of a DTT network up to 50 times.

But, there are problems holding back SFN deployment, with not all countries in Europe or deploying it now and others waiting to see how it works out elsewhere. A major problem occurs at the network edge, because SFN will break down here without bilateral or multilateral arrangements between neighboring networks. This problem is amplified at national boundaries, because cross-border cooperation is required. Another objection is that because a national SFN would use the same frequency for each channel across the whole country, a multiplex would spread out within the DTT 470-862MHz range, requiring households in those states with narrowband antennas to replace them with wideband ones.

There is also the need to broadcast the same content within each channel across all transmitters at once, restricting flexibility and scope for local programming; however, it is possible to deploy SFNs on a regional basis in subnetworks. There is also the challenge of ensuring that the signals for each transmitter are synchronized sufficiently accurately in time to avoid interference, but that should be achieved with the right equipment and appropriate configuration.

In the long run, the efforts look worthwhile given the advantages, which include being able to reclaim spectrum for new applications as a result of the efficiencies gained. It is also possible to reduce the power in locations just within the coverage area of two neighboring transmitters and improve reception in some areas where it has been poor in the past.

The Finnish SFN deployment was assisted by TeamCast, a vendor of digital modulation technologies, which supplied its DVB-T2 modulators for the project after conducting tests to ensure that they complied with the DVB-T2 standard and operated efficiently in SFN mode.