04.18.2008 12:00 AM
Increasing Interest for Single Frequency Networks
A successful rollout of broadcast mobile/handheld services depends on broadcasters’ DTV signals being available in all communities in the market. The U.S. broadcast model of a single high power transmitter on a tall tower or mountain works well in many areas, but in many other markets a large number of people live in areas shadowed from the transmitter site. This isn’t an issue for fixed receivers, as most households subscribe to cable or satellite TV. Most broadcasters concentrate their transmission facilities at one location in a market, so people in shadowed areas are often forced to buy cable or satellite services to receive most of the local TV stations.
In some areas, translators are able to provide coverage, but in a market the size of Los Angeles it would be difficult to find enough open channels for even a few stations to provide over-the-air translator coverage to an area like Santa Clarita–Valencia.
One solution is the single frequency network (SFN). Multiple synchronized transmitters operate on the same frequency, with their locations, antenna patterns, power levels and timing chosen to minimize interference between the transmitters.
At the 2008 NAB Show, Merrill Weiss, president of Merrill Weiss Group LLC, presented a paper, “Implementing an 8-Transmitter Distributed Transmission Network” that attracted a large audience and many questions. The system was deployed in southeast Pennsylvania and provides coverage including Reading, Pa., (the city of license), and Philadelphia, as well as the surrounding communities. Myron Fanton, chief engineer for RF technology at ERI, presented a paper explaining how transmit antenna characteristics affect SFN performance.
An on-channel booster may be sufficient to provide coverage in small shadowed areas. Boosters receive an off-air signal and re-transmit it on-channel. Axcera showed an on-channel booster at the NAB Show that included echo-cancellation, which allows reduced isolation between receive and transmit antennas. While the isolation in the Axcera unit isn’t as great as has been reported for DVB-T boosters, echo-cancellation performance should improve.