Single frequency networks may be the technology that makes it possible for stations compete with wireless in reaching viewers on their mobile devices. Proponents say these booster sites can provide strong signals throughout a station’s coverage area. However it comes at a price: capital costs to implement such booster networks could hit $2 million per market.
Repeaters Integral To Next-Gen Mobile Future
In the future world of next-generation broadcasting, TV stations wishing to blanket their markets to ensure reliable mobile reception will have to supplement the signals from their big sticks with strategically placed booster stations and likely face a new capital expense approaching $2 million in each market.
While the single broadcast antenna on tall tower or building is a good start, it isn’t enough when the goal is mobile coverage, says Jay Adrick, an independent consultant and former VP of Harris Broadcast (now GatesAir).
“There is no single stick solution for mobility that will give you adequate coverage across a market in any mode of operation, OFDM or 8VSB,” he says. “That was proven by Qualcomm when they did MediaFLO. They had to put multiple transmitters in the markets they served.”
As now envisioned by Adrick and others, the booster stations would operate on the same frequency as the main transmitter and be located on the periphery of a station’s coverage contour with directional antennas pointed back at the center of the market.
To date, few TV broadcasters in the United States have deployed such a transmission strategy, known as a single frequency network, or SFN, largely because setting up an SFN to transmit using a single-carrier broadcast modulation system like today’s 8VSB is no easy feat.
Inherently, SFNs create severe multipath conditions that make it difficult for receivers to choose among competing signals.
In areas where the signals are equal in strength, zones of interference are created that prevent the receiver from locking onto any one signal.
But the work of the Advanced Television Systems Committee on the next-generation digital TV system is focused on OFDM-based modulation. OFDM, or orthogonal frequency-division multiplexing, is a multicarrier system.
“OFDM modulation makes use of single frequency networks easier,” says Merrill Weiss, founder of Merrill Weiss Group, a Metuchen, N.J., -based consulting engineering firm. By taking advantage of the guard intervals to overcome interference, an OFDM-based SFN creates transmission diversity, Adrick says.
With boosters on the edge of a station’s contour transmitting back into the center of the market, “you have signals coming from at least two different directions, maybe even three or four directions,” he says.
“You have a contour that would start out quite high in the middle,” explains Adrick. “It would go out and dip down to a point and then build back up to the point where you are at the edge of the contour and then fall off sharply after that because you are using a highly directional antenna at the booster sites, antennas with strong front-to-back ratios,” he says.
An added benefit of SFNs is improved reception in homes and other buildings. Signals coming from multiple directions increase the likelihood that there will be windows facing a direction of at least one signal source, making it easier for the signal to reach an indoor antenna.
“This will allow broadcasters to reach into the home without the outdoor antenna,” Adrick says. “So you are back to where you were in the days of analog, but delivering much better pictures and many more channels.”
Broadcasters will be required to make a capital investment in modifications to the existing big stick site as well as incur the expense of the booster sites to make an SFN a reality, says Adrick.
To modify the existing main transmission site for an OFDM-based SFN will require a new exciter, a new encoder and a different set of signal announcement servers, the future equivalent of today’s PSIP (Program and System Information Protocol) generators. Adrick estimates the expense will range from $50,000 to $300,000 and adds that this expense is separate from any costs associated with the coming FCC repack of the TV spectrum.
To ring a market with four booster stations, each with a directional panel antenna with 120-degree beam width on a leased 300- to 400-foot tower, would cost broadcasters about $1.5 million, he says.
That includes four 5 kW transmitters, typically 10 dB less than the power level of the main transmit site; four panel antennas with horizontal and vertical polarization; four transmission lines; four microwave studio-to-transmitter links; an SFN gateway that generates individually timed signals for each transmitter in the network; and SFN monitoring and system engineering.
Plus, there are the additional operating expenses, topped by the leasing of the booster towers. Of course, broadcasters who can’t find towers to lease may have to build them, adding to the capex budgets.
Rival broadcasters in a market could reduce the total outlay for booster sites by sharing towers and some technology, such as antennas and possibly STL systems, Adrick says.
“I have been singing from that songbook for 20 years, trying to get broadcasters to realize that there are economies of scale to be had by working together,” notes Weiss.
Adrick adds that it is not necessary to add all booster sites at the same time and that staging their deployment could make the capital expense easier to handle. “If broadcasters are really looking at mobility — and I believe they are because they have made it a point to push on mobility — if that is what it’s all about, that is what it is going to take to get the job done.”