Top of mind this week for managers and engineers alike will be hitting the FCC’s July 12 deadline for initial construction permit and repack expense reimbursement filings. But in the rush to get that done, it’s important not to lose sight of future possibilities like single frequency network operation.
Don’t Lose Sight Of SFNs In Deadline Push
With fewer than 20 ATSC 1 single frequency networks (SFNs) authorized for operation around the country, it might seem a bit off point to be thinking about them as nearly 1,000 TV stations race to complete their initial construction permit and expense reimbursement forms by the FCC’s July 12 deadline for stations in the TV spectrum repack.
However, given the expected overlap in the time between ATSC 3.0 completion and authorization and the scheduled July 3, 2020, completion deadline for the repack, failing to think about SFNs now may prove to be quite shortsighted in the future.
“The critical thing is to not paint yourself into a corner,” says Dennis Wallace, managing partner at broadcast technology consultancy Meintel, Sgrignoli & Wallace.
“With a facility you build today for your repack, you want to have the flexibility to modify it as ATSC 3.0 rolls out and SFNs come into being,” he says.
One way to maintain that flexibility is adding circular or elliptical polarization to a station’s main antenna as it’s replaced for the repack, Wallace says.
Even if the expense of adding a vertical component isn’t compensated in the FCC’s like-for-like reimbursement scheme, doing so while the new permanent repack antenna is being installed will prove to be money well spent, he says.
“Doing circular polarization helps you with signal density,” Wallace says. “That will help you have a better 1.0 service today, and lead into the possibility of doing mobile and handheld in 3.0 with that same antenna.”
Once the TV spectrum is repacked, greater signal density will help broadcasters cope with an RF environment where there is “a lot more interference than we previously had,” he says.
Looking ahead to 3.0, broadcasters with a vertical main antenna component will be better positioned to complement an SFN deployment aimed at delivering mobile TV and data across their markets, he adds.
Considering a future SFN while designing today’s repack replacement RF infrastructure may also add flexibility to the decision making process, especially as it relates to service maximization, says Merrill Weiss, owner of the broadcast technology consultancy Merrill Weiss Group.
“If you can’t maximize with a single transmitter, you might be able to maximize with an SFN, depending on being able to do interference showings,” he says. “A station with a small contour might be able to expand that in a maximization and fill in with additional SFN transmitters.”
Whether in or outside the context of the TV spectrum repack, single frequency networks can offer broadcasters several benefits.
“The thing that actually buys you the most bang for the buck in an SFN is what we call path diversity,” Wallace says. “By having multiple transmitters, you have multiple paths to the receiver.”
For instance, path diversity may mean the difference between indoor reception and an empty screen for an urban apartment dweller with no exposure to a broadcaster’s main antenna.
“No matter what side of the building you live on, presumably there is a transmitter that will be able to penetrate that particular dwelling [in an SFN deployment scenario],” he says.
According to the literature and studies examined during the development of 3.0, path diversity can account for about 6 dB of gain, Wallace says.
Another plus for SFNs is increased signal level across the entire DMA, says Wallace. “The idea is wherever you go you have 70 or 80 dBu as opposed to 30 or 40 dBu [signal-to-noise ratio],” he says. “That’s certainly a benefit.”
An SFN also can ensure less signal fading and more reliable signal delivery, Weiss adds. “Even if you increase the signal strength from a single main transmitter and you are going 100 miles, you are going to have a lot of fading,” he says. “However, if you put the transmitter 10 miles away and use a lot less power [in an SFN with multiple lower-power transmitters], you will have a lot less fading.”
Another benefit of an SFN is that the lower power transmitters of neighboring broadcasters are less likely to interfere with one another, which in turn offers broadcasters the ability to position their transmitters closer to one another and offers them the chance to cover more territory, he says.
Still another mark in favor of SFNs is their expediency in enabling ATSC 3 channel bonding to boost the reliability of content delivery, Weiss adds.
To date, SFNs have been more popular in Europe and elsewhere around the world where OFDM-based modulation schemes are employed, says Olaf Fahrenkroog, regional sales manager for Rohde & Schwarz.
“Since OFDM [is] based on multiple carriers, it is much more robust against interference … than single carrier modulation like 8VSB,” he says.
Prior to the FCC freezing new construction permit application filings in April 2013, only 19 U.S. stations had deployed 8VSB-based SFNs, says Weiss, who holds patents on critical 8VSB SFN synchronization technologies.
However, 3.0 opens the door to OFDM for U.S. broadcasters and removes obstacles they faced with 8VSB-based SFNs.
“The way the RF signal is modulated in 3.0, it is not as susceptible to multipath —multiple signals arriving at the receiver at the same moment,” Wallace says.
Rather than relying primarily on the receiver’s equalizer to lock onto the signal from one SFN transmitter and reject those from others in an ATSC 1 deployment, OFDM-based ATSC 3 provides for “redundant information” in the transmission which is used by the receiver to determine which signal to use, Wallace says.
“In OFDM, there is a very distinctive way of dealing with symbols as part of transmission frames,” says Avateq CTO Vladimir Anishchenko.
The guard interval created in this scheme “facilitates OFDM symbol detection and echo immunity” and is “the main feature that distinguishes between single carrier [8VSB ATSC 1] and OFDM approaches,” he says.
It is possible to increase the duration of the guard interval to add greater immunity, but doing so comes at the price of decreasing data-carrying capacity, he adds.
How U.S. broadcasters choose to deploy future SFNs will vary from market to market depending on factors like topography and terrain, Wallace says.
But regardless of the specifics, broadcasters deploying SFNs would do well to share single frequency network sites in their markets, says Wallace. “That is actually what the whole idea of market-wide SFNs is based upon.”
In Europe, combined RF infrastructure of multiple stations at SFN sites is common, says Luca Barello, technical and sales manager at antenna manufacturer I-Tech.
“This is the typical solution adopted in Europe,” Barello says. “The city antennas are small broadband panel antennas typically composed of 2- or 4-bay x 4-sides with a combiner filter that can host many broadcasters.”
Having adjacent channels in the same SFN locations also minimizes interference and is beneficial from an economic point of view, Weiss adds.
While the details of future SFN deployments may not be front of mind the week before construction permit and expense reimbursement filings are due at the FCC, it will pay in the long run not to ignore entirely single frequency networks during this final push to meet the deadline, Wallace says.
“Be sure to leave your options open for flexibility. That’s the main thing.”
(Editor’s note: The Pearl TV consortium of major television station groups commissioned a study of the economics of SFN deployments. For more information on that work, see “ATSC 3.0 SFNs Could Offer Multiple Benefits.”)