Pros and Cons of a TV SFN in US Market 75: Part 1

Interest in TV SFNs is growing because ATSC 3.0 is designed to leverage SFNs for maximum bandwidth, with Pearl and Sinclair scouting the trail to a new way of broadcasting. SFNs aren’t new. Springfield MO Fox affiliate KRBK has been operating a multi-transmitter ATSC 1.0 SFN for several years. How is 3.0 going to be different?

Station KRBK, Osage Beach MO, was the last full-power analog TV Construction Permit the FCC issued, but it never broadcast an analog signal. The station signed on the air in 2009 with a single DTV transmitter to cover its city of license, Osage Beach, MO. In 2011, the station began building out a Single Frequency Network (SFN) and grow cable carriage to expand beyond its city of license to include the much larger Springfield, MO market.

I happen to be the KRBK contract transmitter engineer who installed the first transmitter and helped install the others. I’ll skip through the drama of why the SFN seemed like a good idea, to the story of how KRBK's SFN was built and what was learned.

1.0 v 3.0 SFNs

The difference between ATSC 1.0 and ATSC 3.0 is as vast on the tuner/receiver side as it is on the encoding/modulation/transmission side. ATSC 3.0 uses a sophisticated new tuner design that leverages the different carriers on SFN streams on the same RF channel to additively improve the carrier-to-noise (C/N) ratio.

KRBK learned that an ATSC 1.0 SFN doesn’t exactly live up to its original promise of continuous coverage between all transmitter sites. With all the fundamental improvements in encoding, modulation and receiver design, ATSC 3.0 will do much better.

The ATSC 1.0 SFN exciter synchronizes the ASI input signal by GPS and can adjustably add delay. The SFN baseline delay is defined by the most delayed STL signal received among the transmitter sites. Other exciter’s delays are set to match the baseline delay. In theory, all the SFN transmitters are then synchronized and in phase. If that sounds too easy, it’s because it is.

1.0 Sweet Spots And Nulls

In fact, there are sweet spots and nulls between SFN transmitters due to the propagation delays between the separated transmitting antennas and a single TV receiving antenna. When STL delay is computed and dialed in to exciters, all signals are synchronized at a point equidistant between transmitters. Move closer to either transmitter and the signals go out of phase and interfere with each other. SFN interference becomes moot when the receiver is significantly closer to one transmitter than any others.

Illustration of a three-transmitter single frequency network (SNF). The text above describes how the signals can be phased to better serve a selected population or minimize interference.

Illustration of a three-transmitter single frequency network (SNF). The text above describes how the signals can be phased to better serve a selected population or minimize interference.

There is no specific size of the ATSC 1.0 SFN sweet spot, or universal differential threshold where every ATSC 1.0 receiver ignores a second similar signal. Typically, the ‘ignore’ level is about -20 dB, but it depends on the tuner, its age, the unique receive site, and unique receiving antenna characteristics. Thus, the minimum signal strength ratio between out-of-phase same-channel SFN signals before failure is largely defined at the viewer’s location and not necessarily always mathematically predictable. In the best case, when SFN signals arrive at the receiver at the exact same time, multiple ATSC 1.0 SFN transmitters will supplement the receiver's C/N.

The good news about SFN sweet spots is they can be moved by offsetting exciter delay. For example, delays between SFN transmitters can be tweaked to arrive in-phase in a small town closer to one transmitter than the other. The best news is that timing signals by delay offset is a digital tweak and not a transmission antenna modification.

Delay is computed by simply adding 8.2 µs/mile to allow for propagation delay from the towers to the target TV location. For example, a town covered a SFN transmitter 20 miles away and another 40 miles away can be phased in by adding a 164 µs delay (20x8.2) to the nearer transmitter. Adding that delay to the baseline delay matches the closer signal’s timing to the signal that travels 20 miles further to the same town. The bad news is that nulls move too, and they are generally less predictable due to receive site variables.

Constructive Interference

ATSC I.0 is a single carrier, vestigial sideband (VSB) system known as 8VSB, meaning it converts a binary stream into 8 levels of modulation to maximize the usable bandwidth of a 6MHz TV channel.

ATSC 3.0 is similar to Europe’S DVB-T, and Japan’s ISDB-T. All three are all coded orthogonal frequency-division multiplexing (COFDM) modulation systems that carry digital data on multiple carrier frequencies. COFDM is said to be more resistant to multipath. It’s over-the-air bandwidth performance relies on healthy a C/N ratio that deteriorates with distance.

The primary difference between multipath and SFN signals on the same channel is the delay between signals and echoes. ATSC 3.0 data collection is additive within the guard interval, and guard interval markers are data in the modulation. The guard interval markers define the point of maximum second-signal delay before it is purposely ignored.

For example, at an ATSC 3.0 receiver in an overlap area where the signals from both transmitters reach the receiver with in the guard interval windows, the data from both transmitters is combined to form a decoded signal. The improvement is sometimes referred to as SFN Gain or constructive interference.

Markers can be set to reject the longer reflected multipath echoes and Doppler effects of moving reflections but not the short, stable delay between SFN transmitter signals. When an ATSC 3.0 receiver detects similar signals within the guard interval window, they are additively processed to increase C/N. This is possible because of the multi-carrier COFDM versus the single-carrier 8-VSB.

Higher C/N ratios improve bandwidth. This is the primary motiviaton behind the new interest in ATSC 3.0 SFNs. The ATSC 3.0 SFN scenarios discussed at trade shows and TV engineering events are typically short-range systems on relatively short towers, designed to maximize C/N.

Pearl TV and Sony have developed the first ATSC 3.0 application environment, an on-screen program guide supporting both broadcast and OTT services. Click the image to read the article,

Pearl TV and Sony have developed the first ATSC 3.0 application environment, an on-screen program guide supporting both broadcast and OTT services. Click the image to read the article,"Major ATSC 3.0 Announcements Made at CES," for more information.

But, unlike cell systems, TV SFNs don’t hand off signals from one tower to another. TV SFN transmitters don’t know who is watching or their QoS. The ATSC 3.0 receiver’s equalizer monitors and controls the desirable-to-undesirable (D/U) ratio of the signals being processed, which automatically tweaks and peaks viewer QoS and QoE.

TVs Like Cell Phones?

SFNs are the opposite of the typical TV broadcaster’s shotgun distribution model of quarter-mile high antennas flooding markets with megawatts of ERP.

One of the goals of ATSC 3.0 SFN distribution is the elimination of outdoor TV receiving antennas. If fixed and mobile TVs use built in omni-directional antennas like cell phones, then traditional outdoor antenna differences such as gain, direction, height, and front-to-back ratio are eliminated, and all receivers become more equal. This notion makes signal strength and C/N ratios at receivers the responsibility of the broadcaster. In other words, it makes TV broadcasters more like cell network providers.

Got a null? Add a SFN site. That the business model that works when users are paying for gigabits and minutes. C/N is even more critical with 5G, which will increase the number of antenna sites and drive up vertical real estate prices. Whether SFNs make business sense for free, over-the-air broadcasters depends on each market, and where ATSC 3.0 and 5G eventually go.

Stand by for Part 2 of this feature. It will examine FCC Repack and SFNs, SFN site selection, build-out, SFN monitoring, and the future of KRBK's SFN.  

Thanks to Rich Redmond at GatesAir for help preparing this story.

Editor's note: Part 2 in this series on SFN installation and design will be published Monday, February 12th, 9:00 CET.

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