Transmitter Sites: ings to Consider for ATSC .0

Many stations will replace transmitters, RF systems, line and antennas as part of the channel repack after the incentive auction and it makes sense to select gear that will work for ATSC 3.0. A move to ATSC 3.0 could be as simple as replacing the transmitter's exciter and providing an IP link from the studio, but a system designed for ATSC 1.0 alone may not support single frequency network operation, the full ATSC 3.0 channel bandwidth or the same power level as ATSC 1.0. This month I'll cover some of the things to consider when making changes at the transmitter site.

ATSC 1.0 provides support SFNs (also called distributed transmission systems) but they haven't been widely used in the United States, likely because it is difficult to avoid interference between the transmitters unless they are isolated by terrain. To make matters worse, different ATSC 1.0 receivers will handle the interference differently.

With ATSC 3.0, as I've pointed out in earlier columns, most of those problems go away with proper selection of the OFDM guard interval. However, as with ATSC 1.0, all of the ATSC 3.0 transmitters on a channel in a SFN have to transmit exactly the same signal. Optimization of the SFN requires the ATSC 3.0 signals be emitted from each transmitter at different but precise times.

This means the signal from the studio sent to the transmitters either has to contain all the data, exactly as it is to be transmitted, or that the signal from the studio has to include enough metadata to allow each of the transmitters in the SFN to create and emit exactly the same signal.

TWO APPROACHES

The first approach essentially splits the exciter between the transmitter and the studio. This requires extra bandwidth, as all of the overhead needed to create the constellation has to be added at the studio. At the NAB Show, studio transmitter link (STL) bandwidths of up to 250 Mbps were suggested for this approach.

The second approach, currently being finalized in ATSC S32, allows sending the different program and data streams, signaling information and timing information to the transmitter in a way that the exciter at the transmitter can take all these streams, add the appropriate error correction to them, create the constellation waveform and emit a signal at the correct time that matches all the other transmitters in the SFN. This standardized approach should allow an SFN using exciters from different manufacturers.

The good news is that any microwave or fiber link that supports IP transmission should work for ATSC 3.0, but the data rate required by the first approach might be for many existing microwave STL links. The ATSC 3.0 SFN standard will help solve that problem. Either approach will require additional hardware at the studio and the transmitter site.

The good news is companies are aware of this-we saw an STL using the first approach working at the NAB Show and companies are planning support for the more efficient ATSC 3.0 SFN/STL technology.

Any transmitter that can handle ATSC 1.0 should be able to transmit ATSC 3.0 with a change of exciter. It may not, however, be able to match the output power of ATSC 1.0. The reason is that ATSC 1.0, a single carrier system, has a lower peak-to-average power ratio (PAPR) than ATSC 3.0, a multicarrier OFDM system.

A simple way to check this is to look at the specifications for the transmitter. Most transmitters sold today are offered for both ATSC (8-VSB) and DVB (COFDM) use, but the power levels are not always the same.

Comark specifies the same power levels for both modulations for its PARALLAX transmitter, as does Rohde & Schwarz for its current THU9 transmitter. However, the new Doherty amplifiers R&S showed at the NAB Show have a higher power rating for ATSC, as do GatesAir's Maxiva PowerSmart transmitters. Check the datasheets for specifics and keep this in mind when specifying a replacement transmitter's output power.

The ATSC 3.0 standard includes tools for reducing PAPR, including tone-reservation, but they can have an impact on available data bandwidth. Annex M of the ATSC Proposed Standard A/322-Physical Layer Protocol describes a peak-to-average power reduction algorithm for tone reservation and a possible one for the active constellation extension (ACE) method. Find the latest version at www.atsc.org.

IMPACT ON COMPONENTS

ATSC 3.0's higher PAPR will also have an impact on the components at the output of the transmitter. Even if the average power is unchanged, the higher peak power will result in higher RF voltages, potentially leading to arcs and burn-out in RF system components like filters, transmission line and antennas.

Derek Small, senior engineer with Dielectric, outlined the power handing capability of different filter designs under ATSC 1.0 and ATSC 3.0 in his NAB Show presentation, Efficient UHF Tunable Waveguide TE10 Mode Filter. Broadcasters want tunable filters to allow them to change channels without replacing their RF system. Most tunable filters use tunable coaxial/transitional mode cavities. They are compact compared to waveguide designs, but have greater loss.

Fig. 1: A slide from Small's NAB Show presentation on Dielectric's tunable waveguide filter that has lower loss than the tunable coaxial/transitional mode cavity filters and greater power handling capability.

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