Good article, from Doug Lung. Pros and cons. He didn't address the tighter
packing tradeoffs.
Bert
----------------------------------------------
http://www.tvtechnology.com/resources/0006/transmitter-sites-things-to-consider-for-atsc-30/279011
Transmitter Sites: Things to Consider for ATSC 3.0
July 26, 2016
RF Technology
By Doug Lung
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.
The significant probe penetration in these filters leads to higher electric
field densities, making them more susceptible to breakdown with ATSC 3.0's
higher PAPR and potentially reducing the maximum power they can handle compared
to ATSC 1.0.
Small and Dielectric developed a tunable waveguide filter (Fig. 1) that has
lower loss than the tunable coaxial/transitional mode cavity filters and
significantly greater power handling capability (Fig. 2). Henry Fries, vice
president of operations with Comark, told me that the company plans to use
these filters with its new Parallax transmitter.
The ATSC 3.0 COFDM signal can occupy up to 5.83 MHz of a 6 MHz channel, more
than the 5.38 MHz occupied bandwidth of an ATSC 1.0 signal. Current plans for
ATSC 3.0 in the United States do not change the emission mask or out-of-channel
emission limits-ATSC 3.0 broadcasters will have to comply with the existing
emission mask.
One way to accomplish this is to reduce the number of carriers transmitted,
reducing the occupied bandwidth to 5.51 MHz. See Table 7.1 of ATSC A/322 for
details. Another option is to use a more complex filter.
The same breakdown voltage concerns Small mentioned in his presentation should
apply to tuners in transmission lines and parts of antennas with high electric
field density. Myat has created a document that contains, among many other
things, tables and formulas for calculating the average power and peak power
handling capability of transmission lines. Download the PDF from
www.myat.com/images/stories/pdfs/Engine.pdf. The "Peak Power Rating and
Production Test Voltages" table from the document shows the peak power limit
for different sized line. Note that these values are for 1:1 VSWR and do not
include modulation. Peak and average power ratings will be reduced for
realworld conditions!
Broadcasters looking to reach indoor antennas and portable devices will be
adding elliptical polarization, increasing the amount of power in filters,
transmission lines and antennas. Making sure these components can handle higher
peak powers with ATSC 3.0 will help avoid costly burnouts.
The transmitter site changes required for ATSC 3.0 are small compared with
those required at the studio and in viewers' homes. With a bit of planning when
changing channels for the repack or upgrading facilities, the change at the
transmitter site could be as simple as loading new firmware into the exciter!
Doug Lung is vice president of Broadcast Technology, NBC/Telemundo stations. He
welcomes your comments and questions. Email him at dlung@xxxxxxxxxxxxxxx.
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:
- Using the UNSUBSCRIBE command in your user configuration settings at
FreeLists.org
- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word
unsubscribe in the subject line.
