[opendtv] Broadcaster support for SFNs

After A/110 was published, the FCC wrote FCC 05-192, a notice of
proposed rulemaking on distributed transmission systems. They ask for
opinions on how these DTS systems should be planned. Should they be used
only within a station's existing market area? Should they be used to
expand the market area?

Interesting set of comments, which can be retrieved at

http://gullfoss2.fcc.gov/prod/ecfs/comsrch_v2.cgi

by entering 05-312 in the top left space, under "proceeding."

Looks like lots of enthusiasm. I think that unless the frequency
allottments are recomputed by the FCC, the SFNs have to be limited to
existing market areas, using the existing table of distances for
single-transmitter facilities. Which is the only pratical way to deploy
non-synchronized SFNs anyway, if you want to achieve continuous,
relatively trouble-free coverage over the market area.

If you contemplate synchronizing the towers of an SFN, then, with some
difficulty, you might be able to significantly expand the coverage area.
I see two problems with increasing the coverage area with wide-area
SFNs: interference between markets and interference *inside* each
market.

Between markets, I think you'll see cases where the local, expanded
Channel X will now interfere with Channel X two markets away, where
before it did not. To avoid this problem, use translators to expand
coverage.

The other reason is interference between transmitters in a wide-area
SFN.

When SFN transmitters are synchronized, the goal is to have RF symbols
arriving in sync in areas where the power density from two adjacent
towers is about equal. But there will be many situations in which a
distant tower will provide just about as much signal strength as a
nearby tower. That is, in areas where the distant tower's signal was
supposed to be heavily masked. This could be caused by terrain, but also
for much more innocent reasons, such as location of the receiver in a
home.

In these cases, you will risk exceeding the echo tolerance range of the
receiver and get a dark screen. Yes, this applies to COFDM as well. Oh
yeah, if you use a directional antenna, you might be able to regain
signal, for that station. Sort of defeats the purpose, if reception
becomes more difficult? So the wide area SFN creates the potential for
Schubin apartment reception issues, only now with echoes that even the
LG 5th gen won't tolerate.

The FCC also asks whether LPTV stations which transmit the same
programming should be allowed to create an SFN out of their various
towers. Typically, this would mandate synchronizing the towers, because
these LPTV stations with common programming are usually spaced too far
apart to permit SFN operation unsynchronized. So those interference
problems alluded to above will occur. Aside from that, some broadcasters
say that now the LPTV stations could become something different, more
like a full power station.

I still think that a low-risk approach is to use one strong transmitter
assisted by lower power transmitters in an SFN, ideally where the lower
power transmitters are too far apart *from each other* to interfere
among each other. Or similarly, staying within the echo tolerance of
receivers, use the "umbrella" configuration the French TNT talks about.
This is how the Paris SFN is designed. In Paris, the main transmitter is
on the Eiffel Tower, and there are three smaller, much low power
transmitters N, E, and S. Berlin too has a main tall site at
Alexanderplatz, then one lower tower for most other channels, and an
even lower third tower for a couple of channels only. Always keeping
these close enough together so as not to exceed the GI limit.

Bert
 
 
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