[opendtv] Charles Rhodes on SFNs
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 23 Jun 2010 15:46:47 -0500
http://www.dtg.org.uk/news/news.php?class=countries&subclass=0&id=3805
Charles Rhodes does not seem too positive on the idea of "cellular TV" with
ATSC, and will be doing some simulations and provide the results.
Two interesting graphs in the article.
One shows that the "best" 2004 vintage receivers actually did better than the
"best" 2005 vintage in lagging echo tolerance, with respect to the loudness of
the echo at least. But the 2005 graph is perfectly symmetrical for leading and
lagging echo tolerance, whereas the 2004 receiver had almost no pre-echo
tolerance. That seems odd, because the results for pre-echo seem worse than the
2002/2003 4th gen Linx receiver, which was quite good to at least -10 usec.
The second graph shows tolerance to dynamic echo, where the older receiver does
much better than the 2005 receiver. Again, odd, since at least the Samsung
Gemini chip seems to be much better than what is descibed here. I guess that
Gemini chip never got used in real products, though.
Then again, the results came from a Brazil paper to the IEEE Broadcast
Symposium, so the receivers they had were whatever they were.
He compares these with COFDM 8K 3/4 FEC and 1/16 GI, getting the expected +/-
75 usec performance and dynamic echo tolerance. And he discusses briefly peak
to average power ratio and threshold C/N differences.
But he makes some statements that he should at least develop some, or for sure
he will be misunderstood. For instance:
"It is claimed that proposing the deployment of multiple low-power transmitters
radiating the same program bitstream on the same channel simultaneously to
replace the present single high-power transmitter and its antenna on a very
tall tower would allow the same channel to operate without co-channel
interference than is possible with high-power transmitters radiating from tall
towers."
But NOT BETWEEN ADJACENT MARKETS. If (a) you are a single-market broadcasters
rather than a regional broadcaster, and (b) you want to avoid inteference zones
between markets, then you cannot reuse the same frequency in the two adjacent
markets. *If* you have ample uninhabited space for inteference zones, then it's
no problem even with big sticks.
Then he says:
"In the west, most major population centers are very far from each other so in
many cases, the same channels can be allocated to other cities. But consider
the Boston to Richmond, Va. corridor. That is an excellent example of the
congestion in Europe where SFNs are used and why DVB-T, their DTV standard,
uses COFDM."
1. Unless you deploy a Boston-to-Richmond regionwide SFN, you still cannot
reuse the same frequencies in these different markets. The case today is that
the same channels ARE used, in every other market up the coast, using the
intevening market as the interference zone.
2. *If* they even do this in Europe, it is only because they have regional and
nationwide broadcasters. However in the Rome, Berlin, and Paris SFNs, they
certainly do not reuse the same frequencies in adjacent markets. And the SFNs
are just two or three towers clustered close together, in the middle of the
market area. Which would make it impossible to reuse the same frequencies in
the adjacent market.
The FCC needs to have the WHOLE STORY explained to it, or they will for sure
misunderstand what they can expect out of SFNs.
Bert
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