[opendtv] Re: Seeing Ghosts on a Single Frequency Network

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
  • Date: Tue, 8 Feb 2011 19:23:24 -0600

John Shutt wrote:

> What is the advantage of using two 6 MHz wide channels in a checkerboard
> SFN vs. just using a 12 MHz wide channel and using longer guard intervals,
> slower symbol rates, and more error correction in an SFN?

SFNs have a hard time escaping the fact they are kludges, IMO. Too bad they 
were given so much hype. I think too many people ignored the fact that there 
was a price to be paid. And they conflated the fact that European countries had 
nationwide networks with the SFN concept, coming to the conclusion that it 
would be no problem to create nationwide SFNs. But back to your musings.

Not sure why you would "checkerboard" two SFNs on two different frequencies. 
You mean, for example, up and down the East Coast? Every other city uses the 
same frequency SFN? That still means you're trying to cover a market with the 
SFN. And in order to cover something that's not a straight line, I think you 
need four frequencies, whether it's marketwide SFNs or MFNs.

Creating longer GIs like you suggest helps. Towers can be spread further apart, 
behave more like big sticks. You could either ensure that the towers in a 
market are all within the GI interval, or you could spread the towers so far 
apart that the signal from towers beyond the GI interval is very attenuated. 
The latter is really not much different from deploying big sticks, as far as 
ease of reception goes.

But consider this. The big sticks in the major cities up and down the East 
Coast already do what you suggest, pretty much. A sequence of two frequencies 
repeating, certainly in VHF channels. Instead of trying to cover the area 
better with SFNs, in these markets, why not use relatively low power 
translators? It should be possible to use the same frequency of, say, 500 watt 
or perhaps 1 KW translator in parts of the Wash/Balt area and in the adjacent 
Philadelphia market. To me, that would be a lot more robust than having to tune 
the SFN interference zones where they hopefully should not matter. And there 
should be plenty of frequencies available for such low power transmitters, if 
they can be reused in each adjacent market.

Bert
 
 
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