[opendtv] Re: The "real" problem with OFDM in the U.S.
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 8 Mar 2005 16:07:10 -0500
Tom Barry wrote:
> > If you want to avoid dead zones, as we must here,
> > then adjacent markets won't be able to share
> > frequencies. Even with the small stick approach.
> > But the next market over can use those frequencies,
> > *same as now*.
>
> Is there any topography where someone located equally
> (at any distance) between two stations with the same
> power and channel is not in a dead zone?
On the same channel, I'd say no. That's why if there is
no wilderness between adjacent markets, the frequencies
of one market cannot be used in the adjacent market.
And that buffer zone, where no reception is possible on
a given frequency band, may either consist of a complete
lack of signal or a lot of co-channel interference.
I'm basically rebutting the oft-repeated notion that
SFNs or small sticks will allow greater frequency reuse.
Taken in a vacuum, that statement seems unarguable. But
taken in context of TV transmission in real markets,
where coverage must be ubiquitous, it's just not the
case.
Take VHF Channel 11. It is used in Baltimore and NYC.
If the Philadelphia market were a wilderness, then
Channel 11 could be reused in two adjacent markets.
But the fact is, Phildelphia exists. The Balt Channel
11 must serve communities up into northern MD and must
overlap with signals from the Philadelphia market. And
the NYC Channel 11 must also serve communities which
approach Phildelphia. Because people from these
communities may commute to either market. In Phildephia
itself, Channel 11 can go ahead and be a mess of
co-channel interference or low signal level.
So whether you create the Ch 11 coverage patterns with
big sticks, with small sticks in SFNs, or with a
combination of big sticks and on-channel gap fillers,
the end result is largely the same. Frequency reuse
will depend on HOW LARGE the markets are and on whether
or not adjacent markets are contiguous. As things are
now with big sticks, Channel 11 is used efficiently in
this part of the East Coast.
If you really want to use the *same* frequencies in
adjacent markets, then you must create buffer zones
where signals will be difficult or impossible to
receive. There's no logical excuse to create such
zones today, along the East Coast. Or you could
establish a network of low power translators throughout
the market, but that would require use of more
frequencies, not fewer. It just creates the same
problem on a smaller scale. That's all.
So bottom line, if you have to cover large markets
while making efficient use of frequencies, big sticks
are a reasonable approach. Especially if these can be
assisted by on-channel repearters, to improve the
signal level in challenged locations.
Bert
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