[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: Mon, 7 Mar 2005 19:00:14 -0500
Craig Birkmaier wrote:
> The coverage criteria was based on the same old
> story - market-into-market interference. It had
> almost nothing to do with equaling NTSC coverage,
> other than the reality that you still have to
> protect for interference into neighboring markets.
Correct. Which means, it had a lot to do with
coverage, and the constraints are the usual ones:
preventing interference between adjacent markets.
As always, context matters. So let's ask "as opposed
to what?"
If there are no clearly defined "guard zones" between
markets, where no signal is required, then this
interference issue will *always* emerge. Even with
small sticks in SFNs.
There are no dead zones between markets up and down
the East Coast, for example. Less sparsely populated
areas, sure. But they all belong to at least one
market, if not two markets.
It would be possible to create coverage, with some
amount of realism, with smaller, lower power sticks,
spread reasonably far apart. This requires
synchronization between towers (that no COFDM
country has adopted as a solution yet). But okay,
let's postulate such a scheme. It can be done here
as well as anywhere.
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*.
An example might be Balt/Wash, Philadelphia, NYC.
Baltimore and Washington overlap, and must do so.
Many communities belong to both markets.
The area between the Balt/Wash and Phila markets
must also be covered completely. No justification
for creating dead zones. Whether you deploy big
or small sticks, this means that you can't use
the same frequencies in Balt/Wash and Philadelphia.
And many of those communities would belong to
both Baltimore and Phildelphia markets anyway,
legitimately, which further makes the point.
So the question is, can the next major market over,
NYC, use the same frequencies as Balt/Wash?
NYC and Baltimore (and DC) use the same VHF
frequencies (Channels 2, 4, 5, 7, 11). VHF being
worst case, this shows that if you want to
create continuous coverage, big sticks and
smaller sticks don't make a huge difference in
terms of frequency planning.
It's coverage area without dead zones that creates
the problem. We have big markets to cover here,
and no wilderness areas between them.
I'll agree that a more sharply defined coverage
contour can reduce interference further away from
the boundary zones between adjacent markets, But
where it really *matters*, there is not a big
difference. And the frequency plans would not
be dramatically different. And *certainly* not if
you stick to your idea of three or four towers
only, per major market. That design wouldn't even
create a particularly sharp contour.
Bert
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