[opendtv] Re: Charles Rhodes - FMI: More than Just a Problem on the Fringe
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 15 Feb 2011 06:51:05 -0500
At 5:28 PM -0600 2/14/11, Manfredi, Albert E wrote:
Rather timely, I'd say, because this detail seems to be consistently
overlooked by those who advocate single market SFNs as a means to
allow spectrum reuse closer in. Can't have it both ways. If the big
stick has a hard time providing adequate field strength at the edges
of the market, then any SFN that provides greater signal strength at
the edges of the market will only PREVENT frequency reuse close to
that market boundary.
Once again, Bert comes up with an argument completely decoupled from
the article he has posted to support his position. The article
discusses potential interference problems from FM radio into certain
VHF TV channels using ATSC modulation. It is not clear whether this
problem would also affect COFDM based SFNs, but the real issue here
is the continued, or possibly increased use of VHF frequencies after
a second repacking of the TV spectrum.
Unfortunately VHF is part of the problem here because of the
propagation characteristics of this band. IMHO, the FCC should be
looking at other potential uses of VHF than TV.
Charles Rhodes speculates that:
"Based on the lower effective radiated power limits, which would be
imposed after repacking, and lower tower height above average
terrain, the field strength at many sites well within the community
would be, I believe, significantly reduced."
Clearly this would be the case, however, it may not be a bad thing.
The big sticks have very high power density close in - in some cases
too much power, as was seen with the FCC comparative tests of 8-VSB
and COFDM, where the front ends of the COFDM receivers were
overloaded.
No doubt there is no perfect solution - with terrestrial based
transmitters it is difficult to cover all areas in a market with the
same constant power levels. But I think the article, and Bert, are
jumping on this, when it is not clear that there is a significant
problem. It is instead a factor in the proper design of the SFN for
each market, based on the desired geographic coverage and local
terrain conditions. If anything the SFN should provide more sites
with adequate power levels, not fewer. I would also note that
emission masks can be used at the edges of a market to reduce
emissions into adjacent markets and further enhance spectral reuse.
Furthermore, any SFN that attempts to spread out low power towers
more than it should, aside from the obvious echo tolerance problems
this arrangement creates, will also create all manner of
low/marginal power density areas within the market itself.
Essentially duplicating the "fringe area" problems experienced with
big sticks throughout the market.
MANY of the problems seen with 8-VSB are not at the fringes. In fact,
multipath tends to be less of a concern at the fringes. It is the
heavy multipath close in and terrain blockage that tend to cause the
biggest problems for 8-VSB; and to a lesser extent, overlaod in areas
of very high signal strength. The unstated problem is that big stick
signals are still strong enough BEYOND the intended fringe areas of a
market to prevent spectral reuse - i.e. this requires white spaces in
adjacent markets.
Regards
Craig
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