[opendtv] Re: Broadband DTV interferers

  • From: Albert Manfredi <albert.e.manfredi@xxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Thu, 24 Jan 2008 20:46:32 -0500

Dale Kelly wrote:
>
> Craig wrote:
>> This is not spectrum set aside for TV use. It is spectrum
>> set aside to protect high powered broadcasters who are
>> using the same frequency in an adjacent market. This
>> is the main reason that the current broadcast TV
>> system is SO INEFFICIENT with respect to spectral
>> re-use.
>
> Craig,
> This is an academic argument that you circulate with
> regularity. Surely you aren't seriously suggesting that we
> scrap the billions of dollars invested in the current DTV
> transmission system.

I don't even think that's strong enough of a counter-argument. What Craig 
continues to re-forget, with every iteration of this topic, is that in order to 
cover ubiquitously large, adjacent and contiguous markets, where no excuse 
exists to introduce dead zones, you have to allow for overlap. And furthermore, 
you would need a dense mesh of towers to create any semblance of cookie-cutter 
patterns that he seems to favor so much.

No DTT system in the world has gone to such a scheme, and certainly not for 
FOTA TV. The (viewgraph-engineered) concepts advanced by Qualcomm, for Wash DC 
to NYC coverage, required a dense mesh of 30 or so towers, for a coverage area 
that was far smaller than the 4 big sticks provide now. And yet to be 
demonstrated how changing interference patterns, weather-dependent, would 
affect even that.

What I find ironic is how the 700 MHz spectrum is described as being highly 
coveted, because of the "vast distances" of coverage that it can provide. That 
is indeed a nice attribute, but especially for broadcast systems. Much less so 
by systems that depend on a high level of spectrum re-use, for support of many, 
many two-way links.

The exact quote: "They can transmit across vast distances and through 
buildings, making them highly sought after by wireless companies." Hmm. Maybe 
true about "through buildings," but "vast distances?"

Even in Berlin, where they started out with the idea of deploying an SFN, they 
use only two towers, or three for a few of the channels, and the towers are 
tall, medium-powered sticks, with spectral efficiency reduced to 16-QAM, and 
wide coverage patterns with uneven boundaries.

> "In my most recent experimental studies, I found that
> Triplets of Undesired DTV signals which are asymmetrical,
> such as channels 30, 32 & 37 produce a broad spectrum of
> noise from channel 22 to 45 inclusive. (This will be published
> in a "TV TECHNOLOGY" February issue). The point here is
> that such signals could be either DTV or from unlicensed
> devices or a mix. They all look and act the same. This kind
> of super-broadband interference may jam multiple DTV
> signals.....".
> Here Charlie Rhodes reports on yet another possible source
> of the seemingly inexplicable failure of ATSC reception in
> many locations. Such interference should be reduce with
> the NTSC shutdown but will then increase if unlicensed
> devices are authorized in these so called White Spaces.

Disturbing. I am actually amazed at how well DTT reception does work with 
adjacent high-powered NTSC stations, and adjacent DTV stations. So I've been 
hoping that most of the mysterious problems reported are caused by the 
high-powered NTSC, rather than DTV stations. This kind of introduces some more 
doubt.

(BTW, isn't Ch 37 tabboo?)

Bert

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