[opendtv] Re: The "real" problem with OFDM in the U.S.
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 17 Mar 2005 20:47:54 -0500
Is it even possible to somehow synchronize two antennas such the the
signal received somewhere between them is additive? I don't understand
at all how that would work. It would seem that if you moved the
receiver antenna just a few inches (feet?) then one signal would be out
of phase with the other, and subtract.
- Tom
Manfredi, Albert E wrote:
> Craig Birkmaier wrote:
>
>
>>Yes overlaps are a necessity; this is especially
>>true for SFNs where there are multiple synchronized
>>transmitters. But overlaps between markets SHOULD
>>be avoided or at least minimized.
>
>
> I'll accept that, if it's the FCC mandating such a
> policy. That makes sense. It doesn't make sense to
> expect individual OTA broadcasters to want this of
> their *own* coverage pattern, but it makes good
> sense for OTA broadcasters to demand that the FCC
> impose such policy to protect their markets from
> flagrant intrusion. Whew. One point of possible
> agreement.
>
> Now comes the logical disconnect you keep falling
> into:
>
>
>>>Thank you. Now, if you were to deploy small sticks
>>>in an SFN for Baltimore TV stations [which requires
>>>~60 mile radius of coverage, at least in all
>>>directions except to the SW], to achieve your
>>>desired super-sharp contours, just exactly how many
>>>towers do you expect you would need to cover a 50-60
>>>mile radius with the kind of sharp coverage contours
>>>you want?
>>
>>Probably four synchronized mains around the core
>>Baltimore market, and a handful of translator/gap
>>fillers in the population pockets that are at the edges
>>of the coverage area.
>
>
> The four taller sticks need to get to most of the area,
> and would create a vague pattern. The edges would have to
> be created by an enormous number of small sticks. Because
> if you look at a map, or if you actually go to these
> places, there are no parts with zero population. The
> "pockets" of population are small clusters of homes and
> individual farms. No "pockets" in reality. So in order to
> minize overlap, you'd have to create a sharp contour
> while providing uniform coverage inside the market.
>
> The circumference of a circle with radius 60 miles is 377
> miles. So a very marginal design could use low enough
> power from your 4 main towers, to keep that signal from
> straying too far, then a series of perhaps 30+ smaller
> towers, spaced 10 miles apart. And to provide ubiquitous
> coverage, these 34+ towers would all have to be
> synchronized, *even if* they were COFDM. The reason
> being, the bulk of the area is covered by those 4 main
> towers which must be more than just one GI-equivalent
> round trip delay time apart from one another *and* from
> the smaller edge sticks. But their signal will be strong
> enough where the edge sticks are to create interference,
> unless synchronized.
>
> (The idea of creating the sharp pattern with on channel
> *repeaters* will not work well. These OCRs work well to
> fill signal strength gaps in a larger pattern, and where
> their signal drops off and is overwhelmed by the main
> signal again, before creating interference. To create
> tight patterns and ubiquitous coverage, the small sticks
> at the edge will be operating in low signal zones. They
> will have to be synchronized such that in contours of
> equal power density between two sticks and between each
> stick and the main SFN, symbols will be in phase.)
>
> This is what *all* existing DTT installations want to
> avoid. Huge, synchronized SFNs.
>
> The French don't look for sharp patterns, but they do
> want to create uniform coverage with low power. So they
> use possible tight SFNs in the major markets, then low
> power translators beyond the low power SFN coverage. So
> in terms of using frequencies per given area, you need
> more with this approach, not less.
>
> The Germans use just the main SFN and its vague pattern.
> The US, Australian, and UK DTT systems use single big
> sticks, with varying amounts of power. But vague
> coverage patterns result. These schemes are very
> similar to our own, in terms of efficient use of
> spectrum and in terms of (not) creating sharp contours
> of coverage. The difference, if any, is coverage area
> per big stick. Lower power means less area per stick and
> more sticks to cover the same area.
>
> The common denominator is that no one is creating SFNs
> with dozens of synchronized sticks, as you would require
> to fulfill your coverage ideas. That's why I hear your
> words but see no basis for them.
>
> Bert
>
>
> ----------------------------------------------------------------------
> You can UNSUBSCRIBE from the OpenDTV list in two ways:
>
> - Using the UNSUBSCRIBE command in your user configuration settings at
> FreeLists.org
>
> - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word
> unsubscribe in the subject line.
>
>
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:
- Using the UNSUBSCRIBE command in your user configuration settings at
FreeLists.org
- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word
unsubscribe in the subject line.
Other related posts:
