[opendtv] Re: The "real" problem with OFDM in the U.S.
- From: Cliff Benham <cbenham@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 17 Mar 2005 21:00:54 -0500
You can "phase" two antennas so their signals "add". You also make this
"array" more directional by doing this.
They have to be solidly mounted so they can't be moved to maintain good
quality reception.
I have also been able to use such an arrangement of two antennas
pointing in different directions to "null out"
a signal on the same channel to lower co-channel interference. That is a
much trickier proposition. The phasing
is much more critical as is setting the right amount of signal level
from the second antenna, and also harder to maintian.
Tom Barry wrote:
>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
>>
>>
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>
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