[opendtv] Re: The "real" problem with OFDM in the U.S.
- From: Cliff Benham <cbenham@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 17 Mar 2005 22:46:43 -0500
That's way more sophisticated than what I've ever done. I was speaking
of cutting coax to exact lengths and
then moving the antenna positions for best direction and phasing. When
you are dealing with wavelengths from 6 inches to
several feet it's possible to obtain good results that way.
Tom Barry wrote:
>Cliff Benham wrote:
> > 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.
>
>That is the part I'd like to do with about 4 small indoor antennas, with
>4 leads and the blending done with a computer. It would probably be
>possible to write genetic algorithms that quickly search a space of a
>bazillion possibilities and converge on (one of) the best. But that
>requires circuitry of this sort that can be computer controlled and I
>cannot easily build that.
>
>But I have memories of being a kid playing with multiple antennas, coat
>hangers, etc. If you fiddled with it long enough and were patient there
>was usually some way to get most channels. Computers would be good at that.
>
>- Tom
>
>
>
>
>
>>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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>>
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