[opendtv] Re: Seeing Ghosts on a Single Frequency Network

  • From: "John Shutt" <shuttj@xxxxxxxxx>
  • To: "OpenDTV" <opendtv@xxxxxxxxxxxxx>
  • Date: Thu, 10 Feb 2011 20:35:36 -0500
----- Original Message ----- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx> 



Why those extra towers where there may be adequate signal strength?

The point of the multi-tower approach is to cover a whole area with good
signal strength. With SFNs like Qualcomm's, that attempt to cover large
areas, you cannot avoid situations where several towers provide adequate
signal strength in a location, but are beyond the max distance dictated by
the GI. This will almost certainly occur here and there, as seasons or
weather conditions change. In those locations, you have to install another
tower, to overpower the interference. That's why the MediaFLO system gave
spotty coverage in so many locations that would otherwise have been
covered with the other 50 KW sticks. Look at the MD Eastern Shore, for
example. No coverage.

And that's also why European SFNs have few towers, closely spaced, so that
all towers are within the GI limit. Or the umbrella conguration in Paris
(big stick plus gap fillers).



Charlie's tests (as described in the article referred to in the subject
line,) was for a single 10 uS echo 0.5 dB down from the main signal.

According to Charlie, a 10 uS echo equates to a 6 mile difference in signal
path length between the two sources in an SFN.  So you have to be 6 miles
closer to one transmitter than the other transmitter to see a 10 uS echo.
If you are equidistant to both towers, the echo delay would be 0 uS.  Since
from that equidistant point you could go 6 miles closer to antenna A, or you
could go 6 miles closer to antenna B, a 10 uS echo tolerance gives you 12
miles of overlap between transmitters in your SFN to play with.

With two big stick transmitters of equal power, spaced 50 miles apart, the
overlap area would be from 19 miles away from transmitter A to 31 miles away
from transmitter A, which would correspond to the inverse distances from
transmitter B.

The only case I can see for having a 10 uS echo and at the same time have
that echo only 0.5 dB down from the main signal is with either your Paris
Umbrella scheme, or with an on frequency low power gap filler, where the
farther transmitter is much more powerful than the near transmitter.

With equal power transmitters, by the time you're at 19 miles away from
transmitter A, transmitter B's 10 uS echo would be many dB down from
transmitter A's signal.  I see no problem meeting the 10 uS echo tolerance,
and I don't foresee too many instances where the main and the echo would
only be 0.5 dB different in amplitude.  I know there must be a fallacy in my
thinking, so please point it out.  I feel you are over dramatizing just a
bit how closely together high power SFN transmitters would have to be.

Oh, and Charlie conveniently left out multiple echoes and complex echoes in
his ATSC testing.  The more complex the echo ensemble, the better a
multi-carrier scheme with long guard interval performs compared to a single
carrier scheme with a complex receiver equalizer.  The guard interval is
equally happy ignoring a hundred echoes as it is ignoring a single echo, as
long as the longest echo is less than the guard interval of course.  Echoes
longer than the guard interval have to be equalized out just as all echoes
have to be with a single carrier system.

And if the ultimate goal of this entire exercise was not to provide seamless
mobile service, but instead to simply provide uniform coverage to fixed
receivers, then I agree with you that an SFN isn't necessary.   In that case
a simple system of off-channel gap fillers would be adequate, simpler,
cheaper, and is in fact already in use.  Dual front end equipped receivers
would also be unnecessary, since the viewer would simply choose the channel
with the better reception and be done.  And the broadcaster wouldn't have
the added complexity of synchronizing transmitted symbols.

John




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