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

From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
Date: Wed, 19 Jan 2011 19:39:45 -0600
Al Limberg wrote:

> The crux of the problem is how the 8-VSB receiver combines the
> signals from a pair of 8-VSB transmitters using identical (or
> complementary) modulation.  The SFN proponents originally
> contemplated combining the signals at the antenna of a single
> 8-VSB receiver.  This can result in as much as a 3 db
> improvement in SNR of the combined received signals if they be
> in phase.  However, combining the signals at the antenna of a
> single 8-VSB receiver is also apt to cause catastrophic loss of
> reception should the received signals be in anti-phase.

Yes, but that also holds true for any other modulation, doesn't it? The trick 
has to be to avoid areas where the signals are 180 out of phase *and* of the 
same signal strength. Even with multicarrier systems, you can only wipe out so 
many carriers and still achieve reception. COFDM, used with 2/3 FEC, won't 
survive loss of anything more than 1/3 of the carriers. A 0 dB signal 180 out 
of phase will kill it too!

I thought that Charles Rhodes' graphs showed cases of separate signals, 10 usec 
and 0.5 dB apart, at a single receive antenna. In principle these could have 
come from two different towers. And yet you could see that an equalizer with 
enough taps could salvage those low spots (and did, in four of the receivers 
tested). The deepest nulls were still about 20 dB above the noise floor, it 
looks like.

> If the 8-VSB signals from a pair of 8-VSB transmitters using
> identical baseband modulating signal are to be constructively
> combined irrespective of the siting of the 8-VSB receiver, the
> 8-VSB signals must be kept separate from each other until the
> conversion to baseband where both signals have in effect been
> de-rotated to a common phase.  The two baseband signals can
> then be combined, either in a critically sampled digital regime
> or in an over-sampled digital regime that is quasi-analog in
> nature.

Very interesting twist on the concept. Strictly can't be called SFN anymore. It 
requires two receivers and cuts spectral efficiency in half, unfortunately, but 
it would be good even for legacy receivers. They could pick and choose the best 
signal source. Then again, if you use something like the 1/4 GI that some of 
the Euro SFNs use, you cut spectral efficiency too, by 1/4 in that case. SFNs 
don't come for free.

The other, two towers time sharing on the same RF channel idea, I don't see how 
that can work with legacy receivers? But I agree, it amounts to the same thing 
for spectral efficiency. Especially looks similar if you consider two or more 
broadcasters sharing these RF channels.

> The proponents of SFNs for 8-VSB signals seek to transmit
> respective 8-VSB RF signals resulting from identical baseband
> modulating signals from a pair of 8-VSB transmitters that
> transmit with 100% duty cycles using the same RF channel
> allocation.  It appears that the proponents of SFNs for 8-VSB
> signals now contemplate keeping the respective 8-VSB RF
> signals separate from each other by directional antennas or
> by different antenna polarization.

Again, very interesting. I didn't know about this new initiative. I agree with 
you on the polarization concept not being good for mobile devices, or for that 
matter, for antennas in a cramped fireplace like mine are, but I'll keep an eye 
out for this new non-SFN SFN idea.

Bert
 
 
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Follow-Ups:
- [opendtv] Re: Seeing Ghosts on a Single Frequency Network
  - From: Mark A. Aitken
[opendtv] Re: Seeing Ghosts on a Single Frequency Network

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