[opendtv] Re: SFN considerations (was Doug is Missing the Point)
- From: Doug McDonald <mcdonald@xxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 21 Feb 2006 15:01:41 -0600
Richard Hollandsworth wrote:
> Looking forward to practicing (pretending) to read en Francais.... So does
> anyone have a copy of the LINX report????
>
> BTW: I believe that the big breakthrough in Adaptive Equalizer implementation
> (whether for HF communications or DTV) was the realization that
> you needed a Double Length Equalizer, i.e. the equalizer length needed to be
> at least TWICE the maximum "expected" delay spread between
> multipath components.
>
> Sometimes (esp in urban environment), the first arriving signal can be in a
> signal fading condition and the equalizer can lock onto a strong
> delayed component (if there are any). In a classical "short" equalizer, it
> would be initialized so that this strong signal would be towards
> the beginning of the equalizer. Eventually, the early arriving signal would
> come out of the fade, resulting in a very large "pre-echo". The
> Pre-Echo capability would therefore need to be equal to the Post-Echo
> capability.
>
> You might think that you could detect when the excessive Pre-Echo came out of
> a fade and very carefully right shift everything in the
> equalizer....but that would require an special correlation detector, because
> the equalizer would only know that things are messed up and not
> know exactly what to do about it.
This can be done. Using the training signal it becomes trivial. I have actually
implemented this is simulations and it works fine, no problem. The correlation
is
very easy using hardware-implemented FFTs, and they provide **excellent**
diagnostics
about what the sate of the multipath is. All this requires is a designed
who is ready to build something other than what he learned in school.
Doug McDonald
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