[opendtv] Re: SFN considerations (was Doug is Missing the Point)
- From: Richard Hollandsworth <holl_ands@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 16 Feb 2006 23:25:31 -0800 (PST)
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.
Multiple resync retries could be attempted, but they would occur after the
equalizer has already declared a valid solution on the late arriving multipath
component, making it difficult to determine when to stick with the current
setting and when to go fish.
<holl_ands>
===================================
"Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx> wrote:By the way, here's
some interesting stuff. It turns out that this French
DTT (TNT) document
http://www.csa.fr/pdf/Rapport-GT2-Aspects_Radiofrequence_de_la_TNT.pdf
covers lots of SFN design issue quite well, both for receiver and
transmitter considerations. Much like A/111 does. They discuss both
true, synchronized SFNs, as well as on-channel repeaters. The majority
of their discussion points apply equally to COFDM and 8-VSB. But a
couple that are really almost humorous are (starting p. 22):
1. After defining what a pre-echo is, they caution that the presence of
pre-echo can disupt reception on receivers that are so-called "1st or
2nd generation." That sounds familiar.
2. They mention that various synchronization strategies are possible for
receivers, but they don't all favor pre-echo tolerance. As long as all
echoes arrive within the GI, a strategy for pre-echo tolerance devised
by Philips is to sync up on the first received signal, and treat all
other echoes as trailing echoes. The interesting point here being that
this was the essence of what Linx did, in their 4th generation demod.
Linx discussed this in a white paper that disappeared along with their
web site. Linx filtered the incoming signal in such a way that the
equalizer was presented with a strong initial pulse followed by a long
series of lower level trailing echoes, which equalizers can handle well.
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