[opendtv] Re: Backward Compatibility of Robust DTV Transmissions

  • From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Tue, 7 Nov 2006 18:20:45 -0500

Convolutional byte interleaving screws this up.

Intermittent short sequences of known data don't provide adaptive equalizers
that rely on Kalman or Wiener auto-regression technique much help to open
the data-slicing eye.

Long repetitive PN sequences allow instant channel-impulse-response
computations that can pop open the eye.  Recomputation of the CIR using
long-signature analysis on a sliding window of a few thousand symbols of
ordinary data can the support equalizer tracking even during fast fades.
Auto-regression techniques are too slow.

Al
----- Original Message ----- 
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Tuesday, November 07, 2006 3:41 PM
Subject: [opendtv] Re: Backward Compatibility of Robust DTV Transmissions


> John Shutt wrote:
>
> > Once a receiver is built, how much can be done ex post facto to
> > improve the performance? With so many receivers designed around
> > "blind equalization" exclusively, I don't understand what sort
> > of "training wheels" can be added to ATSC to help them.
> >
> > I can see how one could create pseudo-training sequences ala
> > A-VSB that could help new receivers improve their performance
> > without upsetting existing hardware, but I think the best one
> > can hope for with legacy receivers is similar to the Hippocratic
> > oath, "first do no harm."
>
> Hmmm. Since blind equalization works best with a strong signal, why
> wouldn't it help these legacy receivers if the 8-VSB transmissions
> included some variable number of "bogus" segments, each of which is, for
> example, always modulated at 100 percent (i.e. all large amplitude
> symbols)?
>
> -------Quote from A/54 p. 96---------
> Tracking dynamic echoes requires tap adjustments more often than the
> training sequence is transmitted. Therefore, the prototype Grand
> Alliance receiver was designed so that once equalization is achieved,
> the equalizer switches to a decision-directed equalization mode that
> bases adaptation on data symbols throughout the frame. In this
> decision-directed equalization mode, reception errors are estimated by
> slicing the data with an 8-level slicer and subtracting it from the
> equalizer response.
>
> For fast dynamic echoes (e.g., airplane flutter) the prototype receiver
> used a blind equalization mode to aid in acquisition of the signal.
> Blind equalization models the multi-level signal as binary data signal
> plus noise, and the equalizer produces the error estimate by detecting
> the sign of the output signal and subtracting a (scaled) binary signal
> from the output to generate the error estimate.
>
> To perform the LMS algorithm, the error estimate (produced using the
> training sequence, 8-level slicer, or the binary slicer) is multiplied
> by delayed copies of the signal. The delay depends upon which tap of the
> filter is being updated. This multiplication produces a
> cross-correlation between the error signal and the data signal. The size
> of the correlation corresponds to the amplitude of the residual echo
> present at the output of the equalizer and indicates how to adjust the
> tap to reduce the error at the output.
> -------------------------------------
>
> I've noticed, for example, that in certain cases, DTT channels with a
> lot of echo take longer to show up on the screen. It seems to me that
> there must be some particular sequence of data symbols that makes the
> blind equalization process easier, i.e. more robust. And once synched
> up, the receiver tends to stay locked in, so this happy sequence of
> symbols wouldn't need to be repeated extremely frequently.
>
> Of course, it takes up some bandwidth.
>
> Bert
>
>
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