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From: | Tom Rondeau |
Subject: | Re: [Discuss-gnuradio] Question on threshold mathematics in correlate_and_sync block |
Date: | Fri, 2 Jan 2015 09:57:02 -0500 |
On Wed, 2014-12-31 at 15:48 -0500, Andy Walls wrote:
> On Wed, 2014-12-31 at 14:00 -0500, Andy Walls wrote:
> > Hi.
> >
> > Can someone give me a brief clue about the threshold testing in the
> > correlate_and_sync block?
I found my answer in this post:
https://gnuradio.org/redmine/projects/gnuradio/wiki/Hackfest1310
"The correlation peak is tested by first observing that the width of the
correlation peak will be sps number of samples wide. So we look at
sample i and sample (i-sps). If the difference between these two symbols
is greater than the threashold we calculated in the constructor, we
declare a correlation."
That probably works for most preambles.
Unfortunately it becomes a problem with the data sequence
"101010101010..." and a rectangular pulse filter, as the absolute value
(magnitude) peaks of the correlation increase somewhat linearly as the
sequences align, and there will be a peak at every sps samples. Thus
the threshold test won't trigger as expected.
The correlation absolute value peak happens every sps samples because,
using NRZ bit mapping (0 => -1, 1 => 1), the positive correlation is
about as strong as the neighboring negative correlation that is sps
samples away.
I'm not sure if I should file a bug report for this corner case or not.
I'm writing my own correlator block based on the correlate_and_sync
block; except that it works on real, not complex, input samples. For
myself, I just use the unmodified output of the real valued correlation
filter and just set the threshold test to be relative to 0.
> >
> > Then in the work function (corr is a totally different array variable
> > here):
> >
> > // Calculate the correlation with the known symbol
> > d_filter->filter(noutput_items, in, corr);
> >
> > // Find the magnitude squared of the correlation
> > std::vector<float> corr_mag(noutput_items);
> > volk_32fc_magnitude_squared_32f(&corr_mag[0], corr, noutput_items);
> >
> > int i = d_sps;
> > while(i < noutput_items) {
> > if((corr_mag[i] - corr_mag[i-d_sps]) > d_thresh) {
> >
> > This "if" test confuses me slightly. We check to see if the value of
> > the output of the matched filtering has crossed the threshold relative
> > to one symbol previous? Why not just check relative to 0?
>
>
> An additional note: The "if" test doesn't work too well, if the preamble
> sequence is "101010101010...", since then the correlation will have
> peaks at the symbol spacing, d_sps. Maybe
>
> if((corr_mag[i] - corr_mag[i-d_sps/2]) > d_thresh) {
>
> would be better, since the correlation should sag at the half symbol?
Regards,
Andy
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