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## Re: [Discuss-gnuradio] Question on threshold mathematics in correlate_an

 From: Andy Walls Subject: Re: [Discuss-gnuradio] Question on threshold mathematics in correlate_and_sync block Date: Wed, 31 Dec 2014 15:48:39 -0500

```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?
>
> In the constructor:
>
>       for(size_t i=0; i < d_symbols.size(); i++)
>         corr += abs(d_symbols[i]*conj(d_symbols[i]));
>       d_thresh = 0.9*corr*corr;
>
> corr looks like the value (at offset 0) of the discrete autocorrelation
> of the matched filter.
>
> d_thresh looks like 90% of the value of the autocorrelation of the
> matched filter squared.
>
> So far this makes sense to me.
>
>
> 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

```