Re: Correlation Estimator block threshold settings in Absolute/Dynamic mode

[Christoph Mayer]

How close to 1 did you use thresholds for the relative threshold method?
For the relative threshold method I use thresholds between 1-exp(-5)
.. 1-exp(-8).

Cheers.
Christoph

On Wed, Apr 1, 2020 at 5:58 PM Fabian Missbrenner
<address@hidden> wrote:
>
> Hi everyone,
>
> I am currently developing a wireless transceiver and try to get the
> transmission detection running at the receiving end. To help with
> detection and synchronization, a known word is prepended to every
> message. I now want to use the Correlation Estimator block to detect
> the presence of the known word on a noisy, bursty channel. The block
> computes the (magnitude squared of the) correlation of the incoming
> stream of complex baseband samples with the known word:
> https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/corr_est_cc_impl.cc#L219
>
> Attached you can find the Flowgraph I am using for testing purposes.
>
> With Threshold Method set to Absolute, once the correlation exceeds a
> user-defineable share of the auto-correlation peak of the known word
> (Threshold parameter: between 0 and 1),
> https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/corr_est_cc_impl.cc#L178,
> the known word is considered detected and a tag is added to the stream.
>
> Unfortunately, the block does not quite behave like expected. The
> threshold needs be set to a value that is off by a factor of 1000 in
> my case:
> cf. PDF attachment, slide 1.
> With a Threshold of 0.8/1000=800u, detection works reliably. But I
> don't understand where this factor of 1000 comes from.
>
> When using an AGC3 block before the Correlation Estimator (yes, I made
> sure to correlate against the normalized known word now), there is no
> correction by factor 1000 necessary anymore:
> cf. PDF attachment, slide 2.
> Here, with a threshold of 0.5, detection works reliably.
>
> With the Dynamic mode, the semantic of the Threshold parameter
> changes: Now it means a probability of false alarm:
> https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/corr_est_cc_impl.cc#L175.
> Unfortunately, regardless of configuring a Threshold close to 0 or
> close to 1, there are many false-positive detections:
> cf. PDF attachment, slides 3 and 4.
>
> I'm not quite sure if I am missing some important point about using
> the block or if the described oddities are bugs. Maybe you can help me
> out? I also saw a discussion about this block in a Github issue
> mentioning its lack of documentation:
> https://github.com/gnuradio/gnuradio/issues/1207#issuecomment-279573149
>
> Best regards
> Fabian Missbrenner