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Re: [Discuss-gnuradio] test_corr_and_sync example

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] test_corr_and_sync example
Date: Wed, 4 Mar 2015 11:21:41 -0500

On Tue, Mar 3, 2015 at 5:30 PM, Richard Bell <address@hidden> wrote:
Hi all,

I'm trying to figure out what the test_corr_and_sync example (located .../src/gnuradio/gr-digital/examples/demod/test_corr_and_sync.grc) is showing me. I see the tags added by the correlator, but they seem to jump around a bit. If I add some timing offset with the slider, I don't see much relation to that and the time_est tag value. I can't find any correlation between the four sliders and any of the tag values.

I also see the constellation being smeared out considerably. It looks to me like the rrc_taps variable might be set incorrectly. The number of taps portion of the filter does not match the constellation modulator number of taps (5*sps vs 11*sps). Though changing this to what I believe is correct does not help the smear all that much. Shrug.

If this is working as intended, what net effect does this have on the pfb_sync and costas filters? Is it a considerable boost in sync performance?


The constellation "smearing" is partly some noise, partly some ISI, but mostly still settling issues of the loops.

But yes, the correlate_and_sync block makes a huge difference on the lock-in time of these blocks. Tracking loops aren't great when you have burst signals like this, since they go off into the weeds when there's not signal to track, so the start of each burst forces them to reacquire. The correlate_and_sync provides a good hint to the state they should be in, which gets them close, but they still need time to settle properly. Also, since the information is valid only at the correlation peak, which occurs at the end of the access code it's correlating against, but I think that the constellation might be showing the entire packet including the access code, you're seeing two parts of the packet in that display: the synchronized payload and the unsynchronized access code. The upper left-hand corner display shows you the received samples in time, and you can see some of the effects that occur there that would lead to the smearing.


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