Re: [Discuss-gnuradio] Correlate and Sync block for square signals

[Francisco Albani]

Oh... thanks for that! I wasn't aware of it.

I will try to modify it later, but after a first inspection, I see I will need to fill the preamble not only modulated by square-gaussian pulses, but also modulated by frequency, and use this block with base band signal instead of Quadrature Demod output (as I'm doing right now). This puzzles me because I will not be able to put the following PFB Clock Sync in baseband since GFSK isn't a linear modulation (can't be stated as a sum of shaped pulses) and there is no proper matched filter.

Should I use the PFB Clock Sync AFTER Quadrature Demod? Will the "time_est" tag make sense?

Many thanks!

2016-01-28 20:03 GMT-03:00 Tom Rondeau <address@hidden>:

On Thu, Jan 28, 2016 at 11:04 PM, Francisco Albani <address@hidden> wrote:

Hi to all.

I want to test if the block Correlate and Sync improves my GFSK receiver, so I started modifying ./gr-digital/examples/demod/test_corr_and_sync.grc .

1) I replaced the BPSK modulator (with rrc filter) by a Chunks to Symbols [-1,+1] followed by a Interpolating FIR with gaussian taps.

2) I replaced the matched filter taps for Correlate and Sync and Polyphase Clock Sync with the ones I think are correct BUT OBVIOUSLY ARE NOT [<-- help needed here].

3) I eliminated frequency offset and Costas Loop because I believe they are not applicable after a FM demodulation (which I did not included in this first exploration).

(Flowgraph attached)

Things I observe:

* Correlate and Sync detects preamble start, but the clock recovery looks wrong (this can be seen zooming in or in the constellation plot).

* QT GUI Time sink showing correlation does not triggers with the hardcoded value (200). I had to change it to 45.

I'm having a hard time figuring out from the c++ code and documentation the right way to design the taps to put into Correlate and Sync and PFB Clock Sync blocks.

I always find myself feeling I get contradictory clues from code, examples and documentation when choosing the taps for blocks that depend on PFB.

Can you help me?

Thanks!

Check out the test_corr_est.grc example, instead. The corr_est block was written to replace the correlate_and_sync to improve the internal performance issues as well as make it more applicable to other modulation types like GMSK and GFSK. See if that example doesn't make more sense for your situation.

Tom