[Discuss-gnuradio] Computationally Efficient Fractional Delay

[Paul Garver]

Is there a computationally efficient fractional delay block in GR? My application is to measure group delay in a B200 for a TDoA system as well as precise ranging. I have a flow graph which simulates transmitting an RRC-shaped BPSK PN sequence and then cross-correlating with a matched filter. This works perfectly well but I’d like to modify it to test time delay estimation with fractional sample delays. There was a previous thread [1] on this topic but it was never resolved.  I have one of the proposed solutions working (see attached flow graph, delay_test) by doing a phase adjustment in the frequency domain. However, to do fractional delays requires very large FFTs.  Even at 32ksps, it is slow. I’m looking at generating this for 25 MSPS to get more precise timing. It also appears the rotator needs an integer delay value or the output is incorrect.

Is the fractional resampler a better tool for the job?  I’ve attached a sample flow graph, frac_delay_test.grc.  Adjusting the phase offset doesn’t seem to work in my experiments, although the example_timing.py script in gr-digital/examples seems to do precisely this.

I also thought about the new corr_est_cc block, but I think with the length of the filters this could be a problem since it constructions a large filter bank.

Thanks for the help.

[1] https://lists.gnu.org/archive/html/discuss-gnuradio/2014-11/msg00139.html

delay_test.grc
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frac_delay_test.grc
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