[Discuss-gnuradio] Constellation Modulator and group delay

[Steven Knudsen]

Hi,

In using the Constellation Modulator block from gr-digital, I noticed that tags are not adjusted to account for the group delay of the RRC filter. There was a bit of a clue in the FIR filter use case on [1].

I’ve confirmed that the Constellation Modulator block tag offset matches the group delay of the pfb_arb_resampler_ccf employed in the generic_mod_demod.py source. That is, a tag at offset zero on the input data stream remains at offset zero, but the output is delayed by the group delay. 

It seems to me that tags on the input stream should be shifted by the group delay. Or, at least, the Constellation Modulator (and other blocks like it) should provide in some way the group delay so that other blocks can use the value to compensate.

Would anyone care to comment?

Just FYI, my application involves syncing burst transmissions and I “noticed” the delay when I used the Correlation Estimator block to find the start of my packet’s header. The demodulated header data was off by the group delay because on the transmitter side various length tags used to form the packet were offset. Sigh…

Thanks for any comments or help!

steven

[1] http://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorial_Programming_Topics#Use-case-FIR-filters

Steven Knudsen, Ph.D., P.Eng.

www. techconficio.ca

www.linkedin.com/in/knudstevenknudsen

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