And here's a flowgraph with some of the recommendations above (attached). -John Malsbury
On Fri, Mar 28, 2014 at 12:12 AM, John Malsbury <[email protected]>wrote: > To expand on Martin's comments, here are some of the issues that *could* > be present: > > - Add a mult_const block or otherwise reduce the amplitude of the i/q > signal going to the usrp_sink from +/- 1.0 to 0.5 or 0.7. This ensures you > are not non-linear in the transmitter. > - tx and rx_gain -> if you have these two devices a few feet apart > (3-5 dB for both tx and rx should work) > - Frequency errors between the two USRP devices (or is this on a > single device?) Ultimately, you could include some automatic, course > frequency correction > - In my experience, the GMSK demod will lock within 100-200 symbols. > Since this is a continuous streaming flow graph driven by the random > source, transient behaviour shouldn't be an issue. You should have a > continuous stream of samples to lock to. So I don't think lock/sync time > for the clock recovery is an issue. > - Have you changed any parameters on the packet encoder/decoder blocks > from the defaults? What is the bit/symbol and samps/symb set to? What's > the samps/symbol set to on the gmsk mod and demod? > - Instead of a random source, test with a vector source and a known > pattern. > - To give a quick sanity check on your encoder/decoder config, connect > the two *without* any channel. If that doesn't work, you definitely have > some config issues. > > Also, once you're through with this experiment, you might consider using > gr-mac, which provides similar functionality. Balint and I have been > hacking on it late tonight, you can find the latest here: > > https://github.com/balint256/gr-mac > > Also, a known working version that is a bit older and less bleeding edge: > > https://github.com/jmalsbury/gr-mac > > http://conferences.sigcomm.org/sigcomm/2013/papers/srif/p31.pdf > > -John > > > On Thu, Mar 27, 2014 at 1:52 AM, Martin Braun <[email protected]>wrote: > >> On 03/26/2014 12:30 PM, Ank wrote: >> > Hello all, >> > I'm working on gmsk transmitter and receiver past 6 months , the >> problem >> > that I'm facing is in the packet decoder. >> > >> > I tried with wav file and signal source it worked good. >> > >> > my block is of this type >> > >> > >> > >> Random_source=>Packet_encoder=>gmsk_modulator=>rational_resamlper_1=>uhd_sink >> > >> > >> > >> uhd_source=>rational_resampler_2=>low_pass_filter=>gmsk_demodulator=>packet_decoder=>scope_sink/file_sink >> >> There are a lot of things that can go wrong in this setup. Maybe the >> sync is not locking fast enough, and destroying the preamble. It's hard >> for us to figure this out from a distance. >> >> Try replacing the UHDs with a channel model, and go crazy with that. See >> what happens. >> >> M >> >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> [email protected] >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> > >
gmsk_link_sim.grc
Description: application/gnuradio-grc
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