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 >
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