Thanks Andy!! This is great! The demo data looks good. Running my data through it doesn't generate the bits I'm looking for, but I can see the noise affecting the symbols. Tomorrow I'll retry with a better signal.
Many many thanks! Best Regards, -Tom On Wed, Mar 23, 2016 at 7:34 PM, Andy Walls <a...@silverblocksystems.net> wrote: > On Wed, 2016-03-23 at 18:41 -0600, Tom Golden wrote: > > Hi Andy, > > > > > > > Thanks so much for the link! I'm digging into the gr-ais code - but > > there's certainly a learning curve there. :) > > > > > > Thanks!! > > -Tom > > Hi Tom, > > Here's a GRC that does 9600 baud GMSK demodulation, that I whipped up > without using your data file. > > To run it with the "square and sync" coarse frequency adjustment branch > in place, you'll need the freqest block from gr-ais, and the attached > ais_frequest.xml file, since it doesn't exist in gr-ais. If you know > you're on freq, you can just disable the blocks in that branch, and > bypass the multiply that applies the coarse frequency correction. > > > So a fun(?) thing you should notice about this flowgraph's output. You > can obviously see the GMSK soft signal before timing recovery is a 4 > level signal, because ISI drags some bits closer to 0 freq deviation. > But the timing recovery block is outputting 6 levels. It's getting the > timing of some of the bits slightly wrong. That has implications for > achievable BER for the SNR. > > Regards, > Andy > > > > > > > On Wed, Mar 23, 2016 at 5:28 PM, Andy Walls > > <a...@silverblocksystems.net> wrote: > > > > > From: > > > Tom Golden > > > Subject: > > > Re: [Discuss-gnuradio] Ettus N210 > > > GMSK 9600 > > > Date: > > > Wed, 23 Mar 2016 13:14:18 -0600 > > > > > > > > > ______________________________________________________________________ > > > Here's my flow-graph along with a snapshot of the > > constellation and > > > FFT. > > > > > > > > > Thanks!! > > > -Tom > > > > Hi Tom, > > > > A couple of things: > > > > a) The polyphase resampler isn't going to work well without a > > filter > > definition in the taps field. > > > > b) The constellation sink will not display anything meaningful > > without > > sample timing synchronization; it is not useful at its current > > position > > in the flowgraph. The constellation sink also doesn't display > > anything > > useful for an FSK modulation normally; (G)MSK being an > > exception, if > > treating it like a PSK modulation vs. FSK. > > > > c) You don't have any coarse or fine frequency > > synchronization. That > > will cause you major problems, if trying to treat GMSK as a > > PSK > > modulation. It will cause you minor problems, if treating > > GMSK as an > > FSK modulation. > > > > d) Timing recovery blocks usually want a signal that has peaks > > (which > > you get by putting the signal through a matched filter), and > > those peaks > > should nominally be scaled to +/- 1.0. You don't have a > > matched filter > > or an AGC before the Clock Recovery block. > > > > e) The USRP's 0 dB gain setting is actually the USRP inserting > > the > > maximum attenuation it can (e.g. 37 dB of attenuation). That > > can kill > > your signal to noise ratio. You may want to consider adding > > "gain" as > > long as the time domain signal doesn't look clipped (sometimes > > hard to > > tell with FSK). > > > > f) You may wish to look at what Nick Foster's gr-ais does to > > demodulate > > a 9600 baud GMSK AIS signal. It will probably give you a nice > > starting > > point; just ignore the stuff about correlating to a preamble. > > https://github.com/bistromath/gr-ais > > > > If you share your datafile somewhere, I might be inspired to > > whip a > > flowgraph that works on it. :) But that could rob you of the > > learning > > process. > > > > Regards, > > Andy > > > > > > > > On Wed, Mar 23, 2016 at 1:01 PM, Marcus D. Leech > > <address@hidden> > > > wrote: > > > On 03/23/2016 02:48 PM, Tom Golden wrote: > > > Hi, > > > > > > I'm a novice gnu radio user. I'm using > > gnuradio with > > > an Ettus N210 cabled to a modem transmitting > > GMSK > > > 9600bps. This is just for a test to verify > > the modem > > > transmit bits. > > > > > > I'm having issues with resampling. The N210 > > clock > > > can't be set to a multiple of 9600, so I'm > > attempting > > > to resample. I've tried various mechanisms > > but the > > > output after resampling to 96000 is too > > noisy to > > > successfully decode bits. I've tried the > > GMSK demod > > > block as well as the combination of > > Quadrature > > > Demod->Clock Recovery MM->Binary Slicer - > > and neither > > > works. > > > > > > I've also played with the Polyphase clock > > sync but I > > > don't see any noticeable difference. Can > > anyone > > > recommend a solution? > > > > > > Thanks!! > > > -Tom > > > > > > For a first step, it would be useful for you to > > share your > > > flow-graph with the list. > > > > > > > > > > > >
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