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. > > > > >
gmsk9600_test2.grc
Description: XML document
ais_freqest.xml
Description: XML document
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