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Re: [Discuss-gnuradio] Ettus N210 GMSK 9600
From: |
Andy Walls |
Subject: |
Re: [Discuss-gnuradio] Ettus N210 GMSK 9600 |
Date: |
Wed, 23 Mar 2016 21:34:11 -0400 |
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
> <address@hidden> 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: application/xml
ais_freqest.xml
Description: application/xml
Message not available
Re: [Discuss-gnuradio] Ettus N210 GMSK 9600, Andy Walls, 2016/03/24
Re: [Discuss-gnuradio] Ettus N210 GMSK 9600, Andy Walls, 2016/03/25