Re: [Discuss-gnuradio] Ettus N210 GMSK 9600

[Andy Walls]

>                              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.