Re: [Discuss-gnuradio] Ettus N210 GMSK 9600

[Andy Walls]

Hi Tom:

On Thu, 2016-03-24 at 08:56 -0600, Tom Golden wrote:
> Feel free to ignore this request - as you've helped me quite a lot.

This is the last one I'll respond to that isn't on list.


> I'm sending packets of N bytes containing repeating <pattern> in the
> data.  Using you're model, I tapped off of the binary slicer to save
> the bits to a file.  I think I'm in a good position SNR wise now - but
> I'm not seeing any repeated patterns in the data.  Any additional sage
> wisdom?

Well, unless you pack the bits into bytes, things will be hard to see in
the file.  

Even then, if you don't use blocks to find your preamble and line things
up, then you might see your pattern shifted in the resulting bytes.

>From looking at the data, it looks like you have a preamble that is
something like 

0x00 0x00 .. 0x00 0x55 0x01 0x04 ...

The long stream of 0's could fool the square and sync coarse frequency
sync, so be mindful of that.  You might have to do a smarter, data-aided
coarse frequency sync using your received preamble compared to a
reference preamble instead.

The MSK Timing recovery block isn't converging rapidly enough on the
0x55 to get you the right bits for that part of the preamble.
You should consider using the corr_est block to detect and tag your
preamble, so the MSK timing recovery block gets reset when it sees the
tag and converges rapidly on your preamble.

The frequency deviation is higher than I would expect for GMSK at 9600
baud with a BT of 0.4, which is +/- 2400 Hz max.  Check your modulation
index is 0.5, otherwise you're GFSK instead of GMSK; in case you care
about that.

Regards,
Andy

> 
> 
> Thanks,
> -Tom
> 
> 
> 
> On Wed, Mar 23, 2016 at 6:41 PM, Tom Golden <address@hidden>
> 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
>         
>         
>         
>         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.
>                 
>                 
>                 
>         
>         
> 
>