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RE: [Discuss-gnuradio] A medley of questions

From: Mike Garcia
Subject: RE: [Discuss-gnuradio] A medley of questions
Date: Fri, 16 Mar 2007 20:07:33 -0700

Hi Eric,

I just joined the gnuradio discussion but I found your question about the
GMSK amplitude distortion interesting. My guess is that it's aliasing. GMSK
has good spectral efficiency but it still has spectral sidelobes that get
folding into your desired passband when you decimate. I would suggest
filtering before decimating. Even better, since your decimating by factors
of 2, you can use a half band filter that reduces the sample rate by 2 each
time. Or you could consider downsampling via a polyphase filter. I'd be
interesting in seeing the phase distortion too. Phase distortion is more of
a concern for GMSK demodulation since it's a phase modulation. Take the
phase of the input signal and the phase of the output signal and difference
the two to get the phase error. You should see higher phase distortion as
you decimate more especially if you don't filter.


You're right, I was normalizing for the purposes of plotting. Here's what
the raw complex samples look like:
+/- 2400 or so.
and here's that other plot regenerated:

signal is going in to RX at -10dBm, default gain setting.
On 3/14/07, Eric Blossom <address@hidden> wrote: 
On Wed, Mar 14, 2007 at 09:47:20PM -0400, Steven Clark wrote:
> Plots of USRP decimation woes: 
> input is GMSK waveform @ ~30ksym/sec, BT = 0.35, no noise added
> using decimation rate of 16:
> http://img339.imageshack.us/img339/4467/d16cz9.jpg 
> top plot is complex baseband, bottom plot is amplitude of top plot
> some amplitude variations noticeable, but fairly minor.
> using decimation rate of 256:
> http://img256.imageshack.us/img256/2850/d256qh2.jpg
> heavy amplitude variations.
> zoomed in version of previous plots:
> http://img490.imageshack.us/img490/6528/d256zoomedku7.jpg 
> Still confused as to what is going on here...

Steve, what's the real amplitude of the received baseband signal?
I suspect that you are normalizing to 1.  Can you regenerate the plots
without normalizing so that we've got an idea of the values coming
across the USB.


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