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Re: [Discuss-gnuradio] GMSK: GSM initial phase for a burst

From: Brian Padalino
Subject: Re: [Discuss-gnuradio] GMSK: GSM initial phase for a burst
Date: Mon, 18 Jun 2007 16:14:07 -0400

On 6/18/07, Jeffrey Karrels <address@hidden> wrote:
Another quick question.  I can now see that after the amplitude ramp
up that there is a short section of negative phase movement (.7
radians clockwise) prior to the remaining counterclockwise rotations.
Should I expect a full (-pi/2) to start off the burst? Is my sampling
rate just not high enough to catch the remaining rotation? Any other

GMSK has a certain level of ISI built into the waveform.  0.7 radians
is about a pi/2 shift (0.2228 versus 0.25?) which would be pretty darn

You can look at the inherent ISI within GMSK in a nice pretty graph in this PDF:

You may want to filter your samples down to 1 sample per symbol and
see if you get 3 dots close to each of your 4 constellation points (if
you have already compensated for any phase rotations).


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