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

From: Jeffrey Karrels
Subject: [Discuss-gnuradio] GMSK: GSM initial phase for a burst
Date: Mon, 18 Jun 2007 14:09:46 -0400

Hello everyone.

I am looking at the frequency correction burst of a GSM signal.
According to spec the original data defines 148 bits of 0 that are
differentially encoded and GMSK modulated.  After the data goes
through the differential encoder it ends up as [-1,1,1, (142 1s),
1,1,1].  I have taken this burst and plotted I vs Q (which is
attached).  I was expecting to see the amplitude ramp up followed by a
negative pi/2 phase shift, followed by 147 positive pi/2 phase shifts,
and a ramp down. I never see the initial negative phase shift though.
Am I missing something in my sanity? Any thoughts or ideas?

Thank you for your time

Attachment: fcch2.png
Description: PNG image

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