Is there an initial condition to the differential encoder? Seems odd that
they wouldn't just send the 148 pi/2 phase shifts. -Clark
From: "Jeffrey Karrels" <[EMAIL PROTECTED]>
Reply-To: [EMAIL PROTECTED]
To: [EMAIL PROTECTED], discuss-gnuradio@gnu.org
Subject: [gsm] 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
Jeff
<< fcch2.png >>
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