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

[Clark Pope]

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" <address@hidden>
> Reply-To: address@hidden
> To: address@hidden, address@hidden
> 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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