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

Great suggestion!

I have attached a couple of things:

- The IQ plot (1e6 sample rate)
- The burst data (MATLAB array)
- The da vs time plot. (1 symbol = 3.69us)

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

suggestions?

Thank you for your time

Jeff

On 6/18/07, Jeffrey K. [email protected] 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

suggestions?

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

close.

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

this PDF:

http://www.ieee802.org/16/tg1/phy/contrib/802161pc-00_11.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).

Brian

On 6/18/07, Brian P. [email protected] wrote:

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

close.

Whoops - looks like I’ve got QPSK on the brain today. Sorry about that.

Not really sure why you couldn’t see at least something close to a pi/2

shift.

Brian

Jeffrey K. wrote:

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

suggestions?

I don’t see anything at all wrong with this data.

Matt

Is there an initial condition to the differential encoder? Seems odd

that

they wouldn’t just send the 148 pi/2 phase shifts. -Clark

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

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