GMSK: GSM initial phase for a burst

GNU Radio
#1

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

#2

Jeffrey K. wrote:

Am I missing something in my sanity? Any thoughts or ideas?
Its hard to read the graph like that. Plot angle vs time or even
better, derivative of angle vs. time.

Matt

#3

Great suggestion!

I have attached a couple of things:

  1. The IQ plot (1e6 sample rate)
  2. The burst data (MATLAB array)
  3. 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

#4

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

#5

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

#6

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

#7

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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