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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