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

on 2007-06-18 20:10

on 2007-06-18 20:20

```
Jeffrey Karrels 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
```

on 2007-06-18 22:05

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

on 2007-06-18 22:15

On 6/18/07, Jeffrey Karrels <karrelsj@gmail.com> 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 2007-06-18 22:21

On 6/18/07, Brian Padalino <bpadalino@gmail.com> 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

on 2007-06-19 09:05

Jeffrey Karrels 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

on 2007-06-20 01:00

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 ><< fcch2.png >> _________________________________________________________________ Get a preview of Live Earth, the hottest event this summer - only on MSN http://liveearth.msn.com?source=msntaglineliveearthhm