Gmsk strangeness


#1

'lo all.

We are trying to use the USRP with the new gmsk demod block, and are
encountering some weirdness.
To generate our GMSK signal, we are using an Agilent signal generator.
BT=0.35, 125 ksymbols/sec, 2 samples/symbol, with a PN15 as the data
source.

We load the produced slicer.dat into Matlab and cross-correlate with the
known PN15 sequence.

Sometimes the output looks good: all our peaks are at 2^15 - 1 = 32767,
indicating that all bits are being demodded correctly,
as you can see here:
http://img472.imageshack.us/img472/2766/fullpeakscb3.jpg

Other times, we encounter “half-height” peaks, which go up to 2^14-1 =
16383, indicating that 25% of the bits are being demodded incorrectly.
You can see this here:
http://img441.imageshack.us/img441/7507/halfpeaksax7.jpg
If you zoom in on the previous image, you can see that each xcorr peak
actually consists of 2 high points, then a lull of 14 or so, then
another
high point. This pattern is always the same when the half-height
phenomena
is observed:
http://img168.imageshack.us/img168/7657/halfpeakszoomeduy7.jpg

Any idea what might be going on, or how to debug this?

-Steven


#2

Other times, we encounter “half-height” peaks, which go up to 2^14-1 =
16383, indicating that 25% of the bits are being demodded incorrectly.

Can you get phase information from those samples? If you plot IQ in
your unit circle at 1 sample per symbol, do you get a correctly
equalized waveform?

Any idea what might be going on, or how to debug this?

Could you post your samples?

Brian