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[Discuss-gnuradio] gmsk strangeness

From: Steven Clark
Subject: [Discuss-gnuradio] gmsk strangeness
Date: Fri, 12 Jan 2007 18:24:00 -0500

'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:

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:
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:

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


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