[Discuss-gnuradio] GMSK Over the Air

[justynnuff]

Hello group,

I am helping a group of students at my University design a basic GMSK
waveform.  I have some experience in GNU Radio, but most of what I do is in
GRC only.

The setup:  I have designed a GMSK waveform that consists of a file
souce=>packet encoder=>GMSK Mod=>GMSK Demod=>packet decoder=>file source.

The files from input to output are not exactly the same.  The end of the
output file seems to be lost during the loopback.  I have speculations as to
why this is, but it's not important.

Now when I try to take this same waveform and perform GMSK transmission over
the air, nothing is received at the output of the decoder block.  
The waveform is as follows:
Tx: file source=>packet encoder=>GMSK mod=>UHD: USRP Sink (USRP 1 with an
RFX 2400)
Rx: UHD: USRP source=>GMSK Demod=>Packet decoder=>file sink.

The Problem: Although I can see the GMSK signal on the Rx end off of the
USRP, nothing is produced at the output of the packet decoder.  The BT param
on the GMSK mod block is ~1, the samples per symbol is 8, and almost
everything else is default.  Is there something I'm missing, or does this
block just not work in an over-the-air scenario?
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