GMSK Over the Air

GNU Radio
1

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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2

On Mon, Feb 27, 2012 at 12:12:52PM -0800, justynnuff wrote:

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?

Hi justynnuff,

it should work (and I’ve tried it). I don’t have a setup right here, so
I can’t give you a working example, but here’s some input:

  • I definitely used different params (probably bt=.3 and sps=2),
    although I don’t know off the top of my head if that could cause
    trouble.
  • The standard sync was a bit flaky (e.g. I never got it working on GSM
    bursts). I had to tune pretty precisely to start with, and then timing
    sync using the M&M didn’t work out that well.

It does, however, work with the benchmark_* examples, which you should
definitely read (if you haven’t done so). It’s probably just a matter of
setting the sync params correctly.

MB

–
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin B.
Research Associate

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Phone: +49 721 608-43790
Fax: +49 721 608-46071
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