# GMSK Over the Air **URL:** \ **Category:** GNU Radio\ **Created:** [February 27, 2012, 9:13pm UTC](https://www.ruby-forum.com/t/gmsk-over-the-air/217312 "2012-02-27T21:13:35Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![dubstep](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/cdc98d/32.png) [@dubstep](https://www.ruby-forum.com/u/dubstep)\ **Post date:** [February 27, 2012, 9:13pm UTC](https://www.ruby-forum.com/t/gmsk-over-the-air/217312/1 "2012-02-27T21:13:35Z")
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? View this message in context: [http://old.nabble.com/GMSK-Over-the-Air-tp33402373p33402373.html](http://old.nabble.com/GMSK-Over-the-Air-tp33402373p33402373.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![justynnuff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/dbc845/32.png) [@justynnuff](https://www.ruby-forum.com/u/justynnuff)\ **Post date:** [February 28, 2012, 9:50am UTC](https://www.ruby-forum.com/t/gmsk-over-the-air/217312/2 "2012-02-28T09:50:21Z")
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 Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT – University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association