GR-GSM TRX

Marc Peña Segarra segarrra at gmail.com
Sun Jul 16 19:17:19 UTC 2017


Hi Vadim & Piotr!

I've managed to get ccch_scan working with a RTL-SDR using Vadim
instructions and this container I've created:
https://hub.docker.com/r/pachulo/xenial-gr-gsm-osmocombb/

With something like this to start a container:
docker run --device=/dev/bus/usb/002 -it --name test-osmocombb-trx
pachulo/xenial-gr-gsm-osmocombb

And something like this to get more consoles inside the container:
docker exec -ti test-osmocombb-trx /bin/bash

Thanks for this great addition to OsmocomBB & GR-GSM!

On Sun, Jul 16, 2017 at 7:00 PM, Vadim Yanitskiy <axilirator at gmail.com>
wrote:

> Hi Piotr,
>
> > I'm very happy to hear from you again.
>
> Glad to see you too!
>
> > It's great that you figured it out. I'm more than interested
> > to help you to push this topic forward. Will you be able to tell
> > me how to compile and run the software so I will be able to test
> > it on my side?
>
> Thanks! I definitely need to create a page on wiki about that.
> This is a brief guideline how to run OsmocomBB on SDR:
>
>   1) You will need to clone a fork of GR-GSM from GitHub:
>      https://github.com/axilirator/gr-gsm/tree/fixeria/trx
>      I am going to send a pull request, as soon as I finish
>      some things and clean up the code according to your
>      requirements.
>
>   2) Build GR-GSM as usually, and you will see a new GNURadio
>      block named 'TRX Interface' in gnuradio-companion.
>
>   3) Clone my branch of OsmocomBB:
>      git clone git://git.osmocom.org/osmocom-bb -b fixeria/sdr_phy
>
>   4) Make sure you have a fresh version of libosmocore, and build
>      OsmocomBB without firmware (we don't need it anymore):
>
>      cd osmocom-bb/src/
>      make nofirmware
>
>   5) Go to the 'apps/trx/' directory, and launch the 'grgsm_trx.py'
>      application. One will init a simple follow graph and actual
>      TRX interface.
>
>   6) Go to the 'osmocom-bb/src/host/trxcon/' directory, and run
>      the 'trxcon' application, which is used to connect OsmocomBB
>      L2&3 applications with transceiver application.
>
>      ./trxcon -d DAPP:DL1C:DSCH
>
>   7) Go to the 'osmocom-bb/src/host/layer23/src/misc/' directory
>      and run the 'ccch_scan' application, and you will see how
>      it actually works ;)
>
>      ./ccch_scan -a ARFCN -i 127.0.0.1
>
> > To answer this I will need to know more about what hardware you plan to
> > use to receive and transmit. With i.e. USRP on a hardware level you have
> > the synchronization. If you plan to use RTL-SDR + some transmitter then
> > we will probably need some way to provide synchronization at a hardware
> > level too. As for GSM Receiver - it sends bursts together with GSMTAP
> > header which contains frame number. So you can identify bursts.
>
> I think, exactly devices like USRP should be our primary hardware
> platform. I use UmTRX - USRP based board, designed exactly for mobile
> networks. At the same time, I would prefer to keep in mind further
> possibility to use the project with separate RX and TX hardware.
>
> > We will need to write some modulator for bursts. We can adapt the code
> > from osmo-trx or use portion of the code from GMSK Mod block. Using GMSK
> > Mod block might not be a good idea because of 8.25 symbols long guard
> > periods between bursts. The quater bit long shifts might be a problem.
>
> Yeah, I had a little discussion on #gnuradio IRC, and some guy
> consulted me a bit. One problem, we will probably deal with, is
> GNURadio scheduling engine and block buffers. At the same time, the
> trxcon application can send TX bursts in advance. As much frames before
> as it's required to normal TX operation. This is how OsmoBTS does.
>
> He also described an exemplary algorithm of TX operation:
>
> 1) Take in your bits (burst)
> 2) Up sample
> 3) Pulse shape with an interpolating fir filter with a Gaussian response
> 4) Feed that into a frequency modulation operation with the correct gain
> 5) Then channel filter those with an FFT filter
> 6) Use a frequency rotator of you need to freq shift the signal within
>    your DSP bandwidth
> 7) And perform a final up sampling and antialias filter before TX.
>
> What do you think?
>
> > Can you tell me what you would like to do on OsmocomBB side and what to
> > do on the GNU Radio side? I'm asking because have some idea how to make
> > a low level part of GSM TRX in GNU Radio.
>
> OsmocomBB's side does all burst transcoding operations itself,
> so we only need to modulate and send requested bursts in time.
> We will also need to perform power measurements, but let's focus
> on TX for now.
>
> > What hardware you want to use (RTL-SDR to receive and Calypso
> > phone to transmit)?
>
> No, we need to build a Calypso independent PHY for OsmocomBB.
> See my answer above.
>
> With best regards,
> Vadim Yanitskiy.
>
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