GR-GSM TRX

Piotr Krysik ptrkrysik at gmail.com
Sat Jul 15 21:29:27 UTC 2017


Hi Vadim,

I'm very happy to hear from you again.

Sorry for the long delay. I don't know why I don't see your e-mail
addressed to me in my e-mail client.


W dniu 22.06.2017 o 15:11, Vadim Yanitskiy pisze:
> Hi Piotr,
> 
> I just wrote a simple (more or less working) GNURadio block for
> connection with OsmocomBB. It is named "TRX Interface" and currently
> only capable to transform (in Osmocom bursts are followed by a bit
> different header) and send received bursts through the UDP link.
> 
(...)
> So, I have copied actual implementation, stripped out everything
> related to TCP and implemented a feature to set UDP source port.
> Now I am able to receive all DL bursts, coming from GR-GSM to my
> trxcon application.
> 
> At the moment I am working on TDMA scheduler, which is almost
> finished. It's time to start writing GR-GSM blocks for burst
> transmission. The following thoughts / questions are in my mind:

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?

>   - TX should be simpler than RX, because we don't need to detect
>     bursts and correct any errors.

Yes, from point of view of digital signal processing TX is much simpler.

>   - Both receiver and transmitter should be time synchronized.
>     I think, the SCH clock from the 'GSM Receiver' block may be
>     easily shared. Moreover, actual clock indications could be
>     forwarded to my block.

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.

>   - If I am correct, to transmit a burst, one should be converted
>     to symbols. How can we do that?
> 
>   - We cannot simply use the existing 'GMSK Mod' block, because
>     GSM uses TDMA. Right? If so, point me to the right direction.


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.

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.

What hardware you want to use (RTL-SDR to receive and Calypso phone to
transmit)?

-- 
Best Regards,
Piotr Krysik


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