SDR_PHY question

Vadim Yanitskiy axilirator at gmail.com
Wed Sep 6 13:20:56 UTC 2017


Hi Adrian,

> I am interested in the sdr_phy work being done currently

Great to hear that.

> I built osmo-trx from the ms branch and implemented a crude
> power measurement command. When running trxcon everything works
> fine, I get power indications for channels, and then when the
> mobile app is trying to sync to an ARFCN I start having issues.

I have never developed / tested trxcon with OsmoTRX so far.
Currently I am using GR-GSM as a L1 back-end, and the virtual
connection between OsmocomBB and OsmoBTS during development.

In general, trxcon should be able to 'speak' with OsmoTRX,
because one do use classical transceiver UDP interfaces. But
there are some CTRL commands, which aren't implemented in OsmoTRX
yet (such as ECHO and MEASURE).

> I had to change the code of trxcon in order for SCH to work,
> because the bit values were inverted (0 was 1 and viceversa).
> I am attaching the changes at the end of the message.

Sounds strange. In case of virtual Um-interface (FakeTRX) and
GR-GSM TRX all SCH messages are decoded correctly. I will look
closer to this issue, thanks anyway!

> It appears as if the deinterleaving in gsm0503_xcch_decode
> does not work... could be the order of bursts?

The order of bursts should be consistent, I mean frame-by-frame.
You need to make sure that a frame number from SCH burst is being
decoded correctly by OsmoTRX. Every burst coming from OsmoTRX
has a header with frame number, which is used by trxcon to
determine a current frame position within a multiframe.
Probably, the problem is due to incorrect / consistent
frame number values. The internal clock counter (sched_clck.c)
is only used to schedule the UL bursts transmission.

> I'd appreciate any help here. I'm also ready to share the
> changes I've made to osmo-trx/ms and trxcon.

For now, have a look at:

  - GR-GSM based transceiver:
    https://github.com/axilirator/gr-gsm/tree/fixeria/trx

  - Virtual Um-interface (brief description):

http://git.osmocom.org/osmocom-bb/tree/src/target/fake_trx/README?h=fixeria/fake_trx

  - The latest version of trxcon (use fixeria/fake_trx branch):
    http://git.osmocom.org/osmocom-bb/log/?h=fixeria/fake_trx

You can also join the development process, feel free to mail me.
I will create the wiki page as soon as possible.


With best regards,
Vadim Yanitskiy.
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