SDR PHY status update

Vadim Yanitskiy axilirator at
Sat Jul 8 18:45:23 UTC 2017

Dear Osmocom community,

I would like to share some good news about development of SDR PHY
for OsmocomBB. In a few words: I was managed to make xCCH and SCH
decoding work, so the ccch_scan application works now!

As already pointed out, I use GNURadio and GR-GSM as a back-end
for signal processing (burst detection and demodulation). Despite
GR-GSM provides some blocks for decoding bursts into L2 packets,
there are some reasons why I don't use them:

  - First of all, I would like to keep OsmocomBB compatible with
    OsmoTRX, which can only work with raw GSM bursts. Moreover,
    one already has TX capability and better stability.

  - GR-GSM is only capable to decode xCCH and TCH/F. Other logical
    channels (like TCH/H and PDTCH) aren't supported yet.

  - GR-GSM uses decoding implementation from OpenBTS project, which
    has lower performance / code quality than implementation from
    libosmocoding. BTW: Piotr Krysik was working on this issue, and
    some work, related to libosmocoding migration, already done, but
    not finished for now.

So, we have a simple GR-GSM follow graph, which sends GSM bursts and
some corresponding info (RSSI, frame number, timeslot index) to the
application I am currently working on - trxcon. I have migrated and
a bit modified TDMA scheduler from OsmoBTS project. At the moment,
the application is capable to collect xCCH and SCH bursts, decode
them and send to higher layer applications (L2 & L3) via L1CTL link.

Regarding to L1CTL protocol, trxcon handles the following requests:


All the results of my work may be found here:

If someone would like to test, one will need:

  - GNURadio framework
  - gr-osmosdr
  - Custom GR-GSM with 'TRX Interface' block (link above)
  - fixeria/sdr_phy branch of OsmocomBB (link above)
  - SDR device supported by gr-osmosdr

Regarding to the latest requirement, UmTRX board requires external
power source / active cooling, and takes some space on my table, so
I use RTL-SDR. Sounds crazy, but OsmocomBB works on RTL-SDR ;)
One could be used until TX capabilities are implemented.

With best regards,
Vadim Yanitskiy.
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