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SDR (Software Defined Radio) hardware

Unlike the obsolete Calypso based phones, SDR has many advantages, providing much more freedom for the implementation of mobile side GSM protocol stack. This page describes all steps required for running the higher layer (L2&3) applications (e.g. mobile or ccch_scan) on SDR hardware.

SDR PHY vs Calypso

What's so special about SDR?

  • General purpose hardware
    • i.e. not limited to any particular technology and/or protocol stack,
    • could be used as PHY for GSM, LTE, IEE802.11, GPS, Bluetooth, etc.
  • Becomes cheap and available for everyone
    • e.g. RTL-SDR, LimeSDR-Mini
  • Becomes even smaller than a credit card
    • e.g. XTRX
  • Open Source friendly
    • GNU Radio, GQRX, OpenBTS, OsmoTRX, srsLTE, etc.
    • some drivers, firmware and PCB schematics are available

Advantages over Calypso

  • No need to rely on a 'black box' DSP
  • No hardware availability problem
  • No reverse engineering required
  • No freq. band limitations

Disadvantages

  • Low output power
  • General purpose hardware
    • No screen, no keyboard,
    • No built-in SIM reader,
    • No built-in audio
    • etc.

What is it for?

  • Another hardware platform for OsmocomBB
  • 100% Open Source GSM L1 implementation
    • GSM TS 05.02 scheduling
    • GSM TS 05.03 channel coding
    • GSM TS 05.04 (de)modulation
    • GSM TS 05.10 synchronization

Supported SDR hardware

At the moment, only USRP hardware is supported via UHD driver. This limitation caused by the requirement of RX / TX time tags persistence.
We are currently working on adding support of RX / TX time tags to both Osmocom SDR Source and Sink...

The current implementation is known to work on the following devices:

  • USRP B2X0, including 'mini'
  • Fairwaves UmTRX

Software installation

The signal processing part is based on GR-GSM (https://github.com/ptrkrysik/gr-gsm) project, which is represented by a set of GNU Radio blocks for GSM. So, first of all you need to install GNU Radio with compiled-in UHD driver support.

1. UHD driver

Follow the "Installing and/or Building UHD" section of official project documentation. After the installation, make sure that your device is recognized properly:

$ uhd_find_devices
$ uhd_usrp_probe

2. GNU Radio

Despite GNU Radio is available as a binary package on some distributions, we recommend to build the latest release version from source. Please follow the corresponding building guide.

During the build configuration process, make sure you have the following components:

  • python-support
  • gr-digital
  • gr-filter
  • gr-blocks
  • gr-uhd

For binary or already installed builds, use the following command to get build info:

$ gnuradio-config-info --enabled-components

3. GR-GSM with transceiver

For now, the transmitter implementation lives in separate branches. This manual compilation from source is required. General building process and dependences are described in the project's wiki.

# Get the source code
$ git clone git://git.osmocom.org/gr-gsm
$ cd gr-gsm/
$ git checkout fixeria/trx

# Configure and compile
$ mkdir build
$ cd build
$ cmake ..
$ make

# Install
$ sudo make install
$ sudo ldconfig

The transceiver logic is represented by the following flow graph:

4. OsmocomBB

You need to compile OsmocomBB with the TRX interface support. Please refer the corresponding wiki page.

Running

First of all, please see known limitations.

Hardware preparation

You need to have two antennas connected: one for RX, another for TX. Both antennas should be chosen for a proper frequency band, otherwise the distance will be limited. Also, it's strongly recommended to use a band-pass filter for TX in order to avoid interference.

Note: There is a bug of USRP B2X0 hardware, which affects the burst transmission quality. It appears when both transmission and reception are performed of the same side of device. To mitigate this, please use an attenuator or antenna with some resistance between the active pin and ground.

Finally, connect your SDR hardware and make sure it's working.

Transceiver

Check available configuration options:

$ grgsm_trx --help

...

  TRX interface specific
   -i --remote-addr  Set remote address (default 127.0.0.1)
   -p --base-port    Set base port number (default 5700)

  Radio interface specific
   -a --device-args  Set device arguments
   -s --sample-rate  Set sample rate
   -g --rx-gain      Set RX gain (default 30)
   -G --tx-gain      Set TX gain (default 10)
      --rx-antenna   Set RX antenna (default RX2)
      --tx-antenna   Set TX antenna (default TX/RX)
      --ppm          Set frequency correction (default 0)

If you already have another transceiver runing, e.g. OsmoTRX, choose a different TRX base port. Keep in mind that a single transceiver process occupies six UDP ports at the same time.

Choose proper RX / TX antennas and corresponding gain values depending on your device. TX gain requires more attention and should be set depending on your environment and antennas used. Feel free to use a spectrum browser, such as GQRX or osmocom_fft, for measuring the proper values.

PPM value is typically measured at runtime, so no need to specify it manually.

$ grgsm_trx --rx-gain 30 --tx-gain 80
linux; GNU C++ version 4.8.5; Boost_105500; UHD_003.010.002.000-0-122bfae1

License GPLv2+: GNU GPL version 2 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

[i] Init Radio interface
-- Detected Device: B200
-- Operating over USB 3.
-- Initialize CODEC control...
-- Initialize Radio control...
-- Performing register loopback test... pass
-- Performing CODEC loopback test... pass
-- Setting master clock rate selection to 'automatic'.
-- Asking for clock rate 16.000000 MHz... 
-- Actually got clock rate 16.000000 MHz.
-- Performing timer loopback test... pass
-- Setting master clock rate selection to 'manual'.
-- Asking for clock rate 26.000000 MHz... 
-- Actually got clock rate 26.000000 MHz.
-- Performing timer loopback test... pass
-- Asking for clock rate 26.000000 MHz... OK
[i] Init CTRL interface
[i] Init complete

At this step transceiver is ready.

The trxcon application

As we don't use Calypso-based phones, the osmocon application is useless here. Instead, you need to run another application called trxcon. You should use the same TRX base port as specified to transceiver.

$ cd osmocom-bb/src/
$ host/trxcon/trxcon

License GPLv2+: GNU GPL version 2 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

<0001> l1ctl_link.c:235 Init L1CTL link (/tmp/osmocom_l2)
<0003> trx_if.c:646 Init transceiver interface
<0005> sched_trx.c:112 Init scheduler
<0000> trxcon.c:277 Init complete

See help details.

L2&3 applications

At this step you should be able to run the higher layer applications.

For example, run ccch_scan:

$ cd osmocom-bb/src/
$ cd host/layer23/src/misc/
$ ./ccch_scan -i 127.0.0.1 -a <ARFCN>

It should synchronize with a BTS on specified ARFCN, and start decoding BCCH messages.

Related pages

Project status

Known limitations (not implemented):

  • Automatic PHY delay measurement
  • AGC (Automatic Gain Control)
  • Power measurement
  • Frequency Hopping

Work in progress:

  • GAPK based audio back-end
  • TCH/H implementation

Future plans:

  • Extend supported hardware list (Soapy?)
  • PC/SC physical SIM interface
  • AMR (Adaptive Multi-Rate)

Further reading / watching

https://media.ccc.de/v/34c3-8952-running_gsm_mobile_phone_on_sdr
https://www.youtube.com/watch?v=nmqzr2-yF04

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