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
- Low output power
- General purpose hardware
- No screen, no keyboard,
- No built-in SIM reader,
- No built-in audio
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
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
During the build configuration process, make sure you have the following components:
For binary or already installed builds, use the following command to get build info:
$ gnuradio-config-info --enabled-components
# 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:
First of all, please see known limitations.
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.
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.
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.
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
- Extend supported hardware list (Soapy?)
- PC/SC physical SIM interface
- AMR (Adaptive Multi-Rate)