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A Closer Look at UmTRX

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The UmTRX is an open source hardware dual-channel SDR transceiver that is designed for use in creating highly cost-effective GSM networks.

Back in August of last year I wrote about the arrival of the low cost GSM network, made possible thanks to open source implementations of key software infrastructure. In that post I mentioned in brief the UmTRX transceiver hardware, which looked promising but had yet to go into beta testing.

In December 2012 Fairwaves — the Russian mobile network technology start-up behind UmTRX — announced the availability of a limited run of 14 sample boards, and it's now possible to order a lab package which includes v2.1 hardware plus a power supply and antennas.

Hardware specifications

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Assembling a UmTRX system at the 29th Chaos Communications Congress

So what do you get for your $1,500? Well, a solution that is similar to the USRP N software radio peripheral and with Gigabit Ethernet for connection to a host running the SDR application, albeit integrated into a single board and optimised for industrial use. More specifically:

  • Two full-duplex RF channels (ARFCN in GSM terminology)

  • Two 300MHz-3.8GHz single-chip transceivers (GSM now, and 3G/LTE capable)

  • TCXO plus GPS for frequency stability, and with external clock option

  • Thermal sensors for temperature-based calibration

  • 100mW @ 900MHz, 50mW @ 1800MHz TX power

Or in short: hardware that when combined with a modest PC running Linux and open source software can be used to create a turnkey GSM network with dual TRX and frequency stability.

Being open source hardware, the schematics, BOM and PCB layout files for UmTRX have been made freely available, along with the FPGA HDL and device driver sources.

Take a look at the design files and the eagle-eyed may notice that the same transceiver chip which is used with the bladeRF and Myriad-RF boards has been employed, the LMS6002D from Lime Microsystems. Fast becoming a favourite with affordable wideband SDR solutions, the UmTRX packs two of these along with the frequency stability etc. features required for production networks.

Software architecture

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OsmoBTS/OpenBSC architecture (source: osmocom.org)

Initially UmTRX was used together with OpenBTS and either the Asterisk or Freeswitch software PBX for connecting calls, which could be routed over SIP and “broken out” to the PSTN via telephony interface cards etc. In this configuration OpenBTS provides the air interface and implements the bare minimum of GSM, transitioning into SIP as soon as possible.

UmTRX can still be used with OpenBTS, however Fairwave's strategic software architecture is now based around OsmoBTS. It's very easy to get lost in terminology and what may seem like confusing details, and it's suffice to say that OsmoBTS is more faithful to GSM in that it implements only the BTS component of the network and relies on an external Base Station Controller (BSC).

It must be pointed out that one thing which OsmoBTS does not provide is a transceiver — in its original use this is provided by hardware DSP — and when used with UmTRX the transceiver from OpenBTS is employed. Confused? Well, what we now have is the SDR transceiver only from OpenBTS, with OsmoBTS sitting on top and providing hooks into a traditional GSM architecture.

What this means is that you get a GSM BTS that is open source from the hardware all the way up through the software stack, and which can be plugged into an existing GSM network. If you don't have an existing network you can add OpenBSC to give you a BSC and much more, and SIP integration and PSTN break-out is possible via the addition of Linux Call Router (LCR).

From lab to network operator

The UmTRX provides the hardware foundations of highly affordable GSM network solutions, with a completely open source hardware design and software stack that is free to be studied, modified and redistributed.

The relatively low cost of the bare board lowers the barrier to experimentation and operating small scale trials, and it is an incredibly useful tool for use by developers of OsmoBTS, OpenBSC, OpenBTS and associated software, and those who wish to simply learn about GSM technology.

Fairwaves are building on these foundations to create integrated commercial offerings that are packaged for use in production networks and with support. Where IP infrastructure may be used to create “profitable networks anywhere” — e.g. in developing nations and sparsely populated areas — or integration with established networks is possible via a more traditional architecture.

Andrew Back

Open source (hardware and software!) advocate, Treasurer and Director of the Free and Open Source Silicon Foundation, organiser of Wuthering Bytes technology festival and founder of the Open Source Hardware User Group.

28 Jun 2013, 15:50