First of all, we need to design a signal processing algorithm. The GNU Radio package contains a frontend graphical interface designated for design signal processing flow graphs that may ease our work. It could be found in Application Center with GRC name, which means GNU Radio Companion. GRC automatically creates python programs for the required SDR blocks.

Let’s start with adding of USRP Source from UHD tab to the workspace of GRC. USRP Source is the abstraction layer that allows communication with a hardware. It produces digital signal which will be consumed by the next blocks in our flow graph. The source block tells our SDR to turn on receive mode.

The next block we are going to add is QT GUI FFT Sink under Instrumentation tab. This one helps to visualize the frequency components. To connect the blocks, just click on the «out» of the Source and then on the «in» of the FFT Sink. USRP Source title changed color from red to black, which means that output properly connected with input of the next block. If QT GUI FFT Sink title is still red, then probably we have WX GUI enabled in Generate Options (top block). If we change that option to the QT, the FFT Sink will also become black. QT (http://www.qt.io/) and WX (https://www.wxwidgets.org/) just different libraries with graphical user interface elements.

It’s necessary to increase sample rate in samp_rate variable to be able to register the GSM signal. 1 million would be enough. Let us also create another variable center_freq to be able to change the central frequency in one place. Now we can set center_freq in both blocks and change it in Variable block if needed.

Our lab Base Transceiver Station (BTS) transmits downlink signals at 949.8 MHz and receives uplink signals at 904.8 MHz. If you don’t know exactly GSM frequencies transmitted near you, you may scan them using GNU Radio uhd_fft tool or software like gqrx (http://gqrx.dk/).

We will listen at 949.8 MHz first to catch some broadcast channel signals.

Push «Execute the flow graph» button to catch some radio waves!

Here it is! The downlink GSM signal at 949.8 MHz.

We will use Ubuntu 14.04 for our purposes. Basically, it’s possible to use any Ubuntu system, but we tested everything on 14.04 and guarantee that it will work fine. UHD library built with loadable modules support could be downloaded from Josh Blum’s PPA. Thus, we need to open Terminal and enter:

$ sudo apt-get install python-software-properties
$ sudo add-apt-repository ppa:guruofquality/pothos

Python-software-properties provides the add-apt-repository binary, which is necessary to install a new ppa (Personal Package Archive, is a collection of software not included in Ubuntu by default, https://help.launchpad.net/Packaging/PPA) easily. Pothos repository contains a proper UHD package for UmTRX. Update the cache of the Software Center repository:

$ sudo apt-get update

To install UHD driver from that repository, we should open Ubuntu Software Center. From drop-down menu of All Software button choose Pothos and scroll down until «uhd» package appears. That’s the one we need to install.

It’s also possible follow the standard procedure described in the UHD build guide to build it from the source (http://files.ettus.com/manual/page_build_guide.html).

Once UHD has been installed the UmTRX module must be built and installed over it. We have just installed Ubuntu 14.04 so some necessary software for building UmTRX module is abscent. It’s easy to get, though!

$ sudo apt-get install git
$ sudo apt-get install libboost-all-dev
$ sudo apt-get install build-essential
$ sudo apt-get install cmake

Git is a widely used version control system for software development. Boost is a set of libraries for the C++ programming language that provide support for tasks and structures such as linear algebra, pseudorandom number generation, multithreading image processing, regular expressions, and unit testing. The build-essentials is a reference for all the packages needed to compile a debian package. It generally includes the gcc/g++ compilers an libraries and some other utilities.

We are ready to install UmTRX module right now:

$ git clone https://github.com/fairwaves/UHD-Fairwaves.git
$ cd UHD-Fairwaves/host
$ mkdir build
$ cd build
$ cmake ../
$ make

In this part we cloned Fairwaves repository to the local folder, created a folder for the installation and built the module. To install the driver with a package:

$ cpack
$ sudo dpkg -i umtrx_*.deb

We highly recommend to use Josh Blum’s UHD driver as we based the development of UmTRX driver on it.

It is suitable moment to setup and boot UmTRX board to ensure if the installation went correctly.

The UmTRX Lab Package includes a power supply, GPS antenna and pigtail wires for GSM antennas. The user have to take care of assembly and cooling. A 3-pin mains lead is required.

Cooling is necessary and the heat generated must be dissipated in a stable manner in order to avoid permanent damage of the UmTRX. Cooling using fans is easy to implement but is mostly suited to indoor use. Two fans may be connected via the connectors X17 (FAN1) and X18 (FAN2). FAN1 is constantly powered and FAN2 is automatically switched on/off by U35 (MAX6665ASA45) when the temperature is around 45°C. The maximum DC current of FAN2 must be less than 400mA.

TX and RX antennas should be selected according to the application. Damage may occur if UmTRX is set to transmit without a suitable 50 ohm load connected! We will use antennas from the box.

As we plugged power and cooling, we need to connect our board to computer via ethernet cable directly or via switch. For now, UmTRX does not support 100M Ethernet and it will not be recognized if your computer does not support gigabit Ethernet. Though, if your PC ethernet port supports only 100 Mbit/s, you may use 1GBit/s network switch. By default UmTRX has a static IP address 192.168.10.2/24, so it is recommended to set your computer’s IP address to 192.168.10.3/24.

It’s easy to check, if UmTRX and UHD were installed properly:

$ uhd_find_devices

If the result was like that, we made everything right:

We may also probe the properties of the attached device:

$ uhd_usrp_probe 192.168.10.2

The output of the command shows different properties of our UmTRX, such as driver version and channel settings.

The next step is installation of GNU Radio package into our Ubuntu system. We recommend to use build-gnuradio script as it would allow to install the latest version of the package and not to overwrite our existing UHD driver. Create a folder in your /home to avoid any issues with file permissions.

$ wget http://www.sbrac.org/files/build-gnuradio && chmod a+x ./build-gnuradio && ./build-gnuradio prereqs gitfetch gnuradio_build

We are using additional parameters prereqs, gitfetch and gnuradio_build to check if our prerequisite software is installed (and install it if necessary), copy the installation files to created folder and build GNU Radio only, without UHD driver. The process takes about 2 hours, so please be patient.

To have more options in experiments with SDR, it’s helpful to install Osmocom GNU Radio Blocks (http://sdr.osmocom.org/trac/wiki/GrOsmoSDR). By using the OsmoSDR block we can take advantage of a common software API in our application(s) independent of the underlying radio hardware.

$ git clone git://git.osmocom.org/gr-osmosdr
$ cd gr-osmosdr
$ mkdir build
$ cd build
$ cmake ../
$ make
$ sudo make install
$ sudo ldconfig

So, the script finished it’s work and we are ready to have some fun with GNU Radio!

Software Defined Radio is a radio system which performs the required signal processing in software instead of using dedicated integrated circuits in hardware. The advantage of that approach is that since software can be easily replaced in the radio system, the same hardware can be used to create many kinds of equipment for many different radio standards. Therefore, one SDR can be used for a variety of applications. SDR is a technology that makes IT and Telecommunications closer to each other.

Basically, our system should consist of two parts: computer with software and SDR transceiver. There are plenty of SDR transceivers: UmTRX, USRP, HackRF, BladeRF, etc. Which one to choose?
As we are going to work with GSM, it would be wise to choose SDR that was created with consideration of GSM specifications. There is such SDR solution on the market. It is named UmTRX and was developed in Fairwaves.

UmTRX is based on the open source hardware (Altium Designer schematic and board layout files are made available under the Creative Commons Attribution-ShareAlike 3.0 Unported license) and can be deployed using open source software (for example, GNU Radio, Osmocom family of open source projects), therefore it benefits from being part of an ever-growing ecosystem of complementary hardware and software for mobile communications. UmTRX was developed to be used as a transceiver for OpenBTS and OsmoBTS GSM base stations considering GSM specifications, but due to its SDR nature it could be used for many other applications as well. It operation based on the Ettus UHD driver. UmTRX has its own GPS clock (26 MHz Voltage Controlled Temperature Compensated Crystal Oscillator) and industrial grade components (such as Lime Microsystems Multi-band Multi-standard Transceiver with Integrated Dual DACs and ADCs LMS6002D). UmTRX has two independent channels, so one can operate on two different frequencies. As it works in different environment conditions all over the world, like hot climate of Africa and high humidity of tropical islands, we can be sure it will work on our table.

As we decided our hardware option, we need to choose our first application. Let’s try to investigate GSM signals using our UmTRX.

We are going to use GNU Radio software to build that project. GNU Radio (https://gnuradio.org) is a free and open-source software development toolkit that provides signal processing blocks to implement software radios. You can use it to write applications to receive data out of dial streams or to push data into digital streams, which is then transmitted using hardware. GNU Radio has filters, channel codes, synchronisation elements, equalizers, demodulators, vocoders, decoders, and many other elements which are typically found in radio systems. More importantly, it includes a method of connecting these blocks and then manages how data is passed from one block to another.

Our hardware part needs to receive radio frequency (RF) analog signals and transform them into the digital form using Analog-to-Digital Converter (ADC). UmTRX system should do that job easily. As for software part, digital signal processing (DSP) is where GNU Radio has a plenty of tricks.

Two weeks ago and in a collaboration between Fairwaves and Rhizomatica, UmTRX was used to bring mobile communications to the rural village of Yaviche in Mexico.

Arriving late on the evening of Thursday 26^th September and after a 5 ½ hour drive, by the end of the following day Fairwaves had installed a GSM network that is now providing up to around 2km coverage, and were able to leave early on the morning of Friday 28^th.

The photo at the top of this post shows one of the first users of the service and below is the message that subscribers receive upon network registration.

English translation: “Your phone number at the AirZapoteco mobile network is 28471. Call 20000 to get help”

At present there are around 330 registered subscribers and the installation is operating as a pilot. Equipment is on loan from Fairwaves and the idea being that in due course this will go into village ownership.

The Yaviche BTS setup: a UmTRX-based UmDESK, UPS and RF amplifiers

Rhizomatica provide ground operations and ensure that connectivity is in place for backhaul and remote management, such as satellite or long distance P2P WiFi links. In addition they provide training on basic operation and troubleshooting, along with systems that enable villages to set their own tariffs and carry out billing, thereby paving the way to sustainability.

Two simple patch antennas were used mounted on a pole

Fairwaves and Rhizomatica are together planning to bring GSM service to many more villages and you can expect future updates via this blog.

The Yaviche team (L-R): Peter Bloom (Rhizomatica), Oswaldo (local BTS host/operator), Ciaby (Rhizomatica), Alexander Chemeris (Fairwaves)