Development [LWN.net]

RF hacking with GNU Radio

By Forrest Cook
September 5, 2007

GNU Radio is an open-source software project which controls some simple hardware in order to create arbitrarily modulated radio signals. According to the FSF listing:

GNU Radio is a collection of software that when combined with minimal hardware, allows the construction of radios where the actual waveforms transmitted and received are defined by software: it turns the digital modulation schemes used in high performance wireless devices into software problems. Waveforms are generated as sampled digital signals, converted from digital to analog via a wideband DAC and then possibly upconverted from IF to RF. The receiver uses a wideband Analog to Digital Converter (ADC) that captures all the channels of the software radio node. The receiver then extracts, downconverts and demodulates the channel waveform using software on a general purpose processor.

The Wikipedia entry for GNU Radio explains some of the project's history:

"Philanthropist John Gilmore initiated and has sustained GNU Radio with the funding of $320,000 (US) to Eric Blossom for code creation and project management duties."

"GNU Radio began as a fork of the Pspectra code that was developed by the SpectrumWare project at MIT. The Pspectra SDR design utilized a modular pipeline structure and the programmability of the Python programming language for easy configuration and flexibility. In 2004 a complete "rewrite" of the GNU Radio was completed but much of the original Pspectra code and structure remains."

To use the GNU Radio software, one must acquire some hardware in the form of the Universal Software Radio Peripheral. The USRP consists of a motherboard with a USB 2.0 interface, four D/A converters, four A/D converters, an Altera FPGA chip, and slots for two transmitter and two receiver daugherboards. The full hardware setup is not inexpensive at $850. Hardware subsets are available for less.

The GNU Radio code is a combination of Python and C++. The wxPython library is used for GUI functionality on the host system, control information is sent to the radio peripheral via USB.

Eric Blossom's 2004 paper Exploring GNU Radio explains the project in more detail. Some of the applications of GNU Radio include:

Making a multi-channel radio signal recorder.
Creation of Time Division Multiple Access (TDMA) waveforms.
A passive radar for tracking aircraft through broadcast TV distortion.
A radio astronomy receiver.
A Terrestrial Trunked Radio system (TETRA).
A Global Positioning System (GPS) receiver.
A platform for distributed sensor networks.
A spectrum analysis device.
Amateur radio equipment.
An ad-hoc mesh network platform.
A Radio Frequency ID (RFID) reader.
much more.

The paper includes some short Python applications for creating a telephone dial tone and making a software FM radio receiver.

A device that can create arbitrary waveforms via software is likely to get the attention of the regulatory agencies. From Eric Blossom's paper: "Every revolution has its political issues. Free software for building radios is troublesome to some people. In the US, we've run into opposition from the Motion Picture Association of America and its attempt with the Broadcast Flag to restrict the kinds of receivers that can be built for over-the-air digital TV. The US Federal Communications Commission has issued a Notice of Proposed Rule Making (NPRM) concerning Cognitive Radio Technologies and Software Defined Radios. Several troublesome issues are raised in the NPRM, including restricting the sale of high-speed digital-to-analog converters, requirements for digital signatures or similar methods to keep unauthorized software out of software radio hardware and new restrictions on radios built for the amateur radio market."

GNU Radio version 3.0.4 was announced last July, it featured one bug fix and a change of license to GPLv3.

To see some of the projects that are being done with GNU Radio, see the Our Users page.

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