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Software-defined radio
From Wikipedia, the free encyclopedia
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A Software Defined Radio (SDR) system is a radio communication system which can potentially tune to any frequency band and receive any modulation across a large frequency spectrum by means of as little hardware as possible and processing the signals through software.
An amateur version of SDR performs significant amounts of signal processing mostly using software running on a general purpose computer (PC) equipped with a soundcard, or a reconfigurable home-made piece of digital electronics. The goal of this designs is to produce a radio that can receive and transmit a different form of radio protocol just by running different software.
Software radios have significant utility for the military and cell phone services, both of which must serve a wide variety of changing radio protocols in real time.
Software-defined radio can currently be used to implement simple radio modem technologies. In the long run, software-defined radio is expected by its proponents to become the dominant technology in radio communications. It is the enabler of the cognitive radio.
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The ideal receiver scheme would be to attach an analog to digital converter to an antenna. A digital signal processor would read the converter, and then its software would transform the stream of data from the converter to any other form the application requires.
An ideal transmitter would be similar. A digital signal processor would generate a stream of numbers. These would be sent to a digital to analog converter connected to a radio antenna.
The ideal scheme is, due to the actual technology progress limits, not completely realizable, however.
The actual practical solution is to let the software processing stage be preceded by a front-end that preconditions the input signals to give them characteristics that enable the subsequent stage to elaborate them.
Current (2007) digital electronics are too slow to receive directly typical radio signals over approx. 40MHz. An ideal software radio has to collect and process samples at more than twice the maximum frequency at which it is to operate. Actual software radios, for frequencies below 40MHz, use a direct-conversion hardware solution. In this solution an analog-to-digital converter (ADC) is connected almost directly to the antenna (some preamplifier and impedance adapting circuitry is present to ensure that the input of the ADC correctly is matched to the antenna). The output stream of digital data obtained from the ADC is then passed to the software defined processing stages.
For frequencies above 40MHz, the actual ADCs doesn't perform sufficient speed so direct-conversion is not possible. To solve the problem a superheterodyne RF front end architecture is adopted, to lower the frequency of the received signals to intermediate frequency values (IF) under the 40MHz convertible limit. This IF is then treated by the ADC.
The superheterodyne architecture consists of a frequency mixer and a reference oscillator to heterodyne the radio signals to the lower frequencies.
The mixer changes the frequency of the signal. The phase information then becomes more difficult to detect in it, and many digital encoding systems depend on phase encoding. The normal solution is to mix and digitize two channels, using a reference oscillator that produces two signals that are the same frequency, but where one of the outputs lags the other by 90 degrees of a cycle. Thus, the two sets of samples provide the needed phase information.
Another related problem is that the information about the bit-timing is lost when the frequency changes. The phase information helps recover that as well.
A good software radio must operate at any sample rate within a wide range of rates in order to be compatible with many protocols, so that adaptive control is crucial. It can be implemented either with a hardware link to the converter or in software.
As follows from the Nyquist–Shannon sampling theorem, any signals present in the input to the ADC above half of the sampling frequency would "interfere" with the sampling, causing spurious signals to appear in the data stream. For this reason, a low-pass analog electronic filter must precede the digital conversion step.
Real analog-to-digital converters lack the discrimination to pick up sub-microvolt, nanowatt radio signals. Therefore a low-noise amplifier must precede the conversion step and this device introduces its own problems. For example if spurious signals are present (which is typical), these compete with the desired signals within the amplifier's dynamic range. They may introduce distortion in the desired signals, or may block them completely. The standard solution is to put band-pass filters between the antenna and the amplifier, but these reduce the radio's flexibility - which some see as the whole point of a software radio. Real software radios often have two or three analog "channels" that are switched in and out. These contain matched filters, amplifiers and sometimes a mixer.
One of the first software radios was a U.S. military project named SpeakEasy. The primary goal of the SpeakEasy project was to use programmable processing to emulate more than 10 existing military radios, operating in frequency bands between 2 and 200 MHz. Further, another design goal was to be able to easily incorporate new coding and modulation standards in the future, so that military communications can keep pace with advances in coding and modulation techniques.
From 1992 to 1995, the goal was to produce a radio for the U.S. Army that could operate from 2 MHz to 2 GHz, and operate with ground force radios (frequency-agile VHF, FM, and SINCGARS), Air Force radios (VHF AM), Naval Radios (VHF AM and HF SSB teleprinters) and satellites (microwave QAM). Some particular goals were to provide a new signal format in two weeks from a standing start, and demonstrate a radio into which multiple contractors could plug parts and software.
The project was demonstrated at TF-XXI Advanced Warfighting Exercise, and met all these goals. There was some discontent with certain unspecified features. Its cryptographic processor could not change context fast enough to keep several radio conversations on the air at once. Its software architecture, though practical enough, bore no resemblance to any other.
The basic arrangement of the radio receiver used an antenna feeding an amplifier and down-converter (see mixer) feeding an automatic gain control, which fed an analog to digital converter that was on a computer VMEbus with a lot of digital signal processors (Texas Instruments C40s). The transmitter had digital to analog converters on the PCI bus feeding an up converter (mixer) that led to a power amplifier and antenna. The very wide frequency range was divided into a few sub-bands with different analog radio technologies feeding the same analog to digital converters. This has since become a standard design scheme for wide band software radios.
The goals were to get a more quickly reconfigurable architecture (i.e. several conversations at once), in an open software architecture, with cross-channel connectivity (the radio can "bridge" different radio protocols). The secondary goals were to make it smaller, weigh less and cheaper.
The project produced a demonstration radio only fifteen months into a three year research project. The demonstration was so successful that further development was halted, and the radio went into production with only a 4 MHz to 400 MHz range.
The software architecture identified standard interfaces for different modules of the radio: "radio frequency control" to manage the analog parts of the radio, "modem control" managed resources for modulation and demodulation schemes (FM, AM, SSB, QAM, etc), "waveform processing" modules actually performed the modem functions, "key processing" and "crytographic processing" managed the cryptographic functions, a "multimedia" module did voice processing, a "human interface" provided local or remote controls, there was a "routing" module for network services, and a "control" module to keep it all straight.
The modules are said to communicate without a central operating system. Instead, they send messages over the PCI computer bus to each other with a layered protocol.
As a military project, the radio strongly distinguished "red" (unsecured secret data) and "black" (cryptographically-secured data).
The project was the first known to use FPGAs (field programmable gate arrays) for digital processing of radio data. The time to reprogram these is an issue limiting application of the radio.
The Joint Tactical Radio System (JTRS) is a program of the US and NATO to produce radios which provide flexible and interoperable communications. Examples of radio terminals which require support include hand-held, vehicular, airborne and dismounted radios, as well as base-stations (fixed and maritime).
This goal is achieved through the use of SDR systems based on an internationally endorsed open Software Communications Architecture (SCA). This standard uses CORBA on POSIX operating systems to coordinate various software modules. The SCA documentation is freely available at the JTRS website.
The program is providing a flexible new approach to meet diverse warfighter communications needs through software programmable radio technology. All functionality and expandability is built upon the Software Communications Architecture (SCA).
The SCA, despite its military origin, is under evaluation by commercial radio vendors for applicability in their domains.
A typical amateur software radio, such as the FlexRadio SDR-1000 or the homemade design described in the ARRL Handbook (1999), uses a direct conversion receiver. The conversion is to the audio frequency band, which is sampled by a standard (or enhanced) PC sound card. A fast PC operates custom (usually amateur-written) software as the signal processor.
Uses include every common amateur modulation: morse code, single sideband modulation, frequency modulation, radioteletype, slow-scan television, and packet radio. Amateurs also experiment with new modulation methods: for instance, the DREAM open-source project decodes the COFDM technique used by Digital Radio Mondiale.
More recently, the GNU Radio Universal Software Radio Peripheral (USRP) uses a USB 2.0 interface, a FPGA, and a high-speed set of ADC/DACs, combined with reconfigurable free software. Its sampling and synthesis bandwidth is a thousand times that of PC sound cards, which enables an entirely new set of applications.
In addition the HPSDR (High Performance Software Defined Radio) project (hpsdr.org) uses a 16bit 135MSPS ADC that provides performance over the range 0 to 55MHz comparable to that of a conventional analogue HF radio. The receiver will also operate in the VHF and UHF range using either mixer image or alias responses. Interface to a PC is provided by a USB 2.0 interface.
The project is modular and comprises a backplane onto which other boards plugin. This allows experimentation with new techniques and devices without the need to replace the entire set of boards. An exciter provides 1/2W of RF over the same range or into the VHF and UHF range using image or alias outputs. The HPSDR project is open source for both hardware and software. A Wiki provides frequent updates as to project progress.
On the low-end (and low-cost): the SoftRock kit gives an easy entry into direct conversion shortwave receiver with software-defined demodulation.
As well as transmitting audio information, SDR may have value in the emerging field of Radio Frequency Identification (RFID), where devices operate on various frequencies using various communication protocols. See also digital radio, PACTOR, AMTOR
The papers presented at the SDR Forum 2004 and 2005 Technical Conferences are now available on their website.
These are useful books:
- Software defined radio : architectures, systems, and functions. Dillinger, Madani, Alonistioti. Wiley, 2003. 454 pages. ISBN 0470851643 ISBN-13: 9780470851647
- Cognitive Radio Technology. Bruce Fette. Elsevier Science & Technology Books, 2006. 656 pags. ISBN 0750679522 ISBN-13: 9780750679527
- Software Defined Radio for 3G, Burns. Artech House, 2002. ISBN 1-58053-347-7
- Software Radio: A Modern Approach to Radio Engineering, Jeffrey H. Reed. Prentice Hall PTR, 2002. ISBN 0-13-081158-0
- Eric Blossom explains Software Radio
- OMG Software Based Communication (SBC) Domain Task Force
- Software Defined Radio Forum
- F4DAN Some amateur radio SDR projects and resources
- Using model-driven engineering, domain-specific languages, and software product lines in developing Software-Defined Radio components and applications
- Evolution and standardization of the software communications architecture
- A Holistic Approach to Designing SDRs for Power
- Software Radio Resource Page Other SDR projects and resources
- Component-based SDR and JTRS-SCA in PocoCapsule Applying inversion-of-control (IoC) and domain-specific-modeling (DSM) to SDR