Software-defined radio


Software-defined radio is a radio communication system where components that conventionally have been implemented in analog hardware are instead implemented by means of software on a computer or embedded system.
A basic SDR system may consist of a computer equipped with a sound card, or other analog-to-digital converter, preceded by some form of RF front end. Significant amounts of signal processing are handed over to the general-purpose processor, rather than being done in special-purpose hardware. Such a design produces a radio which can receive and transmit widely different radio protocols based solely on the software used.
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. In the long term, software-defined radios are expected by proponents like the Wireless Innovation Forum to become the dominant technology in radio communications. SDRs, along with software defined antennas are the enablers of cognitive radio.

Operating principles

s use a VFO, mixer, and filter to tune the desired signal to a common IF or baseband. Typically in SDR, this signal is then sampled by the analog-to-digital converter. However, in some applications it is not necessary to tune the signal to an intermediate frequency and the radio frequency signal is directly sampled by the analog-to-digital converter.
Real analog-to-digital converters lack the dynamic range to pick up sub-microvolt, nanowatt-power radio signals produced by an antenna. Therefore, a low-noise amplifier must precede the conversion step and this device introduces its own problems. For example, if spurious signals are present, 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. Real software radios often have two or three analog channel filters with different bandwidths that are switched in and out.
The flexibility of SDR allows for dynamic spectrum usage, alleviating the need to statically assign the scarce spectral resources to a single fixed service.

History

In 1970, a researcher at a United States Department of Defense laboratory coined the term "digital receiver". A laboratory called the Gold Room at TRW in California created a software baseband analysis tool called Midas, which had its operation defined in software.
In 1982, while working under a US Department of Defense contract at RCA, Ulrich L. Rohde's department developed the first SDR, which used the COSMAC chip. Rohde was the first to present on this topic with his February 1984 talk, "Digital HF Radio: A Sampling of Techniques" at the Third International Conference on HF Communication Systems and Techniques in London.
In 1984, a team at the Garland, Texas, Division of E-Systems Inc. coined the term "software radio" to refer to a digital baseband receiver, as published in their E-Team company newsletter. A 'Software Radio Proof-of-Concept' laboratory was developed by the E-Systems team that popularized Software Radio within various government agencies. This 1984 Software Radio was a digital baseband receiver that provided programmable interference cancellation and demodulation for broadband signals, typically with thousands of adaptive filter taps, using multiple array processors accessing shared memory.
In 1991, Joe Mitola independently reinvented the term software radio for a plan to build a GSM base station that would combine Ferdensi's digital receiver with E-Systems Melpar's digitally controlled communications jammers for a true software-based transceiver. E-Systems Melpar sold the software radio idea to the US Air Force. Melpar built a prototype commanders' tactical terminal in 1990–1991 that employed Texas Instruments TMS320C30 processors and Harris Corporation digital receiver chip sets with digitally synthesized transmission. The Melpar prototype didn't last long because when E-Systems ECI Division manufactured the first limited production units, they decided to "throw out those useless C30 boards", replacing them with conventional RF filtering on transmit and receive and reverting to a digital baseband radio instead of the SpeakEasy like IF ADC/DACs of Mitola's prototype. The Air Force would not let Mitola publish the technical details of that prototype, nor would they let Diane Wasserman publish related software life cycle lessons learned because they regarded it as a "USAF competitive advantage". So instead, with USAF permission, in 1991, Mitola described the architecture principles without implementation details in a paper, "Software Radio: Survey, Critical Analysis and Future Directions" which became the first IEEE publication to employ the term in 1992. When Mitola presented the paper at the conference, Bob Prill of GEC Marconi began his presentation following Mitola with: "Joe is absolutely right about the theory of a software radio and we are building one." Prill gave a GEC Marconi paper on PAVE PILLAR, a SpeakEasy precursor. SpeakEasy, the military software radio was formulated by Wayne Bonser, then of Rome Air Development Center, now Rome Labs; by Alan Margulies of MITRE Rome, NY; and then Lt Beth Kaspar, the original DARPA SpeakEasy project manager and by others at Rome including Don Upmal. Although Mitola's IEEE publications resulted in the largest global footprint for software radio, Mitola privately credits that DoD lab of the 1970s with its leaders Carl, Dave, and John with inventing the digital receiver technology on which he based software radio once it was possible to transmit via software.
A few months after the National Telesystems Conference 1992, in an E-Systems corporate program review, a vice-president of E-Systems Garland Division objected to Melpar's use of the term "software radio" without credit to Garland. Alan Jackson, Melpar VP of marketing at that time, asked the Garland VP if their laboratory or devices included transmitters. The Garland VP said: "No, of course not — ours is a software radio receiver." Al replied: "Then it's a digital receiver but without a transmitter, it's not a software radio." Corporate leadership agreed with Al, so the publication stood. Many amateur radio operators and HF radio engineers had realized the value of digitizing HF at RF and of processing it with Texas Instruments TI C30 digital signal processors and their precursors during the 1980s and early 1990s. Radio engineers at Roke Manor in the UK and at an organization in Germany had recognized the benefits of ADC at the RF in parallel. Mitola's publication of software radio in the IEEE opened the concept to the broad community of radio engineers. His May 1995 special issue of the IEEE Communications Magazine with the cover "Software Radio" was regarded as a watershed event with thousands of academic citations. Mitola was introduced by Joao da Silva in 1997 at the First International Conference on Software Radio as "godfather" of software radio in no small part for his willingness to share such a valuable technology "in the public interest".
Perhaps the first software-based radio transceiver was designed and implemented by Peter Hoeher and Helmuth Lang at the German Aerospace Research Establishment in Oberpfaffenhofen, Germany, in 1988. Both transmitter and receiver of an adaptive digital satellite modem were implemented according to the principles of a software radio, and a flexible hardware periphery was proposed.
In 1995, Stephen Blust coined the term "software defined radio", publishing a request for information from Bell South Wireless at the first meeting of the Modular Multifunction Information Transfer Systems forum in 1996, organized by the USAF and DARPA around the commercialization of their SpeakEasy II program. Mitola objected to Blust's term, but finally accepted it as a pragmatic pathway towards the ideal software radio. Although the concept was first implemented with an IF ADC in the early 1990s, software-defined radios have their origins in the U.S. and European defense sectors of the late 1970s, about a year after the First International Conference in Brussels. One of the first public software radio initiatives was the U.S. DARPA-Air Force 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 2000 MHz. Another SpeakEasy 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.
In 1997, Blaupunkt introduced the term "DigiCeiver" for their new range of DSP-based tuners with Sharx in car radios such as the Modena & Lausanne RD 148.

SpeakEasy phase I

From 1990 to 1995, the goal of the SpeakEasy program was to demonstrate a radio for the U.S. Air Force tactical ground air control party that could operate from 2 MHz to 2 GHz, and thus could interoperate with ground force radios, Air Force radios, Naval Radios and satellites. 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 demonstrated all of these goals in a non-production radio. There was some discontent with failure of these early software radios to adequately filter out of band emissions, to employ more than the simplest of interoperable modes of the existing radios, and to lose connectivity or crash unexpectedly. 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 SpeakEasy architecture was refined at the MMITS Forum between 1996 and 1999 and inspired the DoD integrated process team for programmable modular communications systems to proceed with what became the Joint Tactical Radio System.
The basic arrangement of the radio receiver used an antenna feeding an amplifier and down-converter 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. The transmitter had digital-to-analog converters on the PCI bus feeding an up converter 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 wideband software radios.