Software-defined radio (SDR) is a potentially revolutionary technology, in which the SDR Forum, working in collaboration with the Institute of Electrical and Electronic Engineers (IEEE) P1900.1 group, has worked to establish a definition of SDR that provides consistency and a clear overview of the technology and its associated benefits.
Radio in which some or all of the physical layer functions are software defined
In many cases, especially where several radio services are needed, the actual transmitting-receiving circuitry is in one package, while a common computer, display, audio, and any required graphics are in a separate unit. This is a generalization of the "glass cockpit" that is increasingly the standard in aircraft, with an easier to use, less cluttered control panel, and with the power electronics in a separate bay convenient for maintenance.
Alternatively, military applications may combine several computer-controlled functions, such as radio proper, GPS, and encryption in a single package. The U.S. military (and allied) Joint Tactical Radio System (JTRS), which is the largest conversion of communications technology in its history, is based on converting a wide variety of analog radios into a much smaller number of programmable SDRs. The variation among SDRs, in the JTRS project, is often in the power and mounting requirements; the control computer and software may be largely common between a radio clipped to a soldier's uniform, and a ground-to-air radio in a supersonic aircraft.
With SDR, the power of computing can be applied, in new ways, to communications. Adaptive radio, for example, involves the radio monitoring its own performance and automatically adjusting its parameters to optimize performance. At a higher level is Cognitive Radio, in which the radio recognizes its environment (e.g., radio frequencies in use, utilization of those frequencies, etc.) and, following a preprogrammed scheme, fits itself into a system.
Computer controlled radio paradigms
From a data communications standpoint, cognitive radio is one way of achieving mobile ad hoc networking. The automatic identification system (AIS), a means of improving safety at sea, is an example of such technology. Another example would be a commercial mobile telephone that first attempts to connect to a local cellular telephony system and, only if it cannot connect, will then communicate through a satellite link.
Intelligent radio goes to another level and exhibits learning behavior, remembering the optimal parameters for different circumstances.
In a conventional analog radio using the superheterodyne principle, a radio signal at a relatively high frequency, called radio frequency (RF), is received on a passive antenna optimized for its wavelength, and sent to an RF amplifier to enhance its strength for internal processing. The higher the frequency, the more expensive and specialized the receiving antenna is likely to be. These functions use analog circuitry, which is generally harder to build as a completely analog integrated circuit, since passive components such as inductors, transformers and capacitors are difficult to miniaturize.
RF is converted, using the superheterodyne principle, to a lower, easier to process, intermediate frequency (IF). Sometimes, "double" or "triple conversion" receivers translate the first IF to a second or even a third, amplifying and noise reducing at each stage. Eventually, the IF is sufficiently low for efficient demodulation so that the information it carries, such as voice, data, television, or music can be sent to an appropriate user interface. All or most of these use analog circuitry.
In SDR, the signal may still need to be received with analog techniques. With current commercial methods, analog is still needed above 40 MHz, but the frequency is constantly going higher. As soon as possible in the frequency conversion chain, is converted to digital signals, which can be processed in software, either on general-purpose processors, still mass-produced digital signal processors, or possibly digital components, such as field programmable gate arrays, which are commodities. With digital components, the radio can become much smaller, more immune to noise generated in analog processing, and require less electrical power.
- SDR Forum and Institute of Electrical and Electronic Engineers (IEEE) P1900.1, What is SDR?
- Internet Engineering Task Force, Mobile Ad Hoc Networking Working Group charter
- United States Coast Guard Navigation Center, How AIS Works
- Elbert, Bruce R. (July-August 2007), "Mobility and Hybrid Networks: An ISCe 2007 Retrospective", SatMagazine.com
- SDR Forum, Related Technologies