For the first 100 years or so, radio improvement was driven by the development of better hardware, including improved components, more complex circuitry, and more precise manufacturing.
During this time, each type of radio was designed for a specific purpose, and the waveband in which it operated and the waveform it used were fixed by hardware. Changing the behavior of the device was impossible unless you ripped out and replaced significant parts of the circuit.
The advent of software defined radio (SDR) changed all that.
SDR takes advantage of the processing power of modern computer technology to emulate the behavior of a radio circuit. Software defines how the radio works, making it possible for a radio to emulate and communicate with many different types. Unlike their hardware-based counterparts, these devices can be improved and updated and given new capabilities by changing the software.
It is theoretically possible to reduce the SDR to a computer that decodes everything that arrives at an antenna that is connected to it. In practice, however, this is not feasible: antenna voltages are small, far below anything a computer’s analog-to-digital converters (ADCs) can handle, so at the very least an SDR must have a low-noise amplifier between the antenna and ADC.
“SDR takes advantage of the processing power of modern computer technology to emulate the behavior of a radio circuit.”
Amplification can cause problems, however, as spurious signals from other equipment and even background radiation are also amplified and can distort desired signals or even block them altogether. One solution is to put band-pass filters between the antenna and the amplifier, but these reduce the flexibility of the radio. Some SDRs have multiple switchable channels, each with their own filters and amplifiers, which increases discrimination but reduces flexibility and adds complexity to the circuit.
How it started
The concept of SDR originated from work done in the defense sector in Europe and the US, although the term was not coined until 1991, when Joseph Mitola published the first paper on the topic at the 1992 IEEE National Telesystems Conference.
The SPEAKeasy projects in the US in the 1990s were the first SDR implementations using programmable processing to emulate existing military radios, while General DynamicsThe Digital Modular Radio (DMR) system, incorporating SDR, was adopted about ten years ago by the US Navy. Previously, ships often carried several racks of radio equipment to communicate with aircraft, shore, small boats, and various allies; with an SDR facility on board, this was reduced to a single cabinet, saving space and weight and reducing complexity. General Dynamics claims its DMR system typically replaces 14 different radio cabinets and takes up 50% less space.
Early SDR systems used proprietary software, making it difficult to “port” components from one radio made by one company to another. As a result, the DoD wanted the next generation, the Joint Tactical Radio System (JTRS or “jitters”), to be standardized. JTRS is based on an open software communication architecture (SCA), which is increasingly recognized as an international standard and used by equipment manufacturers worldwide, while the European Security Software Radio (ESSOR) standard is also based on SCA.
“Manufacturers see software as just another component that can be purchased, like batteries and switches.”
JTRS radios have become much smaller and more energy efficient since DMR was first used aboard ships, and the system is now widely used in hand-held radios.
Field-programmable gate arrays (FPGAs), with their inherent flexibility and reprogrammability, have also dramatically increased the capabilities of modern SDR systems and enable them to support emerging and changing SDR waveforms. FPGAs meet the performance needs of the radio while keeping size, weight and power consumption to a minimum, extending battery life.
Software design
Once SCA standardizes how software interacts with radio hardware, it is no longer necessary or desirable for each hardware manufacturer to create unique software. Consequently, manufacturers now view software as just another component that can be purchased, like semiconductors, batteries and switches.
Specialized software companies such as Prism Technologies and Wind River exist to supply this market.
Prism’s Spectra SDR development suite allows SDR manufacturers to create their own compatible waveforms, either new or to match legacy equipment. Wind River produces operating systems for SDR, and the two companies have joined forces to provide a complete, high-performance, commercial-ready SDR solution for a wide range of hardware.
Next step – cognitive radio
Having done much to launch SDR in the 1990s, Joseph Mitola turned his attention to cognitive radio, which he defined in his 2000 PhD thesis:
“The term cognitive radio identifies the point at which wireless personal digital assistants (PDAs) and related networks are computationally intelligent enough with respect to radio resources and related computer-to-computer communications to: (a) detect user communication needs as a function of of use context, and (b) provides radio resources and wireless services that are best suited to those needs.”
What applies to PDAs can apply equally well to military SDR systems. The idea is that radios must be smart enough to find a spot in a crowded spectrum and use it to join any network requested by the operator, breaking the tie between network, geography, and specific location in the electromagnetic spectrum.
The radio spectrum is already crowded, and during times of conflict or disaster relief operations, it becomes even more so. The goal of cognitive radio is to improve interoperability during joint and combined operations and coexistence with commercial and civilian systems. The US DARPA XG (Next Generation) program exists to develop this technology.
DARPA XG
DARPA contracted the Shared Spectrum Company (SSC) to develop XG, and in 2006 SSC successfully demonstrated XG radios at Fort AP Hill, Virginia. The demonstration used six mobile XG radios that operated in the same spectrum as a set of fixed, equipped military and commercial radios. An extensive instrumentation system was used to record XG radio connectivity and the performance of legacy radios.
Field exercises have demonstrated the operational utility of XG: that XG does not harm existing military radios in accordance with emission/regulatory rules; XG will allow additional radio networks or communication capacity that are currently possible using existing procedures; and that the XG can operate in the presence of electromagnetic interference (ie jamming).
Cognitive radio is far from being deployed and technical as well as regulatory issues remain. Radio spectrum is usually divided up by national governments and allocated for different purposes, so a system that arbitrarily uses every available frequency band would be illegal almost everywhere.
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