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What Is a Software Radio?

The term software radio was coined by Joe Mitola in 1991 to refer to the class of repro-grammable or reconfigurable radios. In other words, the same piece of hardware can perform different functions at different times. The SDR Forum defines the ultimate software radio (USR) as a radio that accepts fully programmable traffic and control information and supports a broad range of frequencies, air-interfaces, and applications software. The user can switch from one air-interface format to another in milliseconds, use the Global Positioning System (GPS) for location, store money using smartcard technology, or watch a local broadcast station or receive a satellite transmission.

The exact definition of a software radio is controversial, and no consensus exists about the level of reconfigurability needed to qualify a radio as a software radio. A radio that includes a microprocessor or digital signal processor (DSP) does not necessarily qualify as a software radio. However, a radio that defines in software its modulation, error correction, and encryption processes, exhibits some control over the RF hardware, and can be reprogrammed is clearly a software radio. A good working definition of a software radio is a radio that is substantially defined in software and whose physical layer behavior can be significantly altered through changes to its software. The degree of reconfigurability is largely determined by a complex interaction between a number of common issues in radio design, including systems engineering, antenna form factors, RF electronics, baseband processing, speed and reconfigurability of the hardware, and power supply management.

The term software radio generally refers to a radio that derives its flexibility through software while using a static hardware platform. On the other hand, a soft radio denotes a completely configurable radio that can be programmed in software to reconfigure the physical hardware. In other words, the same piece of hardware can be modified to perform different functions at different times, allowing the hardware to be specifically tailored to the application at hand. Nonetheless, the term software radio is sometimes used to encompass soft radios as well.

The functionality of conventional radio architectures is usually determined primarily by hardware with minimal configurability through software. The hardware consists of the am-plifiers, filters, mixers (probably several stages), and oscillators. The software is confined to controlling the interface with the network, stripping the headers and error correction codes from the data packets, and determining where the data packets need to be routed based on the header information. Because the hardware dominates the design, upgrading a conventional radio design essentially means completely abandoning the old design and starting over again. In upgrading a software radio design, the vast majority of the new content is software and the rest is improvements in hardware component design. In short, software radios represent a paradigm shift from fixed, hardware-intensive radios to multi-band, multimode, software-intensive radios.

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