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The idea of the "Software-Defined Radio" (SDR) has been circulating in industry and academia for almost 20 years, the term having been coined by Joe Mitola in a 1992 paper.¹ Since that time, many commercial and defense-oriented radio SDR products have been developed and released, most using basic technology. Until recently, however, most SDRs have used proprietary middleware to facilitate communications between their different radio componentsónot only within waveform applications, but also to higher layers in the communications protocol. These proprietary middleware components are often narrowly focused, resulting in rigid, monolithic radios that inhibit IP reuse, platform-independence among applications, or innovations that would significantly reduce cost and time-to-market. The Software Communications Architecture (SCA) is a software specification that tries to improve this situation, and focuses on the "software" part of a software-defined radio.

SCA was born out of requirements for the Joint Tactical Radio System (JTRS) program. It standardizes the middleware that governs the interoperation of software across all operating layers within SDRs, and ensures portability and modularity between SDR software components and hardware implementations. Thus, SCA-compliant waveforms can be assembled, loaded, run and networked into systems across radio sets. This interoperability facilitates IP re-use, lowers platform costs and development times, and lengthens the service life of platforms by improving their adaptability. SCA is realizing other secondary, indirect benefits. As an open middleware specification, it has helped create a stable SDR industry ecosystem, enabling third-party vendors to streamline development tools and provide additional middleware components. The overall result is an increase in SDR design efficiency.

Today, the SCA standard is proliferating beyond the original JTRS program into other US DoD programs and Mil/Aero SDRs around the world (such as the ESSOR initiative in Europe). This, along with continued evolution (such as the 'SCA Next' initiative under the auspices of the Wireless Innovation Forum), is evidence of the value SCA is adding to SDR design. SCA is enabling smaller and smaller form-factor radios, and is reaching beyond mil/aero applications into commercial telecom SDRs, such as the ETSI reconfigurable Radio Systems (RRS) effort.

Methodology for Developing SCA-compliant Radio Components
As stated, the Software Communications Architecture governs the structure and operation of software within an SDR, allowing waveform components and full radio applications to share a common control interface and signal path connections. SCA also provides common methods for radio applications to communicate across the middleware boundary. However, the development of waveform components for most SDRs still follows a very traditional design methodology when targeted to FPGA, DSP, or general purpose processors (GPP). While these methodologies produce highly efficient designs for a given target, they typically require additional manual development of interface code, as well as additional code development to revise a design if a new target device is identified. This manual process is time-consuming and prone to overdesign in order to meet given performance targets. Also, due to the disjointed nature of the various tools used in the implementation of waveform components, the final integrated system is not always fully optimized for maximum performance.