Previous tech topics have addressed the fundamental basis for interoperability among various radio networks. First, a number of subjective definitions of interoperability laid a foundation for discussing interoperability at the networking level through the use of Internet Protocols. Although this provides interoperability between disparate radio networks, it nevertheless requires a gateway or interface device and some form of network interconnection between the different gateways.

As an alternative, physical layer interconnection requires conversion of the transmitted signals of one network to the transmitted format of another network – again requiring an interface device to accomplish the translation. This topic continues the discussion of interoperability by suggesting a third alternative. By configuring a flexible radio architecture to meet the transmission characteristics of the desired network, interoperability among an entire class of new radios as well as existing radios can be achieved.

Tech Topic #1 stated that the first objective for interoperability of a radio network -- besides agreements on the human level of course! -- was to use compatible equipment operating with the same transmission parameters, which means common radios operating on the same frequency with the same modulation characteristics, etc. As long as all of the users in the network operate on a common set of transmission parameters, security features, and operational procedures, including agreement on any common messaging formats, etc., then interoperability may be attained and maintained. If a radio can be reconfigured to provide all of these "operational characteristics and parameters" consistent with those of another required radio interface, then the two radio networks could be interoperable!

This concept was the basis for the Department of Defense (DoD) "SPEAKeasy" and "Joint Tactical Radio System" (JTRS) Programs that began in the late 1980's and 1990's.1 These programs were the initial research and development programs for an entirely new class of reconfigurable radios. The concept was simple in its origin, but more complex in the details. The vision for the JTRS Program was to develop a family of affordable, high capacity radios that were interoperable and scaleable and built upon a common open architecture. Such characteristics would promote software and hardware reuse, as well as provide a common architecture or framework for software and hardware development. The radios were intended to be multi-band, multi-mode, across a large frequency band of 2-MHz to 2-GHz, and be reverse compatible with legacy radio systems.

Through the later part of the last century, the development of software and hardware progressed to the point that much of the functionality of a conventional receiver (and corresponding transmitters) could be performed through software programs residing on either standard microprocessors or digital signal processors. The interface to the "real world" still had to be in analog format, that is to say either human voice at one end or continuous radio frequency signals at the other, but most of the functionality within the radio could be accomplished through software once the conversion from analog to digital waveforms was accomplished.

The design of a "standard" radio receiver is shown in the block diagram below. This diagram illustrates the value of software programming and implementation. All of the functionality shown in the purple blocks can be accomplished via software running on digital signal processors or via imbedded software programs running on microprocessors, instead of all the functionality being done solely by inflexible and fixed hardware. Even much of the human-to-machine interface, HMI, is accomplished in software once the analog-to-digital conversion (or vice versa) is made. This was the foundation of the concept of software radio – most of the functionality is performed by software. Hence, the term "software radio" was coined by Dr. Joe Mitola in 1992.2


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The goal of the DoD JTRS program was to build a family of radios that could be built based on reconfigurable software running on a common hardware base. By changing the software characteristics, a new radio could be configured dependent on a software definition of the operating characteristics of the radio. Hence "software radios" became "software-defined radios." The first step in the development of the JTRS family of radios was the development of a common "Software Communications Architecture" (SCA) that formed the basis for development of the entire family of radios, as all further development of DoD radios had to be SCA-compliant.

An overarching diagram of the current SCA is shown below.3 The SCA is composed of three layers; the first is the application layer where all of the functional characteristics of the radio are defined. The application layer rests over a "middleware" layer with Application Program Interfaces (API) defined for each application to connect to the middleware. This middleware layer – initially defined to use CORBA (Common Object Request Broker Architecture) -- provides a common interface to allow interconnection between the various applications and the fundamental operating system of the computing hardware. The operating system that was initially used was POSIX (Portable Operating System Interface).4 By defining the functionality of individual radio components in software, each could be used as necessary to match the software radio with the desired operating characteristics. The middleware provides a common interface architecture between the applications and the separate operating system running on whatever hardware system that may be used. In this way, the development of either software or hardware upgrades would be simple to implement as long as the new developments maintained compliance with the SCA.


The FCC first took up this rapidly expanding technology in 2001,5 and then in 2005 modified its definition of a software-defined radio as follows:6

"a radio that includes a transmitter in which the operating parameters of frequency range, modulation type or maximum output power (either radiated or conducted), or the circumstances under which the transmitter operates in accordance with Commission rules, can be altered by making a change in software without making any changes to hardware components that affect the radio frequency emissions."

In order to further the development of software defined radios and the SCA, industry at large formed the Software Defined Radio Forum.7 This consortium of industry partners has been active in furthering the research and development of SDR and furthermore has continued to grow and evolve. The forum also established a special interest group to encourage the use of SDRs for public safety radio purposes – the subject of our next Tech Topic.8

The concepts of SDRs are not limited to government or commercial systems alone. For example, the GNU Radio Project is an effort to develop software radios that operate with the LINUX operating system.9 This project is an open software development effort and contributions are widely accepted from anyone wishing to contribute.

The amateur radio community is also working to develop software driven radios. An example is the SDR-1000, an HF and VHF radio that was independently developed for/by amateurs.10 Even more advanced systems such as the SDR-5000 have now been developed for the amateur community.

Finally, it should be noted that the development of software defined radios is taking place outside the U.S. as well. In fact, organizations around the world are being formed to pursue this new technology. For example, the E2R, End-to-End Reconfigurability Project, is a European effort to develop software defined radios.11 The E2R Project states that its key objectives are to devise, develop, and trial the architectural design of reconfigurable devices and supporting system functions in order to offer an expanded set of operational choices to users, applications and service providers, operators, and regulators in the context of heterogeneous mobile radio systems.

The FCC has accepted the concept of software defined radios as indicated earlier, and has established a corresponding equipment certification program.12 Commercial mobile cellular base station equipment developed and manufactured by Vanu, Inc. was the first equipment certified by the FCC as software defined radio.13 This equipment is being used to provide for rapid reconfiguration and upgrade for existing cellular base station systems.

The promise of software defined radios for rapidly changing the operating characteristics of radios suggests an exciting new method to create opportunities and means for interoperability among and between any number of different radio systems. In the next Tech Topic, the use of software defined radios in a public safety scenario will be discussed.

1 See

2 Dr. Joseph Mitola is credited with the first use of the terminology in 1992. See Mitola, J., "Software Radios: Survey, Critical Evaluation and Future Directions, "Proceedings of the National Telesystems Conference, NY: IEEE Press, May 1992, and J. Mitola, "The Software Radio Architecture", IEEE Communications Magazine, Vol. 33, No. 5, May 1995, pp. 26-38.

3 See

4 Portable Operating System Interface is the collective name of a family of IEEE standards that define the application programming interface (API) for software compatible with variants of the Unix operating system. The family of POSIX standards is formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945. See

5 See

6 47 C.F.R. Section 2.1. For the FCC order adopting this definition, see Facilitating Opportunities for Flexible, Efficient, and Reliable Spectrum Use Employing Cognitive Radio Technologies, ET Docket No. 03-108, Report and Order, 20 FCC Rcd 5486 (2005), also available at

7 See

8 See

9 See Linux is a free Unix-type operating system originally created by Linus Torvalds with the assistance of developers around the world. Developed under the GNU General Public License, the source code for Linux is freely available to everyone.

10 The SDR-1000™ from FlexRadio Systems was the first complete Software Defined Radio (SDR) transceiver interface to a personal computer for amateur radio use. See QST Magazine, October 2005, Vol. 89, Number 10, page 66, and

11 See

12 See, e.g., 47 C.F.R. Section 2.944.

13 See FCC Approves First Software Defined Radio, News Release (rel. Nov. 19, 2004), available at