0278-6648/04/$20.00 © 2004 IEEE. IEEE POTENTIALS. In the past, radio systems were designed to communicate using one or two wave- forms. As a result, two ...
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for third generation (3G) wireless sercoded form. Advances in SDR developIn the past, radio systems were designed vices, architecture and products. ment (such as the physical layer flexibilito communicate using one or two wavety of wireless devices) have provided forms. As a result, two groups of people impetus for ADC performance improvewith different types of traditional radio Enabling technologies ments. Although there are different kinds were not able to communicate due to A number of enabling technologies of ADCs, their performance can be sumincompatibility problems. For example, have significantly changed the landscape marized by a few parameters: signal-tothe communication equipment of the of software radio techniques. They have noise ratio (SNR), stated resolution (numUnited States’ military branches could permitted increased reliability and funcber of bits per sample), spurious-free not “talk” to each other. This problem tionality with reduced size, weight and dynamic range, and power dissipation. could be costly in times of war (e.g. the power. As advances in technology proDigital signal processors (DSPs) play a Air Force could not communicate with prominent role in SDR. They offer develthe Army) and during peace (e.g. police opment flexibility and are used primarily could not communicate with fire fightfor number crunching operations in sigers). The need nal processing algorithms. Traditionally, to communicate Software-defined radio : DSP techniques were used for pre-moduwith people a brief overview lation and post-detection functions in using different radio receivers. In recent times, DSP techtypes of equipment can only be solved niques have been used extensively for using software programmable radios advanced digital communications transbecause of its flexible architecture. ceiver designs, finding their way into Software-defined radio (SDR), also detection, equalization, demodulation, called software radio (SR), refers to wirefrequency synthesis and channel filtering. less communication in which the transThe Fourier transform is one of the mitter modulation is generated or most common functions performed by defined by a computer. The receiver the DSP. In fact, purely softthen also uses a computer to recover the ware-based spectrum monitorsignal intelligence. SDR is an enabling Matthew N. O. ing has been implemented technology that is useful in a wide range vide increasingly faster and less using the Fast Fourier of areas within wireless systems. The priexpensive digital hardware, more Sadiku Transform (FFT). The FFT mary goal of SDR is to replace as many of the traditional analog functions and extracts frequency domain analog components and hardwired digiof the radio receiver will be Cajetan M. information from a series of tal VLSI devices of the transceiver as replaced by software or digital Akujuobi time domain samples to possible with programmable devices. hardware. While the traditional resolve the signal into a set of frequency These include the air interface, modularadio is hardware based, the new digital bins. If the system is to operate in real tion and coding schemes, analog-to-digiand software radios do the vast majority time (which is usually the case), the data tal converter (ADC), and digital-to-anaof the signal processing, including chanmust be able to get in and out of the log converter (DAC). nel selection, tuning and demodulation, DSP, which can create I/O bottleneck What are the benefits of SDR? And in the digital domain. The ultimate goal problems. who will enjoy these benefits? Here are in radio receiver design is to implement DSPs are evolving by being integratsome examples: all receiver functions in software. ed with microcontrollers, getting net • Subscribers—easier international (Besides the issue of globalization of softfunctions and specialized accelerators. roaming, improved and more flexible ware radio, the goal is almost achieved.) Programmable DSPs are used for all services These receivers can be used in mobile functions possible within the current • Mobile network operator —a tool cellular, satellite and personal communistate of the art. They act as specialized to increase customer retention and procations services systems. microcomputers, VSLI devices designed vide added-value services As shown in Fig. 1, a SDR consists of for implementation of extensive arith• Handset and base-station manufacanalog-to-digital converter (ADC), digimetic computation and perform digital turers—the promise of new scale tal-to-analog converter (DAC), antenna signal processing functions through resieconomies and increased production and other modules. In addition, the SDR dent software. Application-specific flexibility may employ a digital signal processor Software radio has two major advan(DSP) and a general purpose, tages: 1) flexibility and 2) ease of adapcentral processing unit (CPU). Fig. 1 Typical software-defined radio architecture tation. Flexibility (being able to switch For simplicity, this article will channels, change modulation) has focus on the receiver side of always been valuable to the military. For the SDR. ADC/DAC DSP Information instance, SDR can be programmed for The analog-to-digital conemerging standards. This is an attractive verter (ADC) is an important processors and field programmable logic feature for radio designers faced with component in these radio receivers. In devices are used for those functions, several standards. Virtually any aspect of fact, an ideal software-defined radio is such as the fast Fourier transform, a program that implements radio funcoften considered to be an ADC connectwhich may be too complex for efficient tions can be easily changed. Although ed directly to an antenna. What happens implementation in today’s general-purvirtually obscure a few years ago, the is that an ADC converts the continuouspose programmable DSPs. SDR now figures prominently in plans time signals to a discrete-time, binary-
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0278-6648/04/$20.00 © 2004 IEEE
IEEE POTENTIALS
Since the SDR itself is an emerging technology, its prospects are related to other important emerging technologies such as smart antennas, networking, software, semiconductors, signal processing and battery technology. Rapid advances in solid-state integrated circuit (IC) technology are fueling the growth in commercial wireless communications systems. These emerging technologies and advances are making SDR technically and commercially realistic.
Implementation The ideal SDR has a set of features that are not yet attainable in commercial systems due to limitations in current technology and cost considerations. To be specific, the digital signal processing microprocessors are not fast enough to implement all radio functions. In spite of this fact, there has been several SDR implementations. One such implementation is the SPEAKeasy system, which is the successful implementation of the SDR for military use in the US. The application of SDR to military communications was conceived in the 1970s. SPEAKeasy allows general-purpose digital hardware to communicate over a wide range of frequencies, modulation techniques, data encoding methods, cryptographic types and other communication parameters. The SPEAKeasy program is a multi-phase research and development program for the next-generation military radio managed by the Air Force Rome Laboratory (with several locations throughout the US). It has a programmable digital processing capacity on the order of one billion 16-bit integer operations per second and 200 million 32-bit operations per second. The efforts on SPEAKeasy have led to other projects such as SPEAKeasy II, the airborne communication node (ACN), and the joint tactical radio system, all sponsored by the US Department of Defense. Similar research efforts have been made in Europe and Japan. In Europe, the focus for software radio R&D has been the third-generation mobile multimedia system. The Flexible Integrated Radio Systems Technology (FIRST) project examined many aspects of softwarereconfigurable air interface implementation. In Japan, the Software Radio Study Group was formed in December 1998 to promote R&D in software radio. Several companies and laboratories have developed prototypes of multimode transceivers using SDR components.
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In the commercial arena, applications of SDR can be divided into user terminals and radio base stations. Since the computational load of SDR is on the order of billions of operations per second, it is extremely challenging to implement a SDR in a battery-powered terminal. SDR hardware platforms can serve a range of applications including analog cellular, personal communication services and global positioning systems. In spite of several challenges (such as direct digitization and direct digital downconversion), prototypes from many research institutions and companies have been produced. An example is the SPEAKeasy system mentioned earlier.
Conclusion Rapidly evolving, this technology is receiving enormous recognition and generating widespread interest in the telecommunication industry. Over the years, the radio community has realized that as more of the basic radio functions are pushed into software, radio itself can achieve greater flexibility in that it is capable to operate in a multiservice environment without being constrained to a particular standard. Software-defined radio (SDR) (or software radio) has made a transition from obscurity to commercial usage in less than a decade. SDR has emerged from military research to become a cornerstone of third generation strategies for regional and global communications. Some of the issues that appeared to be roadblocks to SDR development have been solved somewhat. One of these is the ability to normalize air interfaces across regions of the world. Others arise due to the laws of physics that limit technical progress in power dissipation, clock speeds, dyanmic range and linearity. SDR has been described as a cornerstone in the evolution of GSM. In addition, industrial leaders such as Nokia, Toshiba and Motorola have declared their intent to migrate from digital radios to software radios as the technology matures. The SDR Forum is an international, nonprofit organization that includes members from academia, the military, vendors, wireless service providers, and regulatory bodies. It is open to all organizations interested in promoting SDR. Together, the members are addressing critical issues such as software downloading, hardware and software model interfaces, and protocols. In the future, software-defined radio will be the technology of choice in several wireless applications such as GSM and AMPS.
Read more about it • J. Mitola II and Z. Zvonar, Software Radio Technologies. New York: IEEE Press, 2001. • J. Mitola II, Software Radio Architecture: Object-oriented Approaches to Wireless System. New York: John Wiley & Sons, 2000. • Special issue of IEEE Personal Communication, Aug. 1999. • Special issue of IEEE Journal on Selected Areas in Communications, vol. 17, no.4, April 1999. • Special issue of IEEE Communications Magazine, Feb. 1999.
About the authors Matthew N. O. Sadiku is presently a professor at Prairie View A&M University. He was a professor at Temple University, Philadelphia and Florida Atlantic University, Boca Raton. He is the author of over one hundred and forty technical papers and over twenty books including Elements of Electromagnetics (Oxford, 3rd ed., 2000) and Numerical Techniques in Electromagnetics (CRC, 2nd ed. 2001), Metropolitan Area Networks (CRC Press, 1995), and Fundamentals of Electric Circuits (McGraw-Hill, 2nd ed., 2004, with Charles Alexander). Some of his books have been translated into Korean, Chinese, Italian, Portuguese, and Spanish. His current research interests are in the areas of numerical techniques in electromagnetics and computer communications networks. Dr. Akujuobi is the founding Director of the Center of Excellence for Communication Systems Technology Research (CECSTR). He is also one of the researchers with the NASA Center for Applied Radiation Research (CARR). He belongs to many professional organizations such as IEEE, ISA (Senior Member), ASEE, SPIE, and Sigma XI, the Scientific Research Society. Dr. Akujuobi has over 20 years experience in engineering education, research and development. His current research interests include mixed signal systems, high-speed (broadband) network access technologies, all areas of signal and image processing and communication systems using such tools as wavelet and fractal transforms. Dr. Akujuobi has a National Diploma (OND) in Electrical and Electronics Engineering, BSEE, MSEE, MBA and a Ph.D. in Electrical Engineering. He is listed in Who’s Who in Science and Engineering, Who’s Who in the World, Who’s Who in America, Who’s Who in American Education and Who’s Who in Industry & Finance.
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