Software Defined Radio Comes of Age
By Jeffery A. Wepman

Within the past decade, the software defined radio (SDR) technology has moved rapidly from concept to reality. SDRs are currently available for cellular and PCS base stations as well as fixed station military applications. While SDR technology has enabled some practical products, the current implementations suffer from limitations that preclude development of a practical and widely deployed handheld SDR. Challenges including reduction of power consumption, size, weight, and cost remain for implementation of practical handheld SDR devices. However, several SDR experts believe that handheld SDRs will be in widespread use within five years. This prediction is supported by the continuing rapid development of SDRs, driven by tremendous advances in the key components necessary for SDRs. These key components include high-speed, high spurious-free dynamic range analog-to-digital and digital-to-analog converters; high-speed digital signal processing devices such as application specific integrated circuits (ASICs); digital signal processors; field programmable gate arrays (FPGAs); general purpose processors; and wideband linear power amplifiers.

While there are differing opinions as to what an SDR actually is, a good general definition of an SDR is found in the recently adopted American National Standard, Telecom Glossary 2000. An SDR consists of a receiver and/or transmitter with the following properties: (a) the received signal is digitized and then processed using software-programmable digital signal processing techniques (digitization may occur at the RF, IF, or baseband); and (b) the modulated signal to be transmitted is generated as a digital signal using software-programmable digital signal processing techniques. The digital signal is then converted to an analog signal for transmission (the conversion to analog may occur at baseband, IF, or RF).

One reason that differing definitions of SDRs exist is that SDRs represent a merging of many divergent fields such as computer science, digital signal processing, digital circuit design, and RF design. Complicating the matter even more, there are many different ways to design an SDR. The primary differences in SDR designs are the digital signal processing platform used which affects the software that needs to be developed, and whether digitization in the receiver and conversion to analog in the transmitter occurs at the RF, IF, or at baseband. The digital signal processing platform can be based on ASICs, digital signal processors, FPGAs, general purpose processors, or any combination of these.

A key factor in SDRs is that software programmability allows easy changes of the radio's fundamental characteristics such as modulation types, operating frequencies, bandwidths, multiple access schemes, source and channel coding/decoding methods, frequency spreading-despreading techniques, and encryption-decryption algorithms. Traditional, hardware-based radios required hardware changes to modify these fundamental characteristics of a radio.

SDRs provide great advantages over traditional radios. Manufacturers could benefit by developing a single SDR platform in which multiple radios can be implemented by loading different software. Because the single SDR would replace multiple hardware radios, a greater economy of scale could be achieved that would drive the cost down. New radios would not have to be designed for the plethora of wireless standards that continue to emerge and all changes could be made in software utilizing a common hardware platform. Interoperability problems experienced by public safety/law enforcement agencies and the military could be eased by SDRs. Users would only have to purchase a single SDR to perform many different radio functions with a change in software.

Growing interest in SDRs in the mid-1990s led to the founding of a major SDR organization in 1996, the SDR Forum, which was originally called the Multimode Multifunction Information Transfer System Forum. The new SDR Forum is a non-profit organization dedicated to promoting the development, deployment, and use of open architectures for advanced wireless systems. Membership in the SDR Forum has been increasing rapidly and now consists of over 120 organizations with international representation from the commercial, government, and academic sectors.

The SDR Forum is comprised of three core committees: the Markets, Technical, and Regulatory Committees. Some key outputs of the Markets Committee are predictions of the SDR market size and business revenue as well as identification of primary market characteristics and drivers for the commercial, military, and civil government sectors. The Technical Committee, subdivided into the Handheld, Base Station, and Mobile Working Groups, promotes the advancement of SDRs by developing open architecture specifications of SDR hardware and software structures. The Regulatory Committee was established to address international regulatory issues for SDRs.

While the ease of changing the SDRs fundamental characteristics holds great advantages for users, it raises a new set of spectrum management and regulatory concerns and challenges. In the United States, both NTIA and the FCC have been monitoring the development of SDRs to assess their impact on regulatory and spectrum management issues. The FCC activity includes a Notice of Inquiry on SDRs issued in March 2000. The FCC requested comments on: 1) the assessment of the state of SDR technology; 2) issues related to approval of SDR hardware and software; 3) the impact of SDRs on spectrum efficiency and sharing; and 4) the potential for improved interoperability using SDRs. In the First Report and Order issued on September 13, 2001, the FCC adopted SDR rules with streamlined equipment authorization procedures. The new rules permit equipment manufacturers to make frequency, modulation, and power changes in SDRs without filing a new equipment authorization application. Electronic labeling is permitted allowing other parties to modify these parameters without requiring the SDR to be returned to the manufacturer for re-labeling. No changes were made to the spectrum allocation rules.

Great strides have been made in the development of SDRs, and the spectrum management and regulatory issues are beginning to be addressed. While current and near-term SDR implementations permit easy changes to the radio's fundamental characteristics through software changes, the development of an adaptable SDR that surveys the spectrum and automatically determines what frequency and other types of radio characteristics it should use to communicate is a long-term goal for the future. It is this futuristic type of adaptable SDR that poses the greatest challenge to spectrum managers and regulatory agencies.

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