Software-defined radios can be reprogrammed quickly to transmit and receive on multiple frequencies in different transmission formats. This reprogramming capability could change the way users traditionally communicate across wireless services and promote more efficient use of radio spectrum.
In a software-defined radio, functions that were formerly carried out solely in hardware, such as generation of the transmitted radio signal and tuning of the received radio signal, are performed by software. Because these functions are carried out in software, the radio is programmable, allowing it to transmit and receive over a wide range of frequencies and to emulate virtually any desired transmission format.
The concept of software-defined radio originated with the military, where it was used quickly for electronic warfare applications. Now the cellular/wireless industries in the United States and Europe have begun work to adapt the technology to commercial communications services in the hope of realizing its long-term economic benefits.
If all goes according to plan, future radio services will provide seamless access across cordless telephone, wireless local loop, Personal Communications Services (PCS), mobile cellular, and satellite modes of communication, including integrated data and paging.
Generations of Radio Systems
First-generation hardware-based radio systems are built to receive a specific modulation scheme. Ahandset would be built to work over a specific type of analog network or a specific type of digital network. The handset worked on one network or the other, but not both, and it could certainly not cross between analog and digital domains. Second-generation radio systems also are based in hardware.
Miniaturization enables two sets of components to be packaged into a single, compact handset. This enables the unit to operate in dual mode—for example, switching between Advanced Mobile Phone Service (AMPS) or TDMA modulation as necessary. Such handsets are implemented using snap-in components: Two existing chip sets—one for AMPS and one for TDMA, for example—are used together.
Building such handsets typically costs only 25 to 50 percent more than a single-mode handset but offers network operators and users far more flexibility. Handsets that work across four or more modes/bands entail far more complexity and processing power and call for a different architecture altogether. The architecture is based in software and programmable digital signal processors (DSPs). This architecture is referred to as “software-defined radio” or just “software radio.”
It represents the third generation of radio systems. As new technologies are placed onto existing networks and wireless standards become more fragmented—particularly in the United States—the need for a single radio unit that can operate in different modes and bands becomes more urgent. Asoftware radio handset could, for example, operate in a GSM-based PCS network, a legacy AMPS network, and a future satellite mobile network.
As noted, a software radio is one in which channel modulation waveforms are defined in software. Waveforms are generated as sampled digital signals, converted from digital to analog via a wideband digital-to-analog converter (DAC), and then up-converted from an intermediate frequency (IF) to the desired radiofrequency (RF).
In similar fashion, the receiver employs a wideband analog-to-digital converter (ADC) that captures all the channels of the software radio node. The receiver then extracts, down-converts, and demodulates the channel waveform using the software loaded on a general-purpose processor.
As competing technologies for wireless networks emerged in the early 1990s, it became apparent that subscribers would have to make a choice: The newer digital technologies offered more advanced features, but coverage would be spotty for some years to come. The older analog technologies offered wider coverage but did not support the advanced features. A compromise was offered in the form of wireless multimode/ multiband systems that offered subscribers the best of both worlds.
At the same time, wireless multimode/multiband systems allow operators to economically grow their networks to support new services where the demand is highest. With multimode/ multiband handsets, subscribers can access new digital services as they become available while retaining the capability to communicate over existing analog networks.
The wireless system gives users access to digital channels wherever digital service is available while providing a transparent handoff when users roam between cells alternately served by various digital and analog technologies. As long as subscribers stay within cells served by advanced digital technologies, they will continue to enjoy the advantages provided by these technologies.
When they reach a cell that is supported by analog technology, they will have access only to the features supported by that technology. The intelligent roaming capability of multimode/multiband systems automatically chooses the best system for the subscriber to use at any given time. Third-generation radio systems are frequency-agile and extend this flexibility even further by supporting more modes and bands.
It is important to remember, however, that software radio systems may never catch up to encompass all the modes and bands that are available today and that may become available in the future. Users will always be confronted by choices. Making the right choice will depend on calling patterns, the features associated with the different technologies and standards, and the type of systems in use at international locations visited most frequently.
Multimode and multiband handsets have been available from several manufacturers since 1995. These handsets support more than one technology for their mode of operation and more than one frequency band. An example of a multimode wireless system is one that supports both AMPS and Narrowband AMPS (N-AMPS). Narrowband AMPS is a system-overlay technology that offers enhanced digital-like features, such as Digital Messaging Service, to phones operating in a traditional analog-based AMPS network.
Among the vendors offering dual-mode AMPS/N-AMPS handsets is Nokia, the world’s second-largest manufacturer of cellular phones. An example of a multiband wireless system is one that supports GSM at both 900 and 1800 MHz in Europe. Among the vendors offering dual-band GSM handsets is Motorola. The company’s International 8800 Cellular Telephone allows GSM 1800 subscribers to roam on either their home or GSM 900 networks (where roaming agreements are in place) using a single cellular telephone.
Of course, handsets can be both multimode and multiband. Ericsson, for example, offers dual-band/dual-mode handsets that support communication over both 800-MHz AMPS/Digital AMPS (D-AMPS) and 1900-MHz D-AMPS networks. Subscribers on a D-AMPS 1900 channel can hand off both to and from a D-AMPS channel on 800 MHz as well as to and from an analog AMPS channel.
Multimode and multiband wireless systems allow operators to expand their networks to support new services where they are needed most, expanding to full coverage at a pace that makes economic sense. From the subscribers’ perspective, multimode and multiband wireless systems allow them to take advantage of new digital services that are initially deployed in large cities while still being able to communicate in areas served by the older analog technologies.
With its multimode capabilities, the wireless system preferentially selects a digital channel wherever digital service is available. If the subscriber roams out of the cell served by digital technology—from one served by CDMAto one served by AMPS, for example—a handoff occurs transparently. As long as subscribers stay within CDMAcells, they will continue to enjoy the advantages the technology provides, such as better voice quality and soft handoff, which virtually eliminates dropped calls.
When subscribers reach a cell that supports only AMPS, voice quality diminishes and the chances for dropped calls increases. However, these multimode/multiband handsets are not software-programmable. They rely instead on packaging dual sets of hardware in the same handset.
Miniaturization of the various components makes this both practical and economical, but this approach has its limitations when the number of modes and frequencies that must be supported goes beyond two or three. Beyond that point, a totally new approach is required that relies more on programmable components.
Despite the promising concept of software-defined radios, the rollout of consumer products that use the technology has been slow. In September 2001, the FCC adopted rule changes to accommodate the authorization and deployment of software-defined radios. Under the previous rules, if a manufacturer wanted to make changes to the frequency, power, or type of modulation for an approved transmitter, a new approval was required, and the equipment had to be relabeled with a new identification number.
Because software-defined radios have the capability of being reprogrammed in the field, these requirements could be overly burdensome and hinder the deployment of software-defined radios to consumers. Under the new rules, software modifications in a software-defined radio can be made through a “permissive change” that has a streamlined filing process.
The FCC identification number will not have to be changed, so equipment in the field will not have to be relabeled. These permissive changes can be obtained only by the original grantee of the equipment authorization. To allow for changes to equipment by other parties such as software developers, the FCC will permit an optional “electronic label” for software-defined radios, in which the FCC identification number could be displayed on a liquid-crystal display (LCD) or similar screen.
It will allow another party to obtain an equipment approval in its name and become the party responsible for compliance instead of the original grantee. The FCC also requires that a grantee take adequate steps to prevent unauthorized software modifications to radios, but it declined to set specific security requirements at this time. This will allow manufacturers flexibility to develop innovative equipment while at the same time provide for oversight of the adequacy of such steps through the equipment authorization process.
Software radio architectures not only reduce the complexity and expense of serving a diverse customer base, they also simplify the integration and management of rapidly emerging standards. With software-based radio systems, access points, cell sites, and wireless data network hubs can be reprogrammed to meet changing standards requirements instead of replacing them or maintaining them in parallel with a newer infrastructure.
From the perspective of users, the same hardware would continue to be used—only the software gets upgraded. This could signal the end of outdated cellular telephones. Consumers will be able to upgrade their phones with new applications—much as they would purchase new programs to add new capabilities to their computers. Although the benefits are clear, commercial software-defined radio systems are still a few years away. Until they become available, users will have to make do with the current generation of multimode/multiband handsets.