Characteristics and Benefits of a Software Radio
Implementation of the ideal software radio would require either the digitization at the antenna, allowing complete flexibility in the digital domain, or the design of a completely flexible radio frequency (RF) front-end for handling a wide range of carrier frequencies and modulation formats. The ideal software radio, however, is not yet fully exploited in commercial systems due to technology limitations and cost considerations.
A model of a practical software radio is shown in Figure 1.1. The receiver begins with a smart antenna that provides a gain versus direction characteristic to minimize interference, multipath, and noise. The smart antenna provides similar benefits for the transmitter. Most practical software radios digitize the signal as early as possible in the receiver chain while keeping the signal in the digital domain and converting to the analog domain as late as possible for the transmitter using a digital to analog converter (DAC). Often the received signal is digitized in the intermediate frequency (IF) band. Conventional radio architectures employ a super heterodyne receiver, in which the RF signal is picked up by the antenna along with other spurious/unwanted signals, filtered, amplified with a low noise amplifier (LNA), and mixed with a local oscillator (LO) to an IF. Depending on the application, the number of stages of this operation may vary. Finally, the IF is then mixed exactly to baseband. Digitizing the signal with an analog to digital converter (ADC) in the IF range eliminates the last stage in the conventional model in which problems like carrier offset and imaging are encountered. When sampled, digital IF signals give spectral replicas that can be placed accurately near the baseband frequency, allowing frequency translation and digitization to be carried out simultaneously. Digital filtering (channelization) and sample rate conversion are often needed to interface the output of the ADC to the processing hardware to implement the receiver. Likewise, digital filtering and sample rate conversion are often necessary to interface the digital hardware that creates the modulated waveforms to the digital to analog converter. Processing is performed in software using DSPs, field programmable gate arrays (FPGAs), or application specific integrated circuits (ASICs). The algorithm used to modulate and demodulate the signal may use a variety of software methodologies, such
Figure 1.1: Model of a Software Radio.
as middleware, e.g., common object request broker architecture (CORBA), or virtual radio machines, which are similar in function to JAVA virtual machines. This forms a typical model of a software radio.
The software radio provides a flexible radio architecture that allows changing the radio personality, possibly in real-time, and in the process somewhat guarantees a desired QoS. The flexibility in the architecture allows service providers to upgrade the infrastructure and market new services quickly. This flexibility in hardware architecture combined with flex-ibility in software architecture, through the implementation of techniques such as object-oriented programming and object brokers, provides software radio with the ability to seam-lessly integrate itself into multiple networks with wildly different air and data interfaces. In addition, software radio architecture gives the system new capabilities that are easily implemented with software. For example, typical upgrades may include interference rejection techniques, encryption, voice recognition and compression, software-enabled power minimization and control, different addressing protocols, and advanced error recovery schemes. Such capabilities are well-suited for 3G and 4G wireless requirements and advanced wireless networking approaches. In summary, five factors are expected to push wider acceptance of software radio.
MultifunctionalityWith the development of short-range networks like Bluetooth and IEEE 802.11, it is now possible to enhance the services of a radio by leveraging other devices that provide complementary services. For instance, a Bluetooth-enabled fax machine may be able to send a fax to a nearby laptop computer equipped with a software radio that supports the Bluetooth interface. Software radio's recon-figuration capability can support an almost infinite variety of service capabilities in a system.
Global mobilityA number of communication standards exist today. In the 2G alone, there are IS-136, GSM, IS-95/CDMA1, and many other, less well known standards. The 3G technology tried to harmonize all the standards. However, there are many standards under the 3G umbrella. The need for transparency, i.e., the ability of radios to operate with some, preferably all, of these standards in different geographical regions of the world has fostered the growth of the software radio concept. Military services also face a similar issue with incompatible radio standards existing between as well as within branches of the military.
Compactness and power efficiencyMultifunction, multimode radios designed using the "Velcro" approach of including separate silicon for each system can become bulky and inefficient as the number of systems increases. The software radio approach, however, results in a compact and, in some cases, a power-efficient design, especially as the number of systems increases, since the same piece of hardware is reused to implement multiple systems and interfaces.
Ease of manufactureRF components are notoriously hard to standardize and may have varying performance characteristics. Optimization of the components in terms of performance may take a few years and thereby delay product introduction. In general, digitization of the signal early in the receiver chain can result in a design that incorporates significantly fewer parts, meaning a reduced inventory for the manufacturer.
Ease of upgradesIn the course of deployment, current services may need to be updated or new services may have to be introduced. Such enhancements have to be made without disrupting the operation of the current infrastructure. A flexible architecture allows for improvements and additional functionality without the expense of recalling all the units or replacing the user terminals. Vocoder technology, for example, is constantly improving to offer higher quality voice at lower bit rates. As new vocoders are developed, they can be quickly fielded in software radio systems. Furthermore, as new devices are integrated into existing infrastructures, software radio allows the new devices to interface seamlessly, from the air-interface all the way to the application, with the legacy network.
Users/customers expect service regardless of the geographical areas in which they travel and the wireless technologies that are in use in different regions in the world, but carrying several devices that cover the broad range of technology alternatives is impractical. Users expect one device to utilize services in all regions, which is possible only by reconfiguring the receiver to the air-interface standards used in the respective regions. By dynamically downloading the software to cover the needed air-interface standard, perhaps through transmission of the software configuration to the remote terminal, such over-the-air updates will allow for speedy implementation of software upgrades and new features.