Single-platform convergence is on track

Software radio has now been widely accepted as the implementation of choice for a variety of platforms and applications. It simplifies multichannel, multimission scenarios where reuse of the equipment is essential for achieving a wide range of functionality and size and weight reduction. This feature of software radio serves to integrate radio, signal intelligence, r adar and communications functionality in a common platform. This is an incredible advantage over conventional systems and serves to reduce footprint and weight for critical systems. Software radio is making such convergence possible.

The key elements of a software radio are high-speed and high-performance analog-to-digital and digital-to-analog converters placed as closely as possible to the antenna, followed by digital downconverters (DDCs) and digital upconverters (DUCs). The DDCs and DUCs offer a very convenient way to digitally downconvert and upconvert the channel and thus reduce the baseband data rate. That lets processing be performed in the digital domain, thereby enabling software control of the radio functions.

Software radio inherently simplifies the front-end RF equipment. This could consist of a very simple RF conditioner (comprising a low-noise amplifier/power amplifier and antialiasing bandpass filters) or a simple block downconverter or upconverter. The RF conditioner is used for direct-sampling transceivers for the VHF band.

Current technology for high-speed (more than 100-MHz) 14-bit converters limits the spurious-free dynamic range for the A/D and the D/A to about 90 dB and 65 dB, respectively. Most A/Ds include a wide-bandwidth front end to permit direct undersampling of VHF signals. Today's digital down- and up-converters can handle programmable flat-top bandwidths up to 40 MHz. Increasing user demand, coupled with innovation, is continuously improving those limits.

The DDCs and DUCs may be implemented either in the baseband FPGA or in specific DDC and DUC chips with the flexibility to make a software-configurable radio. The latter approach frees up logic resources in the FPGA, which can then be used for other configurable functions.

On the other hand, implementing the DDC or DUC in FPGA allows the user more control over the converter functionality at the expense of consuming the FPGA's logic resources.

The baseband-processing block performs t he modulation, demodulation, coding and decoding functions. The baseband processing may be implemented in and FPGA, DSP or general-purpose processor (GPP). FPGAs are becoming increasingly popular for such high-speed, compute-intensive, reconfigurable applications as fast Fourier transform, finite-impulse response and other multiply-accumulate operations. Reconfigurable cores are available from FPGA vendors and other board vendors to enable implementation of modulator, demodulator and codec functionality in the FPGA.

Increasingly, the industry is moving toward implementing the front-end data acquisition and pre-processing as open-architecture PCI mezzanine card (PMC) modules. Vendors such as Interactive Circuits and Systems Ltd. offer a variety of software-radio PMC modules. The offerings contain the A/D, DDC, D/A, DUC and a large on-board FPGA and serve as truly open-architecture reconfigurable software radio platforms (see figure). Increasingly, for software-radio apps, the carrier board hosting the PMC module has multiple general-purpose processors that perform the higher-layer functions of a communication link as well as the housekeeping functions. Such partitioning is instrumental in implementing radios that are single-slot, multichannel, multimode, software-configurable and high-performance.

As the packing density of FPGAs increases (more gates per unit area), this will become the dominant trend. The on-board FPGAs in the PMC modules together with the GPP in the carrier board serve to reduce slot count and provide the most bang for the buck in implementing a compact yet versatile radio.

The expanding capabilities of FPGAs also mean a huge demand for application development for large FPGAs. Although there has been a huge improvement in application development tools, FPGA design should still be considered a hardware (albeit flexible) development exercise and requires a different skill set than software development. As a result, users are turning to vendors with the required expertise to provide complete system solutions.

In the future we will see an ever-increasing need for software radio, and radio designers for multichannel, multimission applications will ignore software radio at their own peril.

Angsuman Rudra is director of radio products at Interactive Circuits and Systems Ltd. (Ottawa).

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