Software-defined radio dials in

Years ago, high-end radios for military communications applications went digital. While the RF and IF stages of the devices stayed pretty much the same, once the signal reached baseband frequency, it was digitized, and all further filtering, demodulation, error recovery and decoding were done in the digital domain.

But two trends quickly took root in the digital radio sector that today have blossomed into a movement: software-defined radio, or SDR. The first was a relentless drive to move digitization closer to the antenna, on both the transmit and receive paths. Ideally, directly converting the antenna signals would be wonderful, as it would put all the functionality of the radio in the digital domain, where transfer functions can be almost arbitrarily complex and noise can be arbitrarily small. But of course, this pressure is continually met by a back pressure: the limitations of data converters. Today the industry is closing in on the concept of direct conversion of RF. As a precursor, we are seeing increasing digital monitoring and control of RF analog functions, from gain control and filter center points to power amp predistortion.

The second trend is the increasing commoditization and programmability of the digital hardware. Recently, high-end DSP chips have grabbed large chunks of the data plane tasks, at least in systems with relaxed energy efficiency constraints. FPGAs, offering both enormous signal-processing speed and reconfigurability, have displaced many hardwired data paths. The whole digital load for a GSM basestation has been migrated even further, to a multiprocessor server running commercial Linux.

As these trends have grown, the SDR movement has spread beyond its initial soil as well. From meeting military needs for a multiband, multiprotocol mobile radio, SDR is now being applied to cellular basestations, mobile radios and even long-range readers for RFID tags. If the trend continues, we're likely to see cellular handsets and personal satellite radio in the near future. Stay tuned.

EE Times would like to thank the members and staff of the Software-Defined Radio Forum for their assistance in assembling this special section.