RF Technology Enablers for Software-Defined Radios

The last few years have witnessed an emerging consensus that traditional methods of spectrum management are both technically and economically inefficient. In the United States, the FCC has responded to this through several changes in spectrum management, including the wellknown ultrawide-band program, and the “cognitive radio” initiative at the beginning of 2004. The background behind this initiative, and cognitive radio technology in general is given in [1]. Spectral occupancy measurements have demonstrated that many commercially licensed frequency bands at desirable frequencies are lightly occupied. For example, measurements by the NAF showed that less than 2% of the spectrum from 700–800 MHz, and less than 20% of the spectrum from 30 MHz to 1000 MHz, was regularly utilized in a typical urban area [2]. So, although this spectrum has been completely allocated, it is lightly used. Similar results have been demonstrated for frequencies above 1 GHz. These lightly used frequency bands represent a lost economic opportunity for new wireless services and industries, but an opportunity to create a wide range of new services if acceptable solutions can be found to widen the use of the spectrum. In order to take advantage of this opportunity, technologies need to developed that can utilize the spectrum “opportunistically,” without interfering with existing services and devices. The term “cognitive radio” was coined to describe a range of technologies that detect whether a particular segment of the radio spectrum is currently in use, and to jump into and out of the temporarilyunused spectrum very rapidly, without interfering with the transmission of other authorized users. Cognitive radio approaches are being developed for a variety of applications. One simple example of this is in the commercial digital wireless area. In this case, the proliferation of multiple