In other circles, however, the tone of the debate has shifted away from simple spectrum allocation solutions, relying more heavily on the industry's track record of innovation. Recently, for instance, Congress's investigative arm, the General Accounting Office or GAO, provided a checklist for policy makers to use in attempting to free up more spectrum and allocate its utilization optimally. Among the items on that list are identifying technologies capable of operating at above 100 GHz; development of advanced compression algorithms that would reduce spectrum demand; advancement of software-defined radios capable of changing their operating parameters; and the refining of spectrally-efficient waveforms.

Cognitive radio technology adapts its use of spectrum based on the real-time conditions of its operating environment. In the process, which is conceptually simple, the network identifies which users need service, determines which are operating in the best environment, and fixes on the most efficient data transmission scheme to satisfy the user's request (Fig. 1).

Deliberately and continuously applied, this process results in significantly improved spectrum utilization and is the basis for many of today's wireless standards. W-CDMA High Speed Downlink Packet Access (HSDPA), 3G mobile wireless technologies, and CDMA1x EvDO all employ a cognitive modulation process that attempts to get the highest throughput from a limited spectrum. Mobile wireless isn't the only area using an adaptive or cognitive modulation process, however. Wireless LAN technology (802.11a) and fixed wireless (Flash-OFDM) employ similar processes to improve overall spectrum utilization.

Spectrum multi-purposing
The limitation of existing cognitive radio technology is that users competing for access to throughput on the channel can't simultaneously receive service. Spectrum multi-purposing technologies attempt to address this quandary.

The notion of RF spectrum multi-purposing—exploiting spectrum "gray spaces" or unused regions of dedicated spectrum—is a fairly significant departure from the single-use allocation scheme the FCC employs today. AM and FM radio stations, paging services, and cellular services all use RF spectrum allocated by the FCC for one particular use. However, if technological advances enable spectrum dedicated for an FM radio station to simultaneously provide broadband wireless services to a small city without degrading the FM broadcast, the possibilities for wireless deployment would grow exponentially.

Ultra Wideband (UWB), with its low-power transmission profile, is a step in the right direction. However, UWB's sideband emissions aren't completely interference-free, requiring the use of higher frequency spectrum (upwards of 3 to 10 GHz), which has limited propagation characteristics.

The xMax solution
One modulation technique could potentially meet this challenge. Called xMax, the RF modulation scheme is a hybrid technology combining aspects of narrowband carrier systems and low-powered wideband pulse position modulation (PPM) that permits simultaneous spectrum reuse.

While prior schemes tried to move as much power as possible into the sidebands (where the information resides) and away from the carrier signal, xMax does the opposite, placing most of the power in the carrier to keep sideband energy emissions negligible. The xMax modulation is characterized by an RF spectrum utilization profile where adjacent channel spillover is so far below detectable levels that it has no effect on neighboring users (Fig. 2).