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.

As usual, the industry remains a step ahead of regulators. In recent years, developments in two key areas, cognitive radio and RF spectrum multi-purposing, have allowed for increased spectral efficiency while inspiring engineers to push the envelope even further.

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).

The carrier, far from being useless, correlates with the information to enhance reception. By using the carrier to synchronize the transmitter and the receiver, recovery of the relatively weak information pulse is simplified. Compared to UWB, xMax requires less power, as UWB must build the timing function into the information borne by the signal, which increases power.

The wavelet pass filter (WPF) is the key to the xMax system. This device allows the receiver to extract the relatively weak information pulse from the received signal while simultaneously attenuating the narrowband interference and noise from legacy and neighboring users in the adjacent sidebands. Because of the individual RF cycle modulation, the WPF uses the signal's peak power, rather than the average power, to extract the information pulse. Another benefit of individual RF cycle modulation is that nearly all of the power is found in the carrier, resulting in an average power spectral density substantially below that of the FCC mandated UWB spectrum (Fig. 2, again).

The carrier itself occupies little bandwidth while the information-bearing signal is spread over maximum 100-MHz sideband, giving it the appearance of a UWB system. However, the power spectrum is so low in the adjacent bands that the legacy user of that spectrum would experience minimal or insignificant interference. These characteristics enable the use of narrow bandwidth slivers (6-kHz voice channels) for the carrier wave and use up to 50 MHz on either side of the channel without causing interference to users of adjacent spectral bands. Because xMax sideband emissions fall below the noise floor, legacy users can continue normal operation while xMax simultaneously delivers a second information bearing signal, thereby allowing for spectrum reuse.

UWB vs. xMax
An xMax-enabled system has several advantages of over a UWB network. Primarily, whereas UWB emissions require several gigahertz of spectrum, the "narrowband" version of xMax only requires sidebands on the order of several megahertz. The carrier synchronous nature of xMax also bests UWB, which uses thousands of pulses to represent one symbol.

Paradoxically, UWB is often designed as a PAN technology for use in the 3.1- to 10.6- GHz range and other limited uses in higher bands (24 GHz), leading to potentially high transmitter density. Given the amount of power emitted into adjacent bands, the cumulative likelihood of interference is high. In contrast, xMax is designed as a WAN technology, leading to a low transmitter density and lower interference potential. FCC rules also prohibit UWB applications from using spectrum below the 3.1-GHz band, whereas xMax is designed for sub-GHz use.

Lastly, xMax is a more efficient, agile system that requires as little as 6 MHz for broadband data transmission and can frequency-hop to vacant spectrum. As stated, the xMax signal is carrier-synchronous, making detection easier. UWB, on the other hand, doesn't use a carrier; timing must be embedded in the information, requiring large contiguous swaths of spectrum. Note that UWB requires higher signal power when measured using equivalent resolution bandwidth.

Potential applications
The applications for such a system are widespread, particularly when used in a fixed wireless system. With xMax, a provider could deliver high data rates to businesses and homes. And by using lower frequency spectrum, greater signal distance and penetration can be achieved. As a result, providers needn't build as many access points or towers. For example, Qualcomm recently bought nationwide licenses for its proposed "mediacast" service in the 700-MHz band to deliver one-way high-quality video and audio. According to the company, such spectrum permits a nationwide network with "30 to 50 times fewer towers than cellular and higher frequency-based systems." The xMax prototype system uses a narrowband VHF paging channel, offering even greater distance capabilities.