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UWB competitor squeezes more bits through limited spectrum

Posted: 16 Sep 2005 ?? ?Print Version ?Bookmark and Share

Keywords:rf? fcc? spectrum? bandwidth? regulation requirement?

With the increased demand for wireless technologies, industry leaders are looking to see how policymakers will confront the vexing problem of RF spectrum scarcity. Of late, the FCC has opted to relieve the pressure by making additional swaths of licensed spectrum available for commercial use, typically in higher microwave frequencies. FCC's move to make the 3.65GHz to 3.7GHz band available for nationwide license with minimal regulatory requirements is one such example.

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. Among the items on that list are identifying technologies capable of operating at above 100GHz; 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 areascognitive radio and RF spectrum multipurposinghave 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.

Deliberately and continuously applied, this process results in improved spectrum utilization and is the basis for many of today's wireless standards. W-CDMA high-speed downlink packet access, 3g mobile wireless technologies and CDMA1x EvDO all use a cognitive modulation process that attempts to get the highest throughput from a limited spectrum. Mobile wireless is not the only area using an adaptive or cognitive modulation process, however. WLAN technology (802.11a) and fixed wireless (Flash-OFDM) use similar processes to improve overall spectrum utilization.

Spectrum multipurposing

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

The notion of RF spectrum multipurposingexploiting spectrum "gray spaces" or unused regions of dedicated spectrumis a fairly significant departure from the single-use allocation scheme the FCC employs today. AM/FM radio stations, paging 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.

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

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.

The carrier, far from being useless, correlates with the information to enhance reception. By using the carrier to synchronize the transmitter and 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.

The carrier itself occupies little bandwidth, while the information-bearing signal is spread over maximum 100MHz sideband, giving it the appearance of a UWB system. However, the power spectrum is so low in adjacent bands that the legacy user of that spectrum would experience minimal or insignificant interference. These characteristics enable the use of narrow bandwidth slivers (6kHz voice channels) for the carrier wave and use up to 50MHz 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, thus allowing for spectrum reuse.

UWB vs. xMax

An xMax-enabled system has several advantages over a UWB network. Primarily, while 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 is better than UWB because it uses thousands of pulses to represent one symbol.

Paradoxically, UWB is often designed as a PAN technology for use in the 3.1GHz to 10.6GHz range and other limited uses in higher bands (24GHz), 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 3.1GHz band, while xMax is designed for sub-GHz use.

Lastly, xMax is a more efficient, agile system that requires as little as 6MHz 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, does not 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.

Joseph Bobier, President of Operations

xG Technology LLC

Stuart Schwartz, Professor

Princeton University

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