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UWB competitor squeezes more bits through limited
spectrum
Hybrid technology combines
aspects of narrowband carrier systems and low-powered wideband
PPM.
By Joseph Bobier and Stuart
Schwartz, xG Technology
Wireless Net DesignLine
With the increased demand for
wireless technologies, industry leaders are looking to see how
Washington policy makers 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 the higher
microwave frequencies. The Commission''s move this winter to
make the 3.65- to 3.7-GHz 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. 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 combinin |
