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UWB isn't exactly new and some may argue that the technology goes all the way back to Marconi and his early impulse transmissions using spark gaps. Realistically, however, the roots of UWB research reach back to the early 1960s when tools and techniques became available to study electromagnetic pulses in some depth.
Alternatively referred to as impulse, carrier-free, baseband, nonsinusoidal and time-domain signaling, the term UWB wasn't actually coined until 1989 by the U.S. Dept. of Defense (DoD). The DoD developed UWB for radar, location and communications applications. UWB's ability to operate below the noise floor — which makes for a low probability of detection at low data rates — was particularly attractive for clandestine communications.
UWB technology is based on the generation of extremely short digital pulses in the subnanosecond range (1 to 1,000 picoseconds). To transmit information, such pulse trains can be modulated any number of ways, including time, phase, amplitude and voltage. However, it is the fact that the pulses can be modulated directly by the baseband signal — instead of using a high-frequency carrier — that gives UWB radios their much-hyped simplicity of design. No longer are expensive, complex, large-footprint analog circuits required for carrier-signal generation on the transmit side and for carrier stripping on the receive side.
Because it is so short, the pulse's associated coherent frequency spectrum can be multiples of gigahertz wide, thereby dispersing the pulse's energy across many narrowband systems (such as GPS, cellular, PCS, satellite radio and the various wireless-network bands). The bandwidth and the center frequency of the pulse are determined naturally by its length, but typically some kind of filtering is used to limit the bandwidth to keep it within the recent FCC ruling.
This wide relative bandwidth allows UWB to penetrate walls and other obstacles, making possible capabilities such as through-wall imaging, while at the same time endowing it with a large degree of immunity to multipath interference relative to narrowband systems, a characteristic particularly appealing to the communications industry.
Also, because the amount of information any signal can carry is a trade-off between bandwidth, power and distance (with various modulation and coding schemes used to optimize the communications channel), the wide bandwidth of UWB signals has the allure of potentially very high data rates of up to 500 Mbits/s. It's important to note that the information-carrying capacity scales linearly with bandwidth, and logarithmically with power, making it much more attractive to designers to scale the bandwidth to achieve higher rates.
The FCC ruling in February 2003 was designed to curtail the interference this wide-bandwidth signal might have on GPS, military, ground/air navigation or cellular/PCS applications. The ruling specifically defines UWB as a signal with a bandwidth of 500 MHz or bigger, or 20 percent fractional at the -10 dBm point. The ruling limits UWB power to Part 15 limits (-41.25 dBm) operation over the 3.1- to 10.6-GHz band, though other bands are allowable below 900 MHz and roll-offs around the limits vary according to the application and whether it's for indoor or outdoor use.
Click here to view the latest FCC ruling on UWB in Word format.
Click here to view the latest FCC ruling on UWB in PDF format.