Ultra-wideband (also UWB, and ultra-wide-band, ultra-wide band, etc.) may be used to refer to anything with a very large bandwidth (e.g.: a type of sampling rate in the Speex speech codec). This article discusses the meaning in radio communications.
Ultra-Wideband (UWB) is a technology for transmitting information spread over a large bandwidth that should, in theory and under the right circumstances, be able to share spectrum with other users. A February 14, 2002 Report and Order by the Federal Communications Commission (FCC)  authorizes the unlicensed use of UWB in 3.1–10.6 GHz. This is intended to provide an efficient use of scarce radio bandwidth while enabling both high data rate personal-area network (PAN) wireless connectivity as well as longer-range, low data rate applications as well as radar and imaging systems. More than four dozen devices have been certificated under the FCC UWB rules, the vast majority of which are radar, imaging or positioning systems. Deliberations in the International Telecommunication Union Radio communication Sector (ITU-R) have resulted in a Report and Recommendation on UWB in November of 2005. National jurisdictions around the globe are expected to act on national regulations for UWB very soon.
Ultra Wideband was traditionally accepted as impulse radio, but the FCC and ITU-R now define UWB in terms of a transmission from an antenna for which the emitted signal bandwidth exceeds the lesser of 500 MHz or 20% bandwidth. Thus, pulse-based systems—wherein each transmitted pulse instantaneously occupies a UWB bandwidth, or an aggregation of at least 500 MHz worth of narrow band carriers, for example in orthogonal frequency-division multiplexing (OFDM) fashion—can gain access to the UWB spectrum under the rules. Pulse repetition rates may be either low or very high. Pulse-based radars and imaging systems tend to use low repetion rates, typically in the range of 1 to 10 mega pulses per second. On the other hand, communications systems favor high repetition rates, typically in the range of 1 to 2 giga pulses per second, thus enabling short-range gigabit-per-second communications systems. Each pulse in a pulse-based UWB system occupies the entire UWB bandwidth, thus reaping the benefits of relative immunity to multipath fading (but not to intersymbol interference), unlike carrier-based systems that are subject to both deep fades and intersymbol interference.
The FCC power spectral density emission limit is the same as for unintentional emitters in the UWB band, but is significantly lower in certain segments of the spectrum.
A significant difference between traditional radio transmissions and UWB radio transmissions is that traditional transmissions transmit information by varying the power/frequency/and or phase in distinct and controlled frequencies while UWB transmissions transmit information by generating radio energy at specific times with a broad frequency range.
One of the valuable aspects of UWB radio technology is the ability for a UWB radio system to determine "Time of Flight" of the direct path of the radio transmission between the transmitter and receiver. With any radio transmission the signals reflect off of metallic objects and results in different radio signal paths that then can arrive at the receiver later in time and interfere with radio transmission that went directly from the transmitter to the receiver. With frequency based transmissions the sinusoidal waves add/subtract at the receiver antenna and make it difficult or impossible to distinguish the direct transmission path from the reflected paths. This is called "multi-path fading" and "multi-path interference". However, with UWB transmissions the time encoding can be randomly dithered and the receiver can then determine which is the direct path. With a bidirectional system or a radar system this allows distances to be determined much more accurately.
Due to the extremely low emission levels, UWB systems tend to be short-range. However, due to the short duration of the UWB pulses, extremely high data rates are possible, and data rate can be readily traded for range by simply scaling the number of pulses per data bit. Conventional OFDM technology can also be used subject to the minimum bandwidth requirement of the regulations. High data rate UWB can enable wireless monitors, the efficient transfer of data from digital camcorders, wireless printing of digital pictures from a camera without the need for an intervening personal computer, and the transfer of files among cell phone handsets and other handheld devices like personal digital audio and video players.
UWB also has the potential to enable "see-through-the-wall" imaging technology and high-precision time-of-arrival-based localization approaches.  It is expected to exhibit excellent efficiency with a spatial capacity of approximately 1,000,000 bit/s/m².
Semiconductor companies providing complete solutions for Certified Wireless USB and Bluetooth 3.0, based upon the WiMedia ultra-wideband (UWB) technology, as well as proprietary solutions.(Errata: There is no Bluetooth spec. 3.0 ready)