Ultra-wideband (UWB) communications systems can be defined as wireless communications systems with very large bandwidth, or very large fractional bandwidth.  Fractional bandwidth is defined as the ratio of the -10 dB bandwidth occupied by the signal to the center frequency of the signal:

Traditional communications systems typically use signals having a fractional bandwidth less than 0.01.  Wideband CDMA has a fractional bandwidth of approximately 0.02.  Many in the radar and communications fields consider signals with 0.25 or greater fractional bandwidth measured at the -3 dB points to be ultra-wideband (see Figure 1 for a comparative illustration of fractional bandwidths). A DARPA report which coined the term “ultra-wideband” used a 0.25 fractional bandwidth definition. In its First Report and Order allowing the commercial use of UWB systems, the FCC defined UWB signals as those which have a fractional bandwidth greater than 0.20 or a bandwidth greater than 500 MHz measured at the -10 dB points.