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This paper studies the performances of ultra wideband (UWB) systems in an AWGN channel when interference in the UMTS/WCDMA band is present. The interfering band in the frequency division duplexing mode has been considered to be fully covered for both uplink and downlink cases. The interference is actually modelled as sinc-pulse jamming having a spectrum in a predefined UMTS band. The uncoded UWB system performance is studied through the bit-error-rate as a function of signal-to-noise ratio as well as jamming-to-signal power ratio. Several modifications of a generic Gaussian pulse have been selected for the deeper study. The UWB systems are based on time hopping and direct sequence spread spectrum techniques, which utilizes a binary baseband pulse amplitude data modulation. The results showed that UWB system performance degradation is the highest when the interfering signal and the nominal center frequency of the UWB system overlap. UWB systems based on the time hopping concept seem to outperform those based on the corresponding direct sequence concepts. However, for high values of interfering power, the UWB performances are converging.