The proposed spectrum-overlay scenario for UWB communication systems brings new technical challenges that must be considered during the UWB system design. The main objectives of this paper are to address the minimization of the possible interference to and from narrow-band systems that are overlaid by UWB signals, and to study the effects of the narrow-band interference on the channel estimation process. UWB systems based on either a conventional DS/CDMA technology or a recently proposed spectral-encoding technique are considered. It is shown that this new technique is able to suppress narrowband interference and to not cause noticeable interference to overlaid systems by using a convenient spectral signature.