Coexistence and compatibility of spectrum are important aspects needed to be considered in impulse radio ultra-wideband (IR-UWB) communications. The proper waveforms of signal can decrease the influence of interference between the IR-UWB and extant systems, and improve the efficiency of spectrum resources. In this paper, based on time-frequency analyses, a novel method to design the waveforms of IR-UWB signals is proposed. The constructed waveforms are with raised cosine roll-off frequency spectrum and less time duration. Furthermore, series of time domain waveforms with orthogonal properties constructed by this method can be adopted at high-level modulated UWB systems.