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In this paper, main issues related to spectral analysis of ultra-wideband (UWB) signals are reviewed and discussed. The effects of pulse shape, code sequences and modulation format are evidenced in the expression of power spectral density of the signal. As for the modulation format, both pulse position modulation (PPM) and pulse amplitude modulation (PAM) are considered, while time hopping (TH) is considered for coding. The main goal of the proposed analysis is to define a framework for UWB signal design, that accounts for coexistence issues with other systems in the presence of uncoordinated operations and may drive a trade-off solution between UWB design objectives (e.g., maximizing link capacity by reducing the impact of narrowband interference and multiple access interference, maximizing ranging accuracy) and constraints (typically the amount of interference towards other systems, such as the emission mask imposed by FCC). The final purpose is to explicitly include coexistence issues in a rigorous design approach of distributed ad-hoc wireless sensor networks that rely on a UWB physical layer.