UWB is an umbrella term that mainly indicates that a very large absolute bandwidth (Ba > 500 MHz) or a very large relative bandwidth (Br = 2[fu – fl]/[fu + fl] > 0.2) in the RF spectrum is used instantaneously by the system. With this definition, no special purpose or application and no special modulation is defined but it implies that the components of the system must be capable of handling this wide spectrum. As already mentioned in the previous chapter for RF frontends, on the whole it is the relative bandwidth that poses new challenges, so system aspects for a very large relative bandwidth are mainly discussed here. This chapter provides a mathematical description of the UWB radio channel including the antennas and measures to characterize the UWB performance of the analog frontend, including the radio channel in the frequency domain (FD) and in the time domain (TD). The chapter presents two methods to exploit an ultra-wide bandwidth: the transmission of short pulses in the baseband (impulse radio transmission), and the transmission by a multi-carrier technique called orthogonal frequency division multiplexing (OFDM). For impulse radio, the most common pulse shapes are introduced together with methods to generate them. Finally, modulation and coding techniques are considered as well as basic transmitter and receiver architectures. The coordinate system is given in Fig. 2.1.