As explained in Chapter 1, a variety of electromagnetic narrow pulses, such as Gaussian, monocycle, Hermite, wavelet, and others, can be used for various UWB applications. The selection of the pulse shape is driven primarily by the desirable frequency band that the pulse is to occupy. However, the main characteristics of all pulses used in UWB systems are high-peak power and fast-rise time, typically less than 1 nanosecond. The generation of subnanosecond pulses with large amplitudes plays a critical role in UWB communications. Pulse generators are used not only in UWB transmitters but also in most UWB receivers for template generation to perform the correlation operation on the received pulsesas we'll see in Chapter 3.
Although a number of narrow-pulse generation techniques have been available for several years, mainly for radar applications, most are not suitable for generating UWB pulses. UWB pulse generation requires fast pulse repetition periods, a requirement that is difficult to achieve with most early pulse generation techniques.
Once the subnanosecond pulses are generated, their efficient radiation becomes a major challenge in UWB communications. Although classical antenna theory and design for narrowband signals is a mature field now, the radiation pattern and the pattern efficiency of antennas that have desired characteristics for UWB applications are active areas of research. The reason is that, with narrowband antennas, the shape of the radiated signal stays similar to the shape of the input signal. This is because antennas differentiate their input signal one or more times, and the derivatives of narrowband sinusoidal signals remain sinusoidal. However, in the case of UWB pulses, their derivatives have no resemblance to the original pulse.
In this chapter, we discuss some common short-pulse generation methods, as well as their advantages and disadvantages for UWB communications. Then we present a comprehensive discussion of UWB antennas, including basic UWB antenna concepts and UWB system and network considerations.