nd accepts a degree of overlap among the spheres. This is not acceptable in our case, which concerns a sphere packing problem. Consequently, we do not introduce oversampling, and the number of samples in the space is fixed directly by the bandwidth W'. M. D. Migliore It is also interesting to note that Eq. 2.12 shows a close relationship between (s) and W'. Indeed, in Eq. 2.12 the value of W' can be chosen at our will. Hence, the spatial bandwidth can assume an arbitrary value. Indeed, the fundamental quantity that characterizes the set of waveforms is the NDF of the bandlimited functions. From this point of view, the bandwidth is the NDF per unit observation interval, and s (s) is scaled by the value chosen for W' For example, in case of time-varying waveforms, the numerical value of the bandwidth B is fixed by the amount of unit time, which is the second in MKSA system of units. In the space domain, the use of a unit quantity dividing the space in uniform steps, as meters or degrees in case of observation angles, is not appropriate since the degrees of freedom are not evenly distributed in the space. A common choice is to scale s such that s ranges from -IT to IT in all the space, but other intervals can be chosen. The numerical value of W' will be fixed by the specific choice. Despite the mathematical complexity, the y and su curves have a simple intuitive explanation. As previously discussed, with reference to the phase of the electromag- netic field at an observation point, its value depends on two different phenomena. The first one relates to information that is encoded in some specific values of the phase of the radiated field. The second one is due to the propagation delay. This second phenomenon does not convey information and must be "discarded" in the sampling representation. With reference to Fig. 2.7, let us consider two radiating point sources, the source Sa and the source Sb, and an observation point P having distance ra and rb from Sa and Sb, respectively. The signals radiated by the two sources r