ase the number of singular values that must be retained is almost independent of the noise level and equal to Ok + 1 in Bucci definition. Obviously, under the same conditions, also the NDFé is almost independent of E. However, in presence of scatterers in the environment, the number of singular values that must be taken can be quite sensitive toward the noise level. This sensitivity is particularly high if the observation domain is in the reactive region of the source [35]. It is also interesting to note that when the electrical dimension of the source tends to infinity, the knee of the singular values becomes a step and the value of the NDFé as defined in this chapter coincides with the value of the NDF by Bucci. The maximum number of field configurations that can be distinguished on S is given by the maximum number of points on Y whose distance is larger than E [46, 47]. Such a set of points will be called Y , and, as previously discussed, is the solution of a "sphere-packing" problem, since it requires the identification of the maximum number of non-overlapping E-balls (i.e., balls having E radius) whose centers are elements of Y. Accordingly: the maximum amount of bits that can be transmitted in presence of an E level of uncertainty is the the Kolmogorov e-capacity of Y [35]. The geometric structure of the functional spaces involved in the propagation model has an important impact on the characteristics of the communication process seen at the level of the PL. For example, the parallel spatial communication subchannels that arise in MIMO communication systems [65] are a consequence of the separability of the Hilbert space. In particular, each coefficient yk of y is able to encode independent information, giving a spatial communication subchannel at the PL level [35]. However, the number of spatial subchannels is limited by the number of coefficients that can be retrieved in presence of noise. Equation 2.5 shows that there M.D. Migliore Fig. 2.5 An intuitive explanation of the meaning of NDF; when k increases 