s associated with points whose distance is less than E are considered indistinguishable from y. Consequently, the maximum number of distinguishable waveforms on the receiver is given by the maximum number of functional points y E Y whose distance is larger than 2e. This number is called the Kolmogorov E-capacity of the set Y [47] and is the solution of a sphere packing problem, which consists in identifying the maximum number of non-overlapping open spheres having radius E. In order to apply the Kolmogorov theory to our problem, let us consider the model of the communication system at the DPL level drawn in Fig. 2.2. The transmitter is a harmonic electromagnetic source placed in the volume D having a surface area , while the receiver is an "observer" who has access to the value of the electromagnetic field on the observation curve S placed at a finite distance from the source D. Information is encoded in spatial variations of the electromagnetic field (Fig. 2.3) obtaining a "pure space communication system" [34]. Let us consider a component of the electric field in the phasor domain. Loosely speaking, the propagation process links the physical quantity associated with information in the transmission process, i.e., the charges in the antenna volume, to the physical quantity associated with the reception, i.e., the electromagnetic field configuration on the receiving antenna generated by the electric charges that are accelerated in the transmitting volume. This physical process is modeled by a compact operator [34, 46, 63, 64], called the "radiation operator": 2 Information Flows at the DPL Level Fig. 2.2 Communication system model; D, electromagnetic source bounded by the surface ; S2, observation domain; J, current density in D; E, electromagnetic field on So Observation curve E.M. sources Fig. 2.3 In the "pure space communication system," each information block is encoded in a different current distribution of the source and hence on a different radiated field spatial configuration on the observation domain S2; 