ng it possible to apply the theory, for example, to smart controllable surfaces [19]. Hereinafter the Number of Degrees of Freedom associated with the spatial variation of a harmonic field will be referred to as SNDF (Spatial Number of Degrees of Freedom). At the end of this paragraph, it is impossible not to notice that in the DPL spatial information is linked to one of the most fundamental properties of our universe, the finite speed of light. Consequently, with reference to the description of a communication system at a macroscopic level, there are no layers with more fundamental limits than the DPL. 2.4 The Space-Time Polarization Number of Degrees of Freedom (STPNDF) In previous section the analysis was focused on the spatial channel. Let us consider now the role of time and polarization in the communication process. 2 Information Flows at the DPL Level Time domain representation was discussed at the beginning of the previous section and will not be repeated. Let us just remember that in case of a signal having effective bandwidth B and observed for a time interval T, we have that the TNDF is given by the time-bandwidth product, i.e., TNDF 2 2BT, wherein B is the bandwidth and T the observation interval. In previous section it was also pointed out that there are strong similari- ties between temporal and spatial communication. Basically, the main difference between the use of temporal and spatial resources in a communication system is the physics behind the set of waveforms that can be used to encode information, which provides stricter limitations for spatial waveforms. In this perspective, the theorems of electromagnetism can provide us with important information on these limitations. For example, Maxwell's equations involve the vectors E and H, i.e., six scalar components. However, the uniqueness theorem states that the configuration of the electromagnetic field outside any closed surface surrounding the sources and radiating in free space is uniquely defined by only two components of the electric or magn