layer Deep physical layer Set X of the currents Set Y of the fields Fig. 2.10 Due to the overlapping of the sphere of uncertainty, the e-channel capacity is equal to 1: any attempt to transmit 2 bits on the spatial channel results in incorrect data reception M. D. Migliore Physical layer Deep physical layer Received Field Radiating currents Set X of the currents Set Y of the fields Electromagnetic field Processing Device (EPD) Fig. 2.11 The use of an EPD allows to preserve information obtaining an e-channel capacity equal Fig. 2.12 Example of the spatial information flow in absence of EPD; only one Sk curve intersects the observation domain S2, providing only a single spatial channel The Sk curves introduced in Sect. 2.3 allow a simple and intuitive visualization of the information flow. Let us consider the communication problem shown in Fig. 2.12, wherein D is the radiating source and S is the observation domain. The curves Sk of the source D are drawn in blue. Only one Sk curve intersects the observation domain (drawn as solid blue line). Accordingly, we have only one SNDF on S and only one spatial subchannel in the PL. In Fig. 2.13 an EPD is placed between the transmitter and the receiver. The Sk curves of the EPD are plotted in red. Two Sk curves of D (drawn as solid blue curves) intersect the EPD, indicating that the EPD is able to receive two spatial subchannels. Furthermore, two curves Sk of the EPD (drawn as solid red curves) intersect the observation domain S2, indicating that it is possible to have two spatial subchannels. However, the geometrical representation does not give a complete view of the problem, since it does not give information on the amplitude of the singular values associated with the channels. 2 Information Flows at the DPL Level Fig. 2.13 Example of the spatial information flow in presence of EPD; two Sk curves intersect the EPD from the source D, providing two spatial channels for D-EPD communication; furthermore, two curves intersect the observation domain So from the EPD, providing 