ing to two terms, the first increasing with the square of the central frequency 3 It is understood that in case of Channel State Information (CSI), it is possible to distribute available power among the subchannels according to the waterfilling algorithm in order to maximize the mutual information [75]. 2 Information Flows at the DPL Level fc and the second increasing with the square of the bandwidth B. From a practical point of view, Eq. 2.15 suggests that there are two main ways to increase the SNDF: increasing frequency and increasing fractional bandwidth. In particular, for a fixed central frequency, an enlargement of the bandwidth B gives, besides a linear increase of the first addend, a cubic increase of the second addend. This observation could be useful in case of 6G sub-6 GHz band communication systems that will be forced to use the space domain in a more aggressive way compared to the ones using millimeter and THz frequencies [76]. 2.5 Control of Information Flows at the DPL Level The channel operator plays a key role in limiting the amount of information transmissible along the wireless channel. This suggests to modify the channel operator using devices capable to manipulate the electromagnetic field. We will call these devices Electromagnetic field Processing Devices (EPDs). Loosely speaking, EPDs modify the shape of the set Y of the fields received on the observation domain. Consider, for example, the two-channel MIMO communication system with BPSK modulation shown in Fig. 2.10. The channel operator maps the set of currents X into a highly elongated set Y. Due to the geometry of Y, there is an overlapping of the spheres of uncertainty which prevents the correct reception of the 2 bits. Any attempt to use two spatial channels results in incorrect data reception. The EPD is placed between the TX and RX (Fig. 2.11) modifying the TX-RX radiation operator and consequently the geometry of set Y. In practice, the EPD collects information before it is lost and retransmits it, as shown in Fig. 2.11. Physical 