 of the most relevant is represented by the antennas. In 5G, antennas are no longer relatively simple devices with fixed beams, but become highly reconfigurable systems. The introduction of such sophisticated antennas represents a major step forward in an area that was previously of little interest in cellular systems. However, 5G antennas are just the starting point of a new trend in personal wireless communication [1-3]. 6G promises a much more aggressive use of the communication channel with an active control of it. In this new and revolutionary vision, the communication channel ends up being an uncontrollable bottleneck for the communication system and becomes a resource to be optimized, with a potential disruptive impact on the performance of the communication system. At the same time, antennas will be highly flexible elements that interact intelli- gently with the environment, which in turn plays an active role in the communication system [4, 5]. Concepts such as far field and radiation pattern, as well as simple propagation models, lose their ability to model communication channels, and more sophisticated analyses must be undertaken, prompting research on physical aspects [6-8]. This opens up new challenges in studying the interaction between radiating systems and the propagation environment which requires an analysis of the communication process at a finer level of detail than that normally considered in the communication system. M. D. Migliore (X) DIEI, University of Cassino and Souther Lazio, Cassino, Italy e-mail: mdmiglio@unicas.it © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. A. Matin (ed.), A Glimpse Beyond 5G in Wireless Networks, Signals and Communication Technology, https://doi.org/10.1007/978-3-031-13786-0_2 M. D. Migliore To clarify this point, let us consider the Open Systems Interconnection (OSI) model (see Fig. 2.1). The OSI model divides the data stream into seven layers starting from the highest level, the "application" layer, down to the lowest level, 