equently limited to spherical sources and spherical observation domains [29, 31, 32]. In 2006, the Degrees of Freedom of the Field were introduced to quantify the amount of information transmitted by sources radiated toward observation domains both having a general shape on the basis of numerical simulations [33] and analytical approach [34]. Successively, Kolmogorov information theory was introduced in 2008 in order to analyze spatial information in case of a small Number of Degrees of Freedom [35]. Limitations regarding different aspects of communication using spatial channels have been further clarified in a number of papers [9, 36-45]. The results reported in the above literature are the backbone of the Deep Physical Layer, introduced in [19] as a common frame for the analysis of information carried by the electromagnetic field. It is worth pointing out that this chapter is focused on the Deep Physical Layer and does not discuss specific technological aspects. Instead, the goal is to identify an upper limit in terms of spatial information that can be reliably transmitted by the electromagnetic field. A list of the main variables and acronyms used in this chapter is shown in Table 2.2. 2 Information Flows at the DPL Level Table 2.2 List of variables and acronyms employed Radiation operator Active Large Aperture Time-varying signal bandwidth Kolmogorov E capacity Channel State Information Deep Physical Layer Source domain Electric field on the observation domain Reduced electric field on the observation domain No Channel State Information Length of a linear electromagnetic source Number of Degrees of Freedom Number of Degrees of Freedom at € level Multiple Input Multiple Output Current density distribution on the electromagnetic source Physical Layer Passive Large Aperture Polarization Number of Degrees of Freedom Reconfigurable Intelligent Surfaces Signal Noise Ratio Single Input Single Output Space Number of Degrees of Freedom STPNDF Space-Time-Polarization Number of Degrees of Freedom Time Number of Degrees 