O, Distributed MIMO, Massive Cell Free MIMO, Radio Stripe, Holographic Massive MIMO, and Orbital Angular Moment Antenna, while PLAs include technologies such as reflective smart surface, reconfigurable reflectors, software-controlled metasurfaces, and smart skins. A short list with some of the main references for each type of technology is shown in Table 2.1. However, it should be emphasized that the literature on the subject is extremely vast and rapidly expanding. Research at the PL is therefore characterized by a plethora of solutions. Despite the complexity at the PL, there is an element that unifies all these techniques. Basically, all these solutions are different ways of using resources available at a lower, and somehow more fundamental, level than the physical level. The description of the impact of this level in the communication process requires to consider a further layer placed below the PL. This additional layer will be called the Deep Physical Layer (DPL) [9]. The introduction of the DPL leads to an extended version of the OSI stack in which the communication process is analyzed in terms of distinguishable electromagnetic field configurations on the receiving antenna. In particular, the DPL contains all the resources potentially available to the telecommunication system in terms of space/time/polarization field configurations, i.e., all the possible spatial/temporal/polarization waveforms that can be distinguished at the receiver, while the PL implements specific solutions to effectively exploit the potential offered by the DPL. Table 2.1 Some directions of research in 6G physical layer Research directions Examples Very high frequency Millimeter and sub-millimeter communication [49] [50] Active Large Aperture (ALA) Very large Massive MIMO [51] Cell-free Massive MIMO [52] Radio Strips [53] Distributed MIMO [54] Active Holographic MIMO [54] OAM antennas [55] Passive Large Apertures (PLA) Holographic beamforming [56] Intelligent reflecting surface [57] Large intelligent surface [58, 59] Reconfigurable 