Antenna interfaces are closely associated with and in various cases may be included as part of the RAN "fronthaul" interface between the baseband processing function/controller (e.g. REC in Figure 4.1.8) and one or more RRH units (or RE in Figure 4.1.8, also referred to as radio unit [RU]), to which antenna subsystems are attached. This is also the case in, for example, the O-RAN low-level split architecture, described in Section 4.5 (in which case the fronthaul interface is between O-DU and O-RU). Figure 4.1.8 gNB architecture with antenna. Antenna 4 NG-RAN Architecture High- High- High- Low-PHY Option 1 Option 2 Option 3 Option 4 Option 5 Option 6 Option 7 Option 8 High- High- High- Low-PHY Figure 4.1.9 gNB spit architectures considered in 3GPP study. (Source: Reproduced by permission of © 3GPP). Generally, fronthaul carries control/monitoring/management messages as well as user- plane traffic. The control/monitoring messages may alternately include explicit antenna interface messages as a subset. User-plane traffic (including multi-stream multiple- input and multiple-output [MIMO]) is generally transported in the form of I/Q samples to RU, and that user-plane traffic is ultimately conveyed to/from an antenna subsystem and transmitted as energy over the air by the antenna subsystem. 4.1.3.1 Before 5G: Where We Have Been Since the 2G cellular days and on into 4G, base stations have offered sectorization rather than simple omnidirectional antenna radiation patterns, where each sector has an antenna providing a static directional beam of quite broad horizontal (azimuthal) beam width of, say, 120 or 60°. While planning, design, and operations remained mostly in the two-dimensional horizontal domain, a vertical (elevation) adjustment or "tilt" of the antenna pointing direc- tion would be chosen at upfront installation. Over time operation teams would more frequently adjust the horizontal and vertical antenna pointing direction to fine-tune coverage. Initially and still in many cases this required rolling a truck to 