-dimension MIMO (FD-MIMO) is a combination of azimuth and elevation beamforming. Depending on the way that the antenna down tilt is changed, 3D beamforming can be classified into static and dynamic schemes. The static 3D beamforming refers to a system where the antenna tilt at the base station is set to a fixed value according to some statistical metrics, for example, the mean value of the vertical angles of users. This method cannot be adapted to the dynamic patterns of users' movements, that is, once the tilt angle is selected, it will remain unchanged. In contrast the dynamic 3D beamforming is a technique that steers the base station antenna tilt angle according to specific user locations. As mentioned earlier, the antennas at the base station are usually configured as a linear array of a limited number of antennas in the azi- muth plane. However, these geometries can shape the radiation pattern only in the horizon- tal plane; hence, to change the beam in the elevation plane for 3D beamforming, more general 2D or 3D array topologies are necessary. Those arrays are active antenna systems that are spaced in both azimuth and vertical planes with different configurations such as pla- nar, circular, spherical, or cylindrical structures. In addition, the array may include co- polarized or cross-polarized antenna elements [23]. In general, adding more antenna elements to the array provides more flexibility in beam steering designs and increases the number of radiation beams of the array. For vertical sec- torization in which the number of vertical sectors is usually small (e.g., two or three), only a small number of antennas are required in the vertical plane. However, in 3D beamforming New Radio Access Physical Layer Aspects (Part 2) 575 with per-user beam pattern adaptation (i.e., user tracking), a large number of antennas are needed. One of the challenges of the 3D beamforming is physical constraints and placement of a large number of antennas at the base station (see Fig. 4.68). This problem may be alle- viated i