rns using multiple antennas at the transmitter. It is possible to steer the transmitted signal toward a desired direction and, at the same time, avoid receiving the unwanted signal from an undesired direction. Traditional beamforming schemes controlled the beam pattern only in the horizontal (azimuth) plane. The three-dimensional beamforming adapts the radiation beam pattern in both elevation and azimuth planes to provide more degrees of freedom in supporting users. Higher average user throughput, less inter-cell and inter-sector interfer- ence, higher energy efficiency, improved coverage, and increased spectral efficiency are some of the advantages of 3D beamforming or full-dimensional MIMO. In order to exploit the vertical dimension, the antenna tilt can be considered in the vertical axis. The antenna tilt angle is defined as the angle between the horizontal plane and the boresight direction of the antenna pattern. Mechanical alignment of the antenna was traditionally used to adjust 574 Chapter 4 Elevation beamforming Antenna Antenna Antenna boresight boresight boresight Azimuth beamforming Electrical tilt Mechanical tilt Vertical beamforming FD-MIMO Figure 4.67 Illustration of mechanical/electrical tilting, vertical, and 3D beamforming [23]. the tilt angle of the antenna along the vertical axis. As depicted in Fig. 4.67, some adjust- able brackets were used to mechanically change the tilting angle of the antenna. It is possible to control the tilt angle electrically by applying an overall phase shift to all antenna elements in the array. An active antenna system (AAS) is a recent technology that allows more individual control on antenna elements, where each array element (or group of elements) is integrated with a separate RF transceiver unit that provides remote control to the elements. By employing AAS at the base station, the vertical radiation pattern can also be adjusted dynamically in each sector, and multiple elevation beams can be generated to support multiple users or cover multiple regions; thus full