n higher frequencies that are used for 5G networks [45]. The study of elevation beamforming and FD-MIMO began in 3GPP LTE Rel-12. In an FD-MIMO system, a base station with two-dimensional active antenna array supports multi- user joint elevation and azimuth or 3D beamforming, which results in much higher cell capac- ity compared to conventional systems. In an FD-MIMO architecture using 2D AASs with Ncolumn Nrow physical antennas, the precoding of a data stream is performed in two stages: (1) antenna-port virtualization that is a stream on an antenna port is precoded on NTXRU trans- ceiver units; and (2) transceiver-unit virtualization where a signal is precoded on Nantenna antenna elements. It is noted that in traditional transceiver architecture modeling, a fixed one-- to-one mapping is assumed between antenna ports and transceiver units, and TXRU virtuali- zation effect is combined into a fixed antenna pattern which captures the effects of both TXRU virtualization and antenna element pattern. Antenna-port virtualization is an operation in the digital domain, and it refers to digital precoding that can be performed in frequency- selective manner. An antenna port is typically defined in conjunction with a reference signal. For example, for precoded data transmission on an antenna port, a DM-RS is transmitted on the same bandwidth as the data and both are precoded with the same digital precoder. For CSI estimation, on the other hand, CSI-RSs are transmitted on multiple antenna ports. For CSI-RS transmissions, the precoder characterizing the mapping between CSI-RS ports to TXRUs can be designed as an identity matrix, to facilitate device's estimation of TXRU vir- tualization precoding matrix for data precoding vectors. The TXRU virtualization is an analog operation; thus it refers to time-domain analog precoding. The TXRU virtualization can be made time-adaptive. When TXRU virtualization is semi-static (or rate of change in TXRU virtualization is slow), the TXRU virtualization weights of a serving cell can be chose