of FD-MIMO systems; practical 2D array antenna configuration; vertical and horizontal beamforming patterns; array partitioning architecture with the conventional CSI-RS transmission; and array connected architecture with beamformed CSI-RS transmission 46]. New Radio Access Physical Layer Aspects (Part 2) 577 NcolumnNrowP is partitioned into rectangular arrays of K1 X K2 elements. On the other hand, in 1D full-connection, the output signal of each TXRU associated with a column antenna array with a same polarization is split into Ncolumn signals, and those signals are precoded by a group of Ncolumn phase shifters or variable gain amplifiers. Then NTXRU weighted signals are combined at each antenna element. In 2D full-connection the output signal of each TXRU is split into Ncolumn NrowP signals, and those signals are precoded by a group of NcolumnNrowP phase shifters or variable gain amplifiers. Then, Q weighted signals are combined at each antenna element. An illustration of 1D subarray partition and full-connection as well as gen- eral FD-MIMO architectures are shown in Fig. 4.69 [47]. Multi-antenna systems with a large number of base station antennas, often called massive MIMO, have received much attention in academia and industry as a means to improve the spectral efficiency, energy efficiency, and processing complexity of the cellular systems. Transceiver array boundary Radiated interface boundary TXRU 1 radio TXRU 2 distribution antenna network array TXRU K Transceiver array boundary Composite antenna Active antenna system connector (TAB) transceiver unit array Figure 4.69 Illustration of transceiver architectures in FD-MIMO. 578 Chapter 4 While massive MIMO is a promising technology, there are many practical and technical challenges on the path to its successful commercialization, including design and implemen- tation of low-cost and low-power base station with large antenna arrays, capacity improve- ment of fronthaul links between remote radio heads and baseband units, measurement and reporting of high dimen