n to provide good coverage to its serving mobiles and to reduce interference to other cells. There will be more challenges, if TXRU virtualization is dynamic in terms of hardware implementa- tion and protocol design [46]. In 1D TXRU virtualization, NTXRU TXRUs are associated with Ncolumn antennas comprising a column antenna array with the same polarization. In 2D antenna arrays with dual-polarized configuration, that is, P = 2, and the addition of Nrow rows, the total number of TXRUs would be Q = NTXRUNrowP. In 2D TXRU virtualization, Q TXRUs can be associated with any of NcolumnNrowP antenna elements. These two different TXRU architectures have different trade- offs in terms of hardware complexity, power efficiency, cost and performance. For each method, subarray partition and full-connection architectures are considered. In subarray parti- tion the antenna elements are partitioned into multiple groups with the same number of ele- ments. In 1D subarray partition, Ncolumn antenna elements comprising a column are partitioned into groups of K elements. In 2D subarray partition the total number of antenna elements Chapter 4 Conventional beamforming or FD-MIMO for a single UE in FD-MIMO for multiple UEs in MIMO in horizontal direction horizontal/vertical direction horizontal/vertical directions Antenna boresight network Cross-polarized 2D Radome antenna array Elevation pattern null direction Azimuth pattern null direction Elevation beamforming pattern Azimuth beamforming pattern RF signals -10 (dB) N Columns FD-MIMO enabled base station 2D antenna array with cross- Example radiation patterns in azimuth and elevation planes polarized antenna elements Column k+1 Column k Row 0 User1 Precoding hix++n User K Class A feedback Transceiver array to Codebook Index antenna mapping Row N-1 2D antenna array Column k+1 Column k Row 0 User1 Precoding hV,x++ User K Class B feedback Transceiver array to Beam index antenna mapping Row N-1 2D antenna array Figure 4.68 Horizontal, vertical, and 3D beamforming FD-MIMO systems: concept 