hitecture, each antenna element is connected to <L TXRUs and an orthogonal CSI-RS is assigned for each TXRU. Denoting as the channel vector and veCNX1 as the precoding vector for each beamformed CSI-RS, the beamformed CSI-RS observation can be expressed as y = hvx + n and the UE measures the precoded channel hv. Due to the narrow and directional 580 Chapter 4 CSI-RS beam transmission with a linear array, the SNR of the precoded channel is maximized at the target direction, that is, SNR = hv()| where P is the beam direction and o2 is the noise variance. In non-beamformed scenario, the UE selects and sends a precoder index which maximizes certain performance criterion to the gNB and adapts to the channel variation. In the beamformed scenario, the gNB transmits multiple beamformed CSI-RSs using the connected array architecture and the UE selects the preferred beam and then feeds back its index. When the gNB receives the beam index, the weight corresponding to the selected beam is used for data transmission to the UE [48]. Let us consider a cellular system consisting of Ncell cells each with one base station and NUE terminals in each cell, as shown in Fig. 4.70. Each gNB is equipped with a 2D antenna array of Nv X NH vertical and horizontal antennas, and each UE has a single antenna. We assume that all gNBs and UEs are synchronized and operate in TDD mode with universal frequency reuse. In the downlink, the 1th base station applies a NvNH X NUE precoder Fn, ,Ncell to transmit a symbol for each user, with a power constraint NUE. Uplink and downlink channels are assumed to be reciprocal. If hnck denotes the NyNH X 1 uplink channel from user k in cell C to the nth base station, then the received signal by this user in the downlink can be Cell 1 Nucolumns Cell 2 Full-dimension Cell n antenna array Precoding matrix N X N H X N UE NU X 1 UE Transmitted signal vector Cell Ncell-1 Nv X N H x1 N(0,1) Cell Ncell Channel vector Additive white (1 indicates the number of UE antennas) gaussian noise Figure 4.70 Illustration of ful