reference signals, because the UE only feeds back the index of the best beamformed ref- erence signal. It can be further shown that there is less downlink pilot overhead when the non- precoded reference signal is used. The non-beamformed reference signal overhead increases with the number of transmit antennas, resulting in substantial loss of sum capacity in the FD- MIMO, whereas the beamformed reference signal overhead is proportional to the number of reference signals and independent of the number of transmit antennas; therefore, the rate loss of the beamformed reference signals is marginal even when the number of transmit antennas increases [46,47]. As we mentioned earlier, an AAS transceiver contains integrated PA and LNA SO that the gNB can control the gain and phase of individual antenna elements. A radio signal distribution/com- bining network between TXRUs and antenna elements was introduced (see Fig. 4.69) whose role is to deliver the transmit signal from the PA to the antenna array elements and the received signal from the antenna array to the LNA. Depending on the CSI-RS transmission and feedback mechanism, two architecture options, array partitioning and array connected, may be used. The former architecture is more suitable for the conventional codebook scheme, and the latter is for the beamforming scheme. In the array partitioning architecture, antenna elements are divided into multiple groups, and each TXRU is connected to one of them, whereas in the array con- nected architecture, the radio distribution network is designed such that the RF signals of multi- ple TXRUs are delivered to the single-antenna element. To combine RF signals from multiple TXRUs, additional RF combining circuitry is needed. In the array partitioning architecture, the total number of antenna elements L is partitioned into several groups of TXRUs, and an orthog- onal CSI-RS is assigned for each group. Each TXRU transmits its own CSI-RS SO that the UE can measure channel h from the CSI-RS observation. In the array connected arc