sional/resolution CSI, etc. One of the main features of FD-MIMO systems is the potential to use a large number of antennas at the base station. Theoretically, as the number of base station antennas increases, the cross-correlation of two random channel realizations approaches zero; thus inter-user interference in the downlink can be controlled via a simple linear precoder. However, such a benefit can be realized only when the perfect CSI is available at the base station. While the CSI acquisition in TDD systems is relatively simple due to the channel reciprocity, that is not the case for FDD systems, because the time variation and frequency response of the channel in FDD systems are measured via the downlink reference signals and fed back to the base station after quantization. Even in TDD mode, one cannot always rely on channel reciprocity because the measurement at the transmitter does not capture the downlink inter- ference from neighboring cells or co-scheduled UEs. As such, downlink reference signals are still required to capture the CQI for the TDD systems. As a result, downlink reference signals and uplink CSI feedback are crucial for operation of both duplex schemes. A common problem in closed-loop MIMO systems, and in particular FDD systems, is that the quality of CSI is affected by limitation of the feedback resources. As CSI distortion increases, the MU-MIMO precoder's capability to control the inter-user interference is degraded, resulting in performance degradation of the FD-MIMO system. In general the amount of CSI feedback, which determines the quality of CSI, needs to be scaled with the number of transmit antennas of the base station to control the quantization error, while limiting the overhead of CSI feedback to avoid adverse impact on the system performance. An important problem related to CSI acquisi- tion at the base station is the reference signal overhead. The UE performs channel estimation using the reference signals transmitted from the base station. Since the reference signals are typica