说明：  CSI feedback allows downlink transmission to be adaptively optimized based on the instantaneous DL channel, so that closedloop beamforming and adaptive link adaptation can be enabled to optimize the system performance. The DL reference signal used for CSI measurement is different in Rel-8 and Rel-10. In Rel-8, CSI measurement is based on CRS, which is also used for data demodulation. In contrast, CSI measurement in Rel-10 is based on a set of newly introduced CSI-RS signals, which is low-dutycycle and low-density, and allows a higher reuse factor than Rel-8 CRS.