说明：  From the figure, we observe that the performance gap between ideal and realistic feedback is much more pronounced for MU-MIMO compared to SU-MIMO, which illustrates the increased sensitivity of MU-MIMO with respect to CSI accuracy. This is due to the fact that more accurate CSI feedback helps to improve the accuracy with which interference nulling can be performed at the eNB and to better predict the MU-CQIs, which are crucial for multi-user scheduling [7]. While Fig. 1 sheds light on the benefits that may be achieved with improved CSI feedback accuracy, in practice a tradeoff between improving downlink performance and increasing uplink overhead needs to be struck. In Rel-10, increasing the granularity of the feedback codebooks was discussed but no agreement