说明：  MU-MIMO is widely considered a key technology for system capacity improvement in modern wireless networks. In contrast to SU-MIMO, where the spatial multiplexing gain is confined to a single user, MU-MIMO allows multiple users to be co-scheduled on the same time-frequency resources to exploit this gain among two or more UE units. This is particularly beneficial as high-rank SU-MIMO transmission is often limited by the number of antennas and antenna design constraints at the user end, whereas high-rank transmission using MU-MIMO is more feasible due to the scattered user distribution. It is well known that the information theoretic optimal MU-MIMO scheme is dirty paper coding (DPC), which unfortunately is a nonlinear/ non-causal beamforming scheme and therefore unrealistic in real-life applications. From a practical deployment point of view, the most