a- tial dimensions is limited by number of antennas at the UE. There is a potential that spatial dimensions are wasted, if the UEs have a smaller number of antennas compared to the base station. MU-MIMO more efficiently exploits the multi-user diversity since all spatial dimen- sions which are supported by the base station can be exploited. It will achieve capacity gain, if UEs have a smaller number of antennas relative to the base station. Stronger spatial dimensions are exploited, particularly in the case of low-rank channel. The uti- lized spatial dimensions may be weak in the case of low-rank channel due to spatial correlation. The advantages/disadvantages of SU-MIMO and MU-MIMO schemes are summarized in Table 4.21. 560 Chapter 4 Table 4.21: Comparison of SU-MIMO and MU-MIMO schemes. SU-MIMO MU-MIMO Advantages High user throughput High system capacity High peak data rates Full exploitation of multiuser diversity Disadvantages Multiple transmit antennas at the base Degradation of peak data rates station are not fully exploited due to interuser interference Multiuser diversity is not fully exploited Single-user maximum rate CSU = C24 MAC maximum sum rate region Single-user maximum rate Figure 4.60 Capacity region of MU-MIMO BC with two users compared to SU-MIMO. An important metric for measuring the performance of any communication channel is the information theoretic capacity. In an SU-MIMO channel, the capacity is the maximum amount of information that can be transmitted as a function of available bandwidth given a constraint on transmitted power. In SU-MIMO channels, it is common to assume that the total power distributed among all transmit antennas is limited. For the multi-user MIMO channel, the problem is somewhat more complex. Given a constraint on the total transmit power, it is possible to allocate varying fractions of that power to different users in the net- work; thus for any value of total power, different information rates are obtained. The result is a capacity region shown in Fig. 4.60 for two-us