both $\mathcal{R}^*_{\text{RUS}}$ and $\mathcal{R}^*_{\text{LUS}}$ are enhanced by the increase of $P$. When $N$ increases, $\mathcal{R}^*_{\text{LUS}}$ rises but $\mathcal{R}^*_{\text{RUS}}$ remains unchanged.

In Fig. \[fig4-1\] the impacts of channel estimation accuracy $\rho$ and channel correlation factor $\delta$ on the system performance are characterized, where we set $P=30$ dBm, $M=32$ and $N=64$. In sub-figures (a) and (b), the optimal number of active-UEs for $K^*$-RUS and $K^*$-LUS are shown, respectively. We can see that both $K^*_{\text{RUS}}$ and $K^*_{\text{LUS}}$ attain their maximum values at $\rho=0,\delta=0$. This observation indicates that the BS should serve more UEs under uncorrelated channel scenarios with perfect CSI estimation than those under correlated channel scenarios with imperfect CSI estimation. The sum rates of $K^*$-RUS and $K^*$-LUS are shown in the sub-figures (c) and (d), respectively. It is clear that the sum rates decrease as $\delta$ and $\rho$ increase.

The sum rate performance of various user selection schemes, including $K^*$-LUS, $K^*$-RUS, SUS and RUS, is evaluated against the transmit power $P$ with $M=32,N=64$ in Fig. \[fig7\]. Four scenarios are investigated, i.e., uncorrelated channel with perfect CSI estimation ($\rho=0, \delta=0$), correlated channel with perfect CSI estimation ($\rho=0, \delta=0.5$), uncorrelated channel with imperfect CSI estimation ($\rho=0.1, \delta=0$) and correlated channel with imperfect CSI estimation 