$K^*_{\text{LUS}}$, we just need to sort UEs in ascending order of their distances from the BS and select the first $K^*_{\text{LUS}}$ UEs for data transmission.

With respect to the number of active-UEs $K$, we determine $K=K^*_{\text{RUS}}$ and $K=K^*_{\text{LUS}}$ according to the given system parameters in the proposed $K^*$-RUS and $K^*$-LUS schemes, respectively. By contrast, there usually exists the scenario of $K=M$, where the full-spatial-multiplexing transmission may be performed in the conventional RUS and LUS schemes. In SUS, the value of $K^*$ depends on specific channel realizations and will not be obtained until the selection procedure is completed (please see Section IV-B and Section VI-A in [@yoo_optimality_2006] for more details).

Computational Complexity Analysis 
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For a system having $M$ BS antennas and $N$ candidate-UEs, although the user selection relying on exhaustive search achieves the best sum rate performance, approximately $\sum_{k=1}^{M}\tbinom{N}{k}k^5M$ complex-valued operations are required to complete one selection[@sjobergh_greedy_2008], which may be unaffordable in practice. For SUS, the computational complexity is roughly $\mathcal{O}(M^3N)$ [@yoo_optimality_2006], which is high for large-$M$ systems. In stark contrast to these conventional schemes, the online computational complexity of the proposed $K^*$-RUS and $K^*$-LUS is independent of $M$ and $N$, and the instantaneous CSI-based complicated online computations are avoided. 