$K^*$-LUS are obvious for the systems relying on pilot-based channel estimation. In particular, when $T$ is not significantly larger than $N$, with regard to SUS, a large portion of the coherence slot would be dedicated to channel estimation, which reduces the resources for the downlink data delivery. By contrast, with the proposed user selection schemes, we only have to estimate the CSI of the $K^*_{\text{RUS}}$ or $K^*_{\text{LUS}}$ active-UEs for precoding. Both $K^*_{\text{RUS}}$ and $K^*_{\text{LUS}}$ are usually much smaller than $N$, hence our schemes are superior to SUS by exploiting more data transmission resources.

The result of $\Theta(\log_2\log_2N)$ in is obtained when we have $N\to\infty$ and a fixed $M$[@yoo_optimality_2006]. Moreover, as shown in [@tomasoni_selection_2009], when the number of candidate-UEs $N$ is linearly related to the number of BS antennas $M$ (i.e. the case we have discussed in this paper), only marginal multiuser diversity gain might be achieved by SUS.

Numerical Simulations and Discussions \[sec:Numerical-Simulations\]
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In this section we present simulation results to show the benefits of the proposed user selection schemes. The cellular model employed is based on that of [@bjornson_optimal_2014] and [@3GPP_1]. Like [@yang_performance_2013], we assume the number of symbols in a coherence slot is $196$, over which the 