active-UE, and $T$ denotes the number of symbols over which the channel is constant. As a percentage, the pre-log factor $(1-K/T)$ implies that the downlink data transmission only occupies a fraction of the coherence slot. In particular, we assume that $K$ UEs are simultaneously served. Since each of the $K$ UEs is assigned one of the $K$ orthogonal pilot sequences, the length of the pilot sequence should not be shorter than $K$ symbols[@rusek_scaling_2013]. For simplicity, in this paper we adopt the shortest available pilot sequence of $K$-symbol length. Because the BS does not transmit data during the uplink pilot transmission for channel estimation, there exist $(T-K)$ symbols left for downlink data transmission at each coherence slot consisting of $T$ symbols. Therefore, the sum rate for downlink data transmission may be evaluated using . Note that in order to guarantee the feasibility of data transmission, we assume $K<T$ in this paper. Otherwise, the pilot transmission would occupy the entire coherence slot $T$.

Asymptotic Sum Rate Analyses-based Low-Complexity User Selection
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In this section, the proposed user selection schemes are presented. First, relying on the RMT-based large system analysis, we derive a deterministic approximation of the ergodic sum rate of the ZF 