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User selection, massive MIMO, low-complexity, system sum rate, user fairness.

Introduction
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The multiuser MIMO (MU-MIMO) technology plays a key role in modern wireless communications due to its substantial performance gains over the conventional single-input single-output (SISO) techniques[@3GPP_4; @3GPP_3]. Relying on MU-MIMO, a multi-antenna base station (BS) can simultaneously serve multiple user equipments (UEs) within a cell using the same spectrum resource, and thus the spectral efficiency is improved. User selection is critical for optimizing MIMO systems’ overall performance in a variety of scenarios and has been extensively studied, such as in cellular networks (see for example [@yoo_optimality_2006] and references therein) and in multi-hop networks[@gao_new_2013; @gao_multiuser_2013; @gao_distributed_2014; @liu_novel_2014]. The semi-orthogonal user selection (SUS) leveraging the degree of channel orthogonality among UEs is probably one of the most popular low-complexity user selection methods for improving system sum rates[@yoo_optimality_2006; @chen_resource_2013; @cheung_spectral_2014]. Additionally, considering the fairness amongst UEs, round robin scheduling[@yoo_optimality_2006] and random user selection (RUS) are regarded as the two simplest methods offering equal opportunities to all the candidate-UEs. Both of them have been widely employed in practical cellular systems as well[@3GPP_4].

In the realm of MU-MIMO, the recently proposed massive MIMO, where the BS is equipped with a 