  In this paper, we focus on the ergodic downlink sum-rate performance of a
system consisting of a set of heterogeneous users. We study three user
selection schemes to group near-orthogonal users for simultaneous transmission.
The first scheme is a random selection policy that achieves fairness, but does
not exploit multi-user diversity. The second scheme is a greedy selection
policy that fully exploits multi-user diversity, but does not achieve fairness,
and the third scheme achieves fairness while partially exploiting multi-user
diversity. We also consider two beamforming methods for data transmission,
namely, maximum-ratio transmission and zero-forcing beamforming. In all
scheduling schemes studied in the paper, there is a key parameter that controls
the degrees of orthogonality of channel directions between co-scheduled users.
We focus on optimally setting this parameter for each scheduling scheme such
that the ergodic downlink sum-rate is maximized. To this end, we derive
analytical expressions for the ergodic downlink sum-rate considering each
scheduling scheme. Numerical results are also presented to provide further
insights.
