  In this paper, a new user-scheduling-and-beamforming method is proposed for
multi-user massive multiple-input multiple-output (massive MIMO) broadcast
channels in the context of two-stage beamforming. The key ideas of the proposed
scheduling method are 1) to use a set of orthogonal reference beams and
construct a double cone around each reference beam to select `nearly-optimal'
semi-orthogonal users based only on channel quality indicator (CQI) feedback
and 2) to apply post-user-selection beam refinement with zero-forcing
beamforming (ZFBF) based on channel state information (CSI) feedback only from
the selected users. It is proved that the proposed scheduling-and-beamforming
method is asymptotically optimal as the number of users increases. Furthermore,
the proposed scheduling-and-beamforming method almost achieves the performance
of the existing semi-orthogonal user selection with ZFBF (SUS-ZFBF) that
requires full CSI feedback from all users, with significantly reduced feedback
overhead which is even less than that required by random beamforming.
