  In this paper, randomly-directional beamforming (RDB) is considered for
millimeter-wave (mmwave) multi-user (MU) multiple-input single-output (MISO)
downlink systems. By using asymptotic techniques, the performance of RDB and
the MU gain in mm-wave MISO are analyzed based on the uniform random
line-of-sight (UR-LoS) channel model suitable for highly directional mm-wave
radio propagation channels. It is shown that there exists a transition point on
the number of users relative to the number of antenna elements for non-trivial
performance of the RDB scheme, and furthermore sum rate scaling arbitrarily
close to linear scaling with respect to the number of antenna elements can be
achieved under the UR-LoS channel model by opportunistic random beamforming
with proper user scheduling if the number of users increases linearly with
respect to the number of antenna elements. The provided results yield insights
into the most effective beamforming and scheduling choices for mm-wave MU-MISO
in various operating conditions. Simulation results validate our analysis based
on asymptotic techniques for finite cases.
