  Millimeter wave (mmWave) communication has attracted increasing attention as
a promising technology for 5G networks. One of the key architectural features
of mmWave is the use of massive antenna arrays at both the transmitter and the
receiver sides. Therefore, by employing directional beamforming (BF), both
mmWave base stations (MBSs) and mmWave users (MUEs) are capable of supporting
multi-beam simultaneous transmissions. However, most researches have only
considered a single beam, which means that they do not make full potential of
mmWave. In this context, in order to improve the performance of short-range
indoor mmWave networks with multiple reflections, we investigate the challenges
and potential solutions of downlink multi-user multi-beam transmission, which
can be described as a high-dimensional (i.e., beamspace) multi-user
multiple-input multiple-output (MU-MIMO) technique, including multi-user BF
training, simultaneous users' grouping, and multi-user multibeam power
allocation. Furthermore, we present the theoretical and numerical results to
demonstrate that beamspace MU-MIMO compared with single beam transmission can
largely improve the rate performance of mmWave systems.
