  Millimeter wave (mm-wave) and massive MIMO have been proposed for next
generation wireless systems. However, there are many open problems for the
implementation of those technologies. In particular, beamforming is necessary
in mm-wave systems in order to counter high propagation losses. However,
conventional beamsteering is not always appropriate in rich scattering
multipath channels with frequency selective fading, such as those found in
indoor environments. In this context, time-reversal (TR) is considered a
promising beamforming technique for such mm-wave massive MIMO systems. In this
paper, we analyze a baseband TR beamforming system for mm-wave multi-user
massive MIMO. We verify that, as the number of antennas increases, TR yields
good equalization and interference mitigation properties, but inter-user
interference (IUI) remains a main impairment. Thus, we propose a novel
technique called interference-nulling TR (INTR) to minimize IUI. We evaluate
numerically the performance of INTR and compare it with conventional TR and
equalized TR beamforming. We use a 60 GHz MIMO channel model with spatial
correlation based on the IEEE 802.11ad SISO NLoS model. We demonstrate that
INTR outperforms conventional TR with respect to average BER per user and
achievable sum rate under diverse conditions, providing both diversity and
multiplexing gains simultaneously.
