  In order to cope with the severe path loss, millimeter-wave (mm-wave) systems
exploit highly directional communication. As a consequence, even a slight beam
misalignment between two communicating devices (for example, due to mobility)
can generate a significant signal drop. This leads to frequent invocations of
time-consuming mechanisms for beam re-alignment, which deteriorate system
performance. In this paper, we propose smart beam training and tracking
strategies for fast mm-wave link establishment and maintenance under node
mobility. We leverage the ability of hybrid analog-digital transceivers to
collect channel information from multiple spatial directions simultaneously and
formulate a probabilistic optimization problem to model the temporal evolution
of the mm-wave channel under mobility. In addition, we present for the first
time a beam tracking algorithm that extracts information needed to update the
steering directions directly from data packets, without the need for spatial
scanning during the ongoing data transmission. Simulation results, obtained by
a custom simulator based on ray tracing, demonstrate the ability of our beam
training/tracking strategies to keep the communication rate only 10% below the
optimal bound. Compared to the state of the art, our approach provides a 40% to
150% rate increase, yet requires lower complexity hardware.
