  This paper studies the user scheduling problem in a multiuser multiple-input
multi-output (MIMO) status update system, in which multiple single-antenna
devices aim to send their latest statuses to a multiple-antenna
information-fusion access point (AP) via a shared wireless channel. The
information freshness in the considered system is quantified by a recently
proposed metric, termed age of information (AoI). Thanks to the extra spatial
degrees-of-freedom brought about by the multiple antennas at the AP, multiple
devices can be granted to transmit simultaneously in each time slot. We aim to
seek the optimal scheduling policy that can minimize the network-wide AoI by
optimally deciding which device or group of devices to be scheduled for
transmission in each slot given the instantaneous AoI values of all devices at
the beginning of the slot. To that end, we formulate the multiuser scheduling
problem as a Markov decision process (MDP). We attain the optimal policy by
resolving the formulated MDP problem and develop a low-complexity sub-optimal
policy. Simulation results show that the proposed optimal and sub-optimal
policies significantly outperform the state-of-the-art benchmark schemes.
