  The concept of age of information (AoI) has been proposed to quantify
information freshness, which is crucial for time-sensitive applications.
However, in millimeter wave (mmWave) communication systems, the link blockage
caused by obstacles and the severe path loss greatly impair the freshness of
information received by the user equipments (UEs). In this paper, we focus on
reconfigurable intelligent surface (RIS)-assisted mmWave communications, where
beamforming is performed at transceivers to provide directional beam gain and a
RIS is deployed to combat link blockage. We aim to maximize the system sum rate
while satisfying the information freshness requirements of UEs by jointly
optimizing the beamforming at transceivers, the discrete RIS reflection
coefficients, and the UE scheduling strategy. To facilitate a practical
solution, we decompose the problem into two subproblems. For the first per-UE
data rate maximization problem, we further decompose it into a beamforming
optimization subproblem and a RIS reflection coefficient optimization
subproblem. Considering the difficulty of channel estimation, we utilize the
hierarchical search method for the former and the local search method for the
latter, and then adopt the block coordinate descent (BCD) method to alternately
solve them. For the second scheduling strategy design problem, a low-complexity
heuristic scheduling algorithm is designed. Simulation results show that the
proposed algorithm can effectively improve the system sum rate while satisfying
the information freshness requirements of all UEs.
