  This paper considers a wireless network with a base station (BS) conducting
timely status updates to multiple clients via adaptive non-orthogonal multiple
access (NOMA)/orthogonal multiple access (OMA). Specifically, the BS is able to
adaptively switch between NOMA and OMA for the downlink transmission to
optimize the information freshness of the network, characterized by the Age of
Information (AoI) metric. If the BS chooses OMA, it can only serve one client
within each time slot and should decide which client to serve; if the BS
chooses NOMA, it can serve more than one client at the same time and needs to
decide the power allocated to the served clients. For the simple two-client
case, we formulate a Markov Decision Process (MDP) problem and develop the
optimal policy for the BS to decide whether to use NOMA or OMA for each
downlink transmission based on the instantaneous AoI of both clients. The
optimal policy is shown to have a switching-type property with obvious decision
switching boundaries. A near-optimal policy with lower computation complexity
is also devised. For the more general multi-client scenario, inspired by the
proposed near-optimal policy, we formulate a nonlinear optimization problem to
determine the optimal power allocated to each client by maximizing the expected
AoI drop of the network in each time slot. We resolve the formulated problem by
approximating it as a convex optimization problem. We also derive the upper
bound of the gap between the approximate convex problem and the original
nonlinear, nonconvex problem. Simulation results validate the effectiveness of
the adopted approximation. The performance of the adaptive NOMA/OMA scheme by
solving the convex optimization is shown to be close to that of max-weight
policy solved by exhaustive search...
