  This paper considers a wireless network with a base station (BS) conducting
timely transmission to two clients in a slotted manner via hybrid
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 minimize the information freshness, characterized by
Age of Information (AoI), of the network. If the BS chooses OMA, it can only
serve one client within a time slot and should decide which client to serve; if
the BS chooses NOMA, it can serve both clients simultaneously and should decide
the power allocated to each client. To minimize the weighted sum of expected
AoI of the network, we formulate a Markov Decision Process (MDP) problem and
develop an 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. We
prove the existence of optimal stationary and deterministic policy, and perform
action elimination to reduce the action space for lower computation complexity.
The optimal policy is shown to have a switching-type property with obvious
decision switching boundaries. A suboptimal policy with lower computation
complexity is also devised, which can achieve near-optimal performance
according to our simulation results. The performance of different policies
under different system settings is compared and analyzed in numerical results
to provide useful insights for practical system designs.
