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Optimizing Information Freshness via Multiuser Scheduling with Adaptive NOMA/OMA

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Document pages: 30 pages

Abstract: This paper considers a wireless network with a base station (BS) conductingtimely status updates to multiple clients via adaptive non-orthogonal multipleaccess (NOMA) orthogonal multiple access (OMA). Specifically, the BS is able toadaptively switch between NOMA and OMA for the downlink transmission tooptimize the information freshness of the network, characterized by the Age ofInformation (AoI) metric. If the BS chooses OMA, it can only serve one clientwithin each time slot and should decide which client to serve; if the BSchooses NOMA, it can serve more than one client at the same time and needs todecide the power allocated to the served clients. For the simple two-clientcase, we formulate a Markov Decision Process (MDP) problem and develop theoptimal policy for the BS to decide whether to use NOMA or OMA for eachdownlink transmission based on the instantaneous AoI of both clients. Theoptimal policy is shown to have a switching-type property with obvious decisionswitching boundaries. A near-optimal policy with lower computation complexityis also devised. For the more general multi-client scenario, inspired by theproposed near-optimal policy, we formulate a nonlinear optimization problem todetermine the optimal power allocated to each client by maximizing the expectedAoI drop of the network in each time slot. We resolve the formulated problem byapproximating it as a convex optimization problem. We also derive the upperbound of the gap between the approximate convex problem and the originalnonlinear, nonconvex problem. Simulation results validate the effectiveness ofthe adopted approximation. The performance of the adaptive NOMA OMA scheme bysolving the convex optimization is shown to be close to that of max-weightpolicy solved by exhaustive search...

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