Age-optimal Power Allocation Policies for NOMA and Hybrid NOMA/OMA Systems

In this paper, we study downlink short-packet communication in wireless networks, where a base station (BS) sends time-sensitive status updates to users via non-orthogonal multiple access (NOMA) or Adaptive NOMA/orthogonal multiple access (OMA). The Age of Information (AoI), namely the amount of time that elapsed since the most recently delivered packet was generated, captures the freshness of the information. We aim to minimize the Expected Weighted Sum AoI (EWSAoI) by optimizing the power allocation. First, a low-complexity power allocation policy, namely Stationary Power Allocation policy is proposed in NOMA. In this policy, the BS allocates fixed power to each user and we obtain the closed-form expression of the optimal allocation factor to minimize the EWSAoI. Then, we propose an Adaptive NOMA/OMA policy based on Lyapunov Optimization in which the BS can adaptively switch between NOMA and OMA and dynamically allocate power for users to keep the EWSAoI low. Numerical results demonstrate the advantages of the proposed policies, which provide useful insights for practical system designs.