Version Age of Information Minimization Over Fading Broadcast Channels

We consider a base station (BS) receiving version update packets from multiple streams and broadcasting them to users using non-orthogonal multiple access (NOMA) over a fading broadcast channel. Sequentially indexed packets arrive randomly in each stream, rendering previous ones obsolete. To measure information freshness, we define the version age of information (VAoI) at a user as the difference in the version index of the latest available packet at the BS and that at the user. Our objective is to minimize a weighted sum of average VAoI across users, subject to an average power constraint at the BS, by optimally scheduling and transmitting update packets from various streams with sufficient power to ensure successful delivery. We consider channel-only stationary randomized policies (CO-SRP), which make transmission decisions solely based on channel power gains. We solve the resulting convex problem and demonstrate that the VAoI achieved with the proposed CO-SRP is within twice the optimal achievable VAoI. Additionally, we obtained a Constrained Markov Decision Process (CMDP)-based solution and analyzed its structural properties. Numerical simulations demonstrate close performance between the CO-SRP and CMDP-based solutions. Time-division multiple-access (TDMA) scheme, allowing transmission to at most one user at a time, matches NOMA's performance under strict power constraints but is outperformed as constraints are relaxed.