Cyclic Scheduling for Age of Information Minimization with Generate at Will Status Updates

We study the scheduling problem in a status update system composed of multiple information sources with different service time distributions and weights, for the purpose of minimizing the weighted sum age of information (AoI). In particular, we study open-loop schedulers which rely only on the statistics (specifically, only on the first two moments) of the source service times, in contrast to closed-loop schedulers that also make use of the actual realizations of the service times and the AoI processes in making scheduling decisions. We consider the generate-at-will (GAW) model, and develop an analytical method to calculate the exact AoI for probabilistic and cyclic open-loop schedulers. In both cases, the server initiates the sampling of a source and the ensuing transmission of the update packet from the source to the server in an open-loop manner; either based on a certain probability (probabilistic scheme) or according to a deterministic cyclic pattern (cyclic scheme). We derive the optimum open-loop cyclic scheduling policy in closed form for the specific case of N = 2 sources and propose well-performing heuristic cyclic schedulers for general number of sources, i.e., N > 2. Numerical examples are provided to validate the existing methods.