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Precoding Design for Minimizing Age of Information in Downlink MU-MISO Systems

The multiple-input multiple-output (MIMO) techniques have been contributed to reducing communication duration by exploiting the spatial degrees of freedom. However, with the growing number of real time applications, the conventional multi-user multiple-input single-output (MU-MISO) design, which minimizes the communication duration of mobile users (MUs), has become unsuitable for the scenario where MUs want to keep fresh data persistently. Therefore, in this paper, we consider a MU-MISO system where a base station (BS) equipped with multiple antennas serves the MUs that want to maintain fresh data. To analyze the data freshness of sensed information, we define the expected age of information (AoI) violation time and approximate it in a tractable form using the smooth maximum/minimum function. We derive a first-order optimality condition of an optimization problem that minimizes the AoI violation probability of multiple MUs, and propose a generalized power iteration (GPI) precoding algorithm to obtain an optimal precoding vector. In simulation results, we prove that the proposed method outperforms conventional methods on the ergodic AoI violation probability.