Joint AoI and Security-Oriented Optimization in Satellite-Terrestrial Integrated Networks

In resource constrained satellite-terrestrial integrated networks (STINs), satellite downlinks supporting earth stations often have to share the same band with terrestrial networks. Under this scenario, achieving secure and timely communication in STINs could be challenging due to the presence of co-channel interference, imperfect CSI, and potential eavesdroppers. Existing security techniques in STINs e.g. robust secure beamforming (BF) often overlook another key performance indicator of STINs, i.e. communication timeliness in terms of Age of Information (AoI). To overcome this issue, this paper proposes a new joint AoI and security-oriented optimization in enhancing the performance of STINs. Specifically, two schemes are developed to achieve low latency and secure BF, including a single-slot scheme and a multi-slot scheme. Specifically, we first establish a continuous-time AoI evolution model and derive a closed-form secrecy margin for the wiretap channel, which enables a unified characterization of information freshness and transmission security. Building on this, we incorporate imperfect channel state information (CSI) to formulate a single-slot joint optimization problem that explicitly targets both AoI-aware freshness and physical-layer security. The objective is to minimize the total transmit power, while meeting the required secrecy-margin constraints, quality of service (QoS) constraints, eavesdropping probability constraints, and pertransmitter power budget constraints. To handle this nonconvex problem, we first apply Bernstein inequalities to convert the probabilistic constraints into deterministic forms. Subsequently, an iterative difference-of-convex programming algorithm is proposed to derive the BF vectors. Furthermore, to capture multi-slot temporal dynamics, we extend the design to a multi-slot framework that considers the impact of random data arrivals on system performance and establishes queue stability conditions based on the data queue. We apply the Lyapunov optimization technique to transform the multi-slot stochastic problem into a per-slot penalized power minimization problem, after which each slot can be solved in the same manner as the single-slot design. Finally, experimental results show that the proposed single-slot scheme satisfies both security and timeliness requirements while significantly reducing system power consumption, whereas the multi-slot optimization retains these performance advantages and further enables a favorable trade-off between power consumption and queue stability.