This paper studies a secure satellite-terrestrial communication system assisted by a hybrid intelligent reflecting surface (IRS). The hybrid IRS, which composes of active and passive reflecting elements, is deployed to enhance the secure communication from the satellite to multiple users against multiple eavesdroppers. A joint design optimization problem for the satellite beamforming and the hybrid IRS interaction is formulated to maximize the system worst-case secrecy rate under time-varying channel conditions. With high system dynamic and complexity, deep reinforcement learning (DRL) is employed to solve the non-convex optimization problem. We propose a fast DRL algorithm, namely deep PDS-DPG, to obtain the robust secure beamforming design for satellite and hybrid IRS. Numerical results show a better learning efficiency of the proposed algorithm as to the state-of-the-art deep deterministic policy gradient (DDPG) algorithm with comparable system secrecy performance.

This paper analyzes the physical layer security performance of cache-enabled hybrid satellite-terrestrial networks with intelligent reflecting surfaces (IRS). Content caches are deployed at both satellite and ground station to help fast content delivery to users. The IRS assists in securing communications from the ground station and satellite to the user against an eavesdropper. The connection probability and the secrecy probability are derived in asymptotic and closed-form expressions for both ground station-IRS-user and satellite-IRS-user links under practical channel models. From the obtained results, the secure transmission probability of the system is evaluated, and then maximized by designing the caching probabilities and the transmission rates. Numerical results verify the accuracy of the analysis and demonstrate the performance gains of having IRS support secure content delivery.

The implementation of the Internet of Things (IoT) is mostly done through cellular networks which do not cover the whole world. In addition, the explosive growth of global Internet access demand introduces the need for integrating satellites with cellular IoT networks for coverage extension and backhaul offloading. Operating hybrid satellite-IoT (SIoT) networks, however, might incur excessive service latency and high satellite bandwidth consumption. To tackle these issues, edge caching technology has been considered in SIoT. This article reviews existing research on edge caching-based SIoT networks with illustrative performance evaluation. Various caching design criteria with a focus on two-tier cache-enabled SIoT are discussed. In addition, open research problems on edge caching in SIoT are identified as future research directions and opportunities.

Enabling global Internet access is challenging for cellular-based Internet of Things (IoT) due to the limited range of terrestrial network services. One viable solution is to deploy IoT over satellite systems for coverage extension. However, operating a hybrid satellite-terrestrial network (STN) might incur high satellite bandwidth consumption and excessive service latency. Aiming to reduce the content delivery latency from the Internet-connected gateway to the users, this work proposes a wireless two-tier cache-enabled model with full-duplex transmissions where content caches are deployed at the satellite and ground station. A closed-form solution for the successful delivery probability (SDP) of the files is derived considering the requested content distributions and channel statistics. Then, the SDP performance under common caching policies can be conveniently evaluated. The results are also used to optimize cache placement under caching capacity constraints. Numerical results demonstrate the performance improvements of the proposed system over those of single-tier cache-aided and half-duplex transmission systems.