Physical Layer Security in Lossy Untrusted Relay Networks with Finite Blocklength

In this paper, we examine the physical layer security (PLS) in lossy untrusted relay networks, focusing on finite blocklength transmissions. In such networks, ensuring secure communication is particularly challenging due to the presence of untrusted relay nodes and the inherent limitations of short-packet transmissions. We propose the reliable-and-secure probability (RSP) as a performance metric to evaluate the likelihood that the destination node successfully recovers the transmitted message while untrusted relays experience an outage. Two basic network topologies are taken into consideration, which are the three-node one way network and the single-source multi-untrusted-relay network without direct source-to-destination link. By optimizing power allocation and relay positioning, the RSP can be significantly improved, even when decoding errors occur at the relays. The study demonstrates that lossy decode-and-forward relaying complicates secure communication but also presents opportunities for performance enhancement through careful resource management. Numerical simulations validate the effectiveness of the proposed optimization strategies in enhancing security, especially for applications in ultra-reliable low-latency communication and massive machine-type communication in 5G/6G networks. Future research will focus on deriving more precise closed-form expressions for RSP in finite blocklength settings and investigating machine learning-based approaches for real-time optimization of secure communications in dynamic wireless environments. This study contributes to the growing body of knowledge on PLS in modern wireless systems, offering insights for future advancements.