Optimal Power Allocation for Secure Transmission in Untrusted Relay Networks with Alamouti Space-Time Block Coding

This work explores the impact of optimal power allocation on the physical layer security of lossy decode-and-forward untrusted relay networks using Alamouti Space-Time Block Code (STBC). We introduce the reliable-and-secure probability (RSP) metric to measure the likelihood of successful message recovery at the destination while ensuring the message remains unintelligible to the relays. In high spectral efficiency scenarios, allocating more power to the untrusted relay significantly enhances the RSP, surpassing low spectral efficiency results. A balanced power allocation between the source and the relay, with Alamouti STBC, achieves a high RSP. However, the peak RSP is achieved when the power contributions from the source and the relay are balanced.