Physical layer security of rate splitting multiple access in underlay cognitive radio networks

This paper studies the physical layer security of rate-splitting multiple access (RSMA) in underlay cognitive radio (CR) networks over Nakagami- m fading channels. In the considered system, a primary user (PU) allows a secondary transmitter (ST) to share its spectrum, while the ST employs RSMA to simultaneously serve multiple secondary users (SUs). To protect the quality of service of the PU, the transmit power of the ST is constrained by an interference threshold. Due to the structure of RSMA, users are capable of decoding not only their intended signals but also parts of other users’ transmissions through successive interference cancellation, which introduces potential security concerns within the system. To address this issue, we analyze both the ergodic sum rate and the secrecy performance of the considered network. In particular, analytical expressions are derived for the ergodic rates of the common and private streams, as well as the ergodic secrecy rate under the considered setting. Based on the derived analytical framework, a secrecy-oriented power-splitting optimization strategy is further developed to balance communication security and transmission efficiency. The analytical results are corroborated by Monte Carlo simulations, which confirm their accuracy. Numerical results further illustrate the trade-off between spectral efficiency and security under interference constraints, and highlight the influence of the power splitting factor in RSMA. These findings provide useful insights for the design of secure and efficient transmission strategies in CR systems employing RSMA.