An Exact Analysis on Security-Reliability Tradeoff in Downlink Rate Splitting Multiple Access

This letter considers the downlink transmission in the rate-splitting multiple access (RSMA) aided system in slow-fading channels. Specifically, a source transmits the intended common and private streams both at constant rates to the served K users at the same time-frequency resources, while an external eavesdropper tries to overhear the messages sent to the K legitimate users. In contrast to the existing works, we first propose a novel outage probability definition to exactly measure the outage performance of this RSMA system. Then, based on the proposed outage probability, we take the Rayleigh fading channel as an example for performance analysis, where we analyze the security-reliability tradeoff by deriving an analytical expression for the optimal average sum outage rate (ASOR) under a target eavesdropping probability. Moreover, we also derive a simple but tight closed-form approximation for the optimal ASOR in the high signal-to-noise ratio (SNR) regime. Finally, numerical results are presented to verify the accuracy of the derived expressions.