Physical-Layer Security for Cache-Enabled C-RANs via Rate Splitting

This letter proposes a rate splitting (RS)-based secure transmit approach against multiple eavesdroppers in cache-enabled cloud radio access networks (C-2-RANs). Unlike existing secure schemes, our approach uses the common stream for dual purposes, serving as the desired message for legitimate users (LUs) and artificial noise (AN) for eavesdroppers. We jointly optimize message splitting, remote radio head (RRH) clustering, and beamforming to maximize the minimum secrecy rate among all LUs and to meet the fronthaul capacity and transmit power per RRH constraints. We transform the formulated problem into two-tier alternating optimization by constructing accurate surrogates for the non-convex objective functions and constraints. We solve the outer-tier and inner-tier problems with the semi-closed-form expressions and the convex framework, respectively. The proposed transmit approach achieves significant gains over existing schemes.