Secure beamforming and deployment design for rate-splitting multiple access-based UAV communications

Rate-splitting multiple access (RSMA) has recently gained attention as a potential robust multiple access (MA) scheme for upcoming wireless networks. Given its ability to efficiently utilize wireless resources and design interference management strategies, it can be applied to unmanned aerial vehicle (UAV) networks to provide convenient services for large-scale access ground users. However, due to the line-of-sight (LoS) broadcast nature of UAV transmission, information is susceptible to eavesdropping in RSMA-based UAV networks. Moreover, the superposition of signals at the receiver in such networks becomes complicated. To cope with the challenge, we propose a two-user multi-input single-output (MISO) RSMA-based UAV secure transmission framework in downlink communication networks. In a passive eavesdropping scenario, our goal is to maximize the sum secrecy rate by optimizing the transmit beamforming and deployment location of the UAV-base station (UAV-BS), while considering quality-of-service (QoS) constraints, maximum transmit power, and flight space limitations. To address the non-convexity of the proposed problem, the optimization problem is first decoupled into two subproblems. Then, the successive convex approximation (SCA) method is employed to solve each subproblem using different propositions. In addition, an alternating optimization (AO)-based location RSMA (L-RSMA) beamforming algorithm is developed to implement joint optimization to obtain the suboptimal solution. Numerical results demonstrate that (1) the proposed L-RSMA scheme yields a 28.97% higher sum secrecy rate than the baseline L-space division multiple access (SDMA) scheme; (2) the proposed L-RSMA scheme improves the security performance by 42.61% compared to the L-non-orthogonal multiple access (NOMA) scheme.