Fair and Secure Beamforming for RSMA-Based UAV Underlay Networks with Imperfect CSI

Non-terrestrial networks (NTNs) using unmanned aerial vehicles (UAVs) as temporary base stations enable flexible coverage in disaster-stricken areas and congested regions. This paper presents a beamforming design for rate splitting multiple access (RSMA) enabled UAV underlay networks that addresses untrusted users (potential eavesdroppers), enforces max-min fairness in achievable user rates, accommodates imperfect CSI, and limits interference to a licensed primary receiver. We transform the non-convex optimization problem via auxiliary-variable-based reformulation and successive convex approximation (SCA), obtaining a tractable convex semi-definite program (SDP). Simulations demonstrate that the proposed framework achieves perfect rate fairness (Jain's fairness index = 1.0) across all users while delivering robust sum-rate performance under varying numbers of users, UAV transmit antennas, and secrecy requirements.