Robust Secure Precoding for NOMA Multi-beam Satellite Systems

We investigate multi-user physical layer security assisted by an unmanned aerial vehicle (UAV) for multi-beam satellite communications in the presence of an eavesdropper (Eve) within the same beam. In particular, the UAV is exploited as a jammer that deliberately generates artificial noise to confuse Eve. To achieve a positive secrecy rate, we consider a non-orthogonal multiple access (NOMA) scheme with imperfect channel state information at the user and Eve. Specifically, we design a robust precoding algorithm to maximize the minimum achievable secrecy rate that satisfies the quality of service for each user and the NOMA decoding order between legitimate users in each beam. In addition, the algorithm is designed under joint total and per-beam transmit power constraints. We first combine the Bernstein-type inequality, the arithmetic-geometric mean inequality with a semi-definite relaxation iterative algorithm to solve the non-convex problem, and further analyze the complexity of the proposed precoding design. Simulation results show that the algorithm significantly improves the security performance.