Novel Signaling Design for MIMO-NOMA Against External and Internal Eavesdroppers

Secure communication over two related multiple-input multiple-output (MIMO) non-orthogonal multiple access (NOMA) networks is investigated. Both networks consist of two legitimate (internal) users and an eavesdropper (external user). In one network, in addition to the external eavesdropper, one of the internal users is also seen as an eavesdropper to the other internal user, resulting in layered confidentiality. The secrecy capacity expressions of these networks are composed of nonconvex functions of transmit covariance matrices, which makes it intractable to find optimal transmit covariance matrices. To overcome this challenge, we form a weighted sum-rate maximization problem for each network and apply the block successive maximization method. This method updates the primal variable blocks successively by maximizing local lower-bounds of the original problem, followed by an update for the dual variable in a closed form. The solutions for these problems design covariance matrices corresponding to the transmitted messages such that they are transmitted securely and reliably. Numerical results illustrate the efficacy of the proposed signaling design for both MIMO-NOMA networks.