Beamforming for the Cooperative Non-Orthogonal Multiple Access Transmission with Full-Duplex Relaying with...

We investigate the cooperative non-orthogonal multiple access (CNOMA) transmission through a full-duplex (FD) decode-and-forward (DaF) mode relay and propose two sub-optimal beamforming schemes for this CNOMA FD relay channel. For the optimization metric, we use the end-to-end information rate based on the mutual information from information theory. In addition to the pure CNOMA relay channel, the proposed beamforming schemes are applied to the security attack case as well, where an unauthorized eavesdropper tries to overhear the CNOMA transmission. The FD operation incurs the self-interference (SI) at the relay and the DaF mode along with CNOMA transmission forces the weakest link among the links toward three involved nodes to determine the end-to-end throughput. These facts lay the foundation for the designing and optimization of the beamforming vectors at the access point (AP) and at the relay. The first proposed sub-optimal optimization algorithm for the beamformer relies on the quadratically constrained quadratic problem (QCQP) in its central part, and this OCQP is iteratively applied with different interference level values at the near CNOMA user as the constraint term until some conditions for the design objectives are met. In addition to the first algorithm, a zero-forcing-based beamforming algorithm is proposed for a reference scheme. The proposed two algorithms are slightly modified to address the security-attacked CNOMA FD relay channel when a illegal user overhears the legitimate transmission. Simulation results are presented to advocate for the efficiency of the proposed algorithms for the CNOMA channel both with and without a security attack from an eavesdropper.