Beamforming for Cooperative Non-Orthogonal Multiple Access with Full-Duplex Amplify-and-Forward Relaying

We consider cooperative non-orthogonal multiple access (CNOMA) transmission through a full-duplex (FD) amplify-and-forward (AF) relay. Two CNOMA users are served by an access point (AP) through the FD-AF relay. FD relaying can be spectrally efficient if it sufficiently suppresses the self-interference (SI) from the FD operation. The feedback structure of the FD-AF relaying makes the convergence of the SI a critical problem because catastrophic system failure may occur if the system fails to converge the SI. The two linear beamforming algorithms proposed in this paper address these challenges, where the first is based on the zero-forcing (ZF) of the SI and the other regularizes the SI to improve the first scheme. The ZF-based algorithm completely nulls out the SI and incurs no convergence problems. In contrast, the second method is based on the regularization of SI to improve the first one, and it carefully locates the operating point of the FD-AF relay such that the system avoids the SI divergence. Numerical simulation results are provided to discuss the advantages and disadvantages of the proposed algorithms by comparing their average sum-rate performances.