Downlink Precoder Design for Two-User Power-Domain MIMO-NOMA with Excess Degrees of Freedom

In this paper, we consider downlink precoder design for two-user power-domain multiple-input multiple-output (MIMO) non-orthogonal multiple access (NOMA) for the scenario where the receivers have fewer antennas than the transmitter. We propose a novel precoding scheme which takes advantage of the excess degrees of freedom (DoFs) at the transmitter to enhance MIMO-NOMA performance. The proposed precoder achieves simultaneous diagonalization of the MIMO channels of both users, assuming self-interference cancellation at one of the receivers, thereby lowering the overall decoding complexity. The performance of the proposed scheme is analyzed in terms of the ergodic rate using results from non-asymptotic random matrix theory. Our results show that the proposed scheme significantly outperforms orthogonal multiple access (OMA) and a benchmark precoder performing simultaneous diagonalization via generalized singular value decomposition (GSVD).