MISO NOMA Downlink Beamforming Optimization with Per-Antenna Power Constraints.

Consider a multiuser downlink beamforming optimization problem for the non-orthogonal multiple access (NOMA) transmission in a multiple-input single-output (MISO) system. The total transmission power minimization problem is formulated subject to both per-antenna power constraints and quality-of-service (QoS) constraints under the NOMA principal. The problem is a non-convex quadratically constrained quadratic program, and the conventional semidefinite program (SDP) relaxation is not tight. In order to tackle the non-convex NOMA beamforming problem, we construct a second-order cone approximation for each signal-to-interference-plus-noise ratio (SINR) constraint and form an iterative algorithm, in which a sequence of second-order cone programs (SOCPs) are solved. The optimal values of the sequence of SOCPs are non-increasing, and it converges to a locally optimal value. However, our extensive simulation results show that the locally optimal value is more or less as good as the globally optimal value. In particular, we show that the SDP relaxation is tight for two-user case if one of the SINR constraints is strict (non-binding) at the optimality. Detailed simulation results are presented to demonstrate the performance gains of the NOMA downlink beamforming with per-antenna power constraints through the proposed approximate algorithm.