In this letter, based on the non-orthogonal multiple access (NOMA) concept, a quality-of-
service optimization problem for two-user multiple-input-single-output broadcast systems is
considered, given a pair of target interference levels. The minimal power and the optimal
precoding vectors are obtained by considering its Lagrange dual problem and via Newton's
iterative algorithm, respectively. Moreover, the closed-form expressions of the minimal
transmission power for some special cases are also derived. One of these cases is termed …

Recently, optimum non-orthogonal multiple access (NOMA) precoding for a two-user
multiple-input single-output (MISO) downlink has been proposed by Chen et al. Reference
demonstrates that strong duality holds for the MISO-NOMA precoding optimization problem.
Hence, the proposed preocoding algorithm is not only locally optimal, but also globally
optimal. However, the proof of this proposition is flawed. In this regard, we provide a
corrected proof in this comment.