On Gray Labeling for Downlink Non-Orthogonal Multiple Access Without SIC

The impact of labeling a multilevel coding scheme with pulse amplitude modulation constellation in downlink non-orthogonal multiple access (NOMA) systems is studied. Particular interest is paid to the case where the strong user's receiver does not perform successive interference cancellation (SIC) before decoding its desired message. With judiciously chosen parameters, it was shown in a recent work that the considered scheme can approach the capacity region to within a constant gap. In this letter, analytic and extensive simulation results are given to show that Gray labeling provides better performance than non-Gray labeling and natural labeling for the scheme in terms of the symbol error rate, block error rate, and epsilon-feasible rate. Simulation results also indicate that the performance gap between the case with SIC and that without SIC for Gray labeling can be further bridged, which suggests the possibility of avoiding SIC in downlink NOMA.