A method of determining a performance metric for a selection of a first user and a second user among a set of candidate users for attribution to a sub-band in a multiple access communications system based on Non-Orthogonal Multiple Access (NOMA), is provided wherein the first user () and the second user () are selected as the pair of candidate users corresponding to an extremum of the ratio between a first term reflecting the total throughput achievable by any pair of the candidate users assigned to the sub-band (s) under consideration, and a second term reflecting the known throughput achieved by that same pair of candidate users over a predetermined preceding period. Implementations include a method of determining a performance metric is presented for attributing users to one or more of a plurality of sub-bands in a multiple access communications system, in which in an initial assignment phase for a specific sub-band, a first user is selected for that band on the basis of one or more criteria such as user priority. Then a second sub-band user maximizing or minimizing the performance metric.