In this letter, a low-complexity waterfilling-based Power Allocation (PA) technique, incorporated within the Proportional Fairness (PF) scheduler, is proposed and applied to a Non-Orthogonal Multiple Access (NOMA) scheme in a cellular down-link system. The aim of the proposed joint PA and scheduling scheme is to maximize the achieved average throughput through a quasi-optimal repartition of the transmit power among subbands, while guaranteeing a high level of fairness in resource allocation. Extensive simulation results show that the proposed technique enhances both system capacity and user fairness, when compared to either orthogonal signaling (OS) or NOMA with static PA.