  This paper introduces a reconfigurable intelligent surface (RIS)-assisted
grant-free non-orthogonal multiple-access (GF-NOMA) scheme. To ensure the power
reception disparity required by the power domain NOMA (PD-NOMA), we propose a
joint user clustering and RIS assignment/alignment approach that maximizes the
network sum rate by judiciously pairing user equipments (UEs) with distinct
channel gains, assigning RISs to proper clusters, and aligning RIS phase shifts
to the cluster members yielding the highest cluster sum rate. Once UEs are
acknowledged with the cluster index, they are allowed to access their resource
blocks (RBs) at any time requiring neither further grant acquisitions from the
base station (BS) nor power control as all UEs are requested to transmit at the
same power. In this way, the proposed approach performs an implicit
over-the-air power control with minimal control signaling between BS and UEs,
which has shown to deliver up to 20% higher network sum rate than benchmark
GF-NOMA and optimal grant-based PD-NOMA schemes depending on the network
parameters. The given numerical results also investigate the impact of UE
density, RIS deployment, and RIS hardware specifications on the overall
performance of the proposed RIS-aided GF-NOMA scheme.
