  Reconfigurable intelligent surface (RIS) empowered communications with
non-orthogonal multiple access (NOMA) has recently become as an appealing
research direction for the next-generation wireless communications. In this
paper, we propose a novel NOMA solution with RIS partitioning, where we aim to
enhance the spectrum efficiency by improving the ergodic rate of all users, and
to maximize the user fairness. In the proposed system, we distribute the
physical resources among users such that the base station (BS) and RIS are
dedicated to serve different clusters of users. Furthermore, we formulate an
RIS partitioning optimization problem to slice the RIS elements between the
users such that the user fairness is maximized. The formulated problem is a
non-convex and non-linear integer programming (NLIP) problem with a
combinatorial feasible set, which is very challenging to solve. Therefore, we
exploit the structure of the problem to bound its feasible set and obtain a
sub-optimal solution by sequentially applying three efficient search
algorithms. Furthermore, we derive exact and asymptotic expressions for the
outage probability. Simulation results clearly indicate the superiority of the
proposed system over the considered benchmark systems in terms of ergodic
sum-rate, outage probability, and user fairness performance.
