  We consider the combination of uplink code-domain non-orthogonal multiple
access (NOMA) with massive multiple-input multiple-output (MIMO) and
reconfigurable intelligent surfaces (RISs). We assume a setup in which the base
station (BS) is capable of forming beams towards the RISs under line-of-sight
conditions, and where each RIS is covering a cluster of users. In order to
support multi-user transmissions within a cluster, code-domain NOMA via
spreading is utilized. We investigate the optimization of the RIS phase-shifts
such that a large number of users is supported. As it turns out, it is a
coupled optimization problem that depends on the detection order under
interference cancellation and the applied filtering at the BS. We propose to
decouple those variables by using sum-rate optimized phase-shifts as the
initial solution, allowing us to obtain a decoupled estimate of those
variables. Then, in order to determine the final phase-shifts, the problem is
relaxed into a semidefinite program that can be solved efficiently via convex
optimization algorithms. Simulation results show the effectiveness of our
approach in improving the detectability of the users.
