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B. Tahir, S. Schwarz, M. Rupp:
"RIS-Assisted Code-Domain MIMO-NOMA";
Talk: 29th European Signal Processing Conference (EUSIPCO), Dublin, Ireland (virtual); 08-23-2021 - 08-27-2021; in: "Proceedings of the 29th European Signal Processing Conference (EUSIPCO)", (2021), ISBN: 978-9-0827-9706-0; 821 - 825.

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.

Non Orthogonal Multiple Access, Reconfigurable Intelligent Surface, NOMA, RIS, IRS, Code-Domain, Optimization

https://publik.tuwien.ac.at/files/publik_296886.pdf