Joint RIS-RSMA Optimization for Asynchronous Multi-User Cell-Free MIMO

Abstract

Cell-free multiple-input multiple-output (CF-MIMO) systems assisted by reconfigurable intelligent surfaces (RISs) and rate-splitting multiple access (RSMA) are promising for interference management in distributed sixth-generation (6G) networks. In practical CF-MIMO deployments, independent access-point (AP) oscillators and unequal propagation delays introduce AP-dependent phase variations that reduce coherent combining and degrade precoding performance. This paper develops a unified RIS-RSMA optimization framework for asynchronous downlink CF-MIMO. The RSMA common precoder, private precoders, and passive RIS phase shifts are jointly optimized within a sum-rate maximization problem subject to total transmit-power, per-user quality-of-service, and RIS unit-modulus constraints. The resulting non-convex problem is addressed through a weighted minimum mean-square error (WMMSE)-based alternating optimization framework with Gauss–Seidel coordinate optimization for the RIS phase subproblem. An analytical common-stream power expression is derived to explain the limited benefit of decoupled common-precoder designs. The analysis shows that independent oscillator phase drifts at the APs attenuate coherent cross-AP combining terms and can restrict the common rate through the weakest-user decoding constraint. In the default overloaded CF-MIMO configuration, the tested heuristic common-precoder schemes provide less than 0.2% gain over the corresponding optimized space-division multiple access (SDMA) baseline when combined with WMMSE-optimized private precoders. In contrast, the proposed joint RSMA design achieves approximately 12–15% sum-rate gain at moderate-to-high transmit powers and retains a similar high-power advantage in a larger-scale validation. Targeted comparisons using WMMSE and regularized zero-forcing (RZF) private precoders, together with the common-power allocation results, support the need for joint common/private precoder design. Additional results show that the gain persists across the tested RIS sizes, oscillator phase-noise variances, Rician factors, loading ratios, channel realizations, and multi-RIS deployments. Distributed multi-RIS deployment also improves spatial coverage uniformity under a fixed total element budget.