Precoding-free Hierarchical Rate-Splitting Multiple Access via Stacked Intelligent Metasurface

TL;DR

The paper tackles interference management in dense multi-antenna systems by proposing a precoding-free hierarchical rate-splitting MA enabled by a stacked intelligent metasurface. It introduces a joint optimization framework that tunes SIM configurations, power allocation, and user grouping to maximize the minimum user rate, solved via an alternating optimization algorithm that leverages SPSA for phase and power updates and clustering-based grouping with greedy refinement. The results show that SIM-aided HRSMA achieves superior min-rate performance and fairness with fewer active antennas compared to conventional hybrid beamforming and non-precoding baselines, owing to the wave-domain degrees of freedom provided by the SIM. The work demonstrates the potential of SIMs as low-cost, energy-efficient, scalable transceiver components for beyond-6G networks and outlines practical design considerations and future extensions to wideband and multi-cell scenarios.

Abstract

Interference management is a central bottleneck in dense multi-antenna wireless networks. Therefore, in this study, we present a digital precoding-free hierarchical rate-splitting multiple access (HRSMA) architecture assisted by a stacked intelligent metasurface (SIM) to achieve high spectral efficiency and user fairness with reduced hardware complexity. In the proposed system, the base station performs only scalar power allocation, while a multi-layer SIM acts as a wave-domain processor that spatially separates users and mitigates interference via nonlinear wavefront reconfiguration. This design eliminates the need for digital or hybrid precoding, drastically reducing the baseband computations. A joint optimization problem is formulated to maximize the minimum user rate by jointly optimizing SIM phase shifts, power allocation, and user grouping. To efficiently solve the resulting non-convex problem, an alternating optimization algorithm is developed, combining simultaneous perturbation stochastic approximation (SPSA) for SIM configuration and power control with clustering-based grouping refinement. Simulation results demonstrate that the proposed SIM-aided HRSMA achieves substantial gains in both spectral efficiency and fairness compared to hybrid beamforming and non-precoding baselines. Specifically, SIM-aided HRSMA attains comparable or superior minimum rates with significantly fewer active antennas by exploiting the additional wave-domain degrees of freedom provided by multi-layer SIMs. These findings highlight the potential of SIM-aided HRSMA as a low-cost, energy-efficient, and scalable solution for beyond-6G networks.

Figures (10)

• Figure 1: Comparison of conventional hybrid beamforming-based HRSMA and SIM-aided HRSMA.
• Figure 2: The proposed SIM-aided HRSMA system.
• Figure 3: Convergence behavior of Algorithm \ref{['alg:AO_framework']}.
• Figure 4: Simulation environment.
• Figure 5: Minimum achievable rate versus the number of SIM layers.