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

TL;DR

This paper introduces a precoding-free hierarchical rate-splitting multiple access system aided by stacked intelligent metasurfaces, significantly improving spectral efficiency and fairness while reducing hardware complexity in dense wireless networks.

Contribution

It proposes a novel SIM-assisted HRSMA architecture that eliminates digital precoding, optimizing wavefront reconfiguration for interference mitigation and user separation.

Findings

Achieves higher spectral efficiency and fairness compared to traditional methods.

Reduces hardware complexity by eliminating digital precoding.

Attains comparable or better user rates with fewer antennas.

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.