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.

翻译：干扰管理是密集多天线无线网络中的核心瓶颈。因此，在本研究中，我们提出了一种由堆叠智能超表面辅助的免数字预编码分层速率分割多址接入架构，旨在以降低的硬件复杂度实现高频谱效率和用户公平性。在所提出的系统中，基站仅执行标量功率分配，而多层SIM充当波域处理器，通过非线性波前重构在空间上分离用户并抑制干扰。该设计消除了对数字或混合预编码的需求，大幅减少了基带计算量。我们构建了一个联合优化问题，通过联合优化SIM相移、功率分配和用户分组来最大化最小用户速率。为高效求解所得非凸问题，开发了一种交替优化算法，该算法结合了用于SIM配置和功率控制的同步扰动随机逼近以及基于聚类的分组细化。仿真结果表明，与混合波束成形和非预编码基线相比，所提出的SIM辅助HRSMA在频谱效率和公平性方面均取得了显著增益。具体而言，通过利用多层SIM提供的额外波域自由度，SIM辅助HRSMA以显著更少的活跃天线实现了相当或更优的最小速率。这些发现凸显了SIM辅助HRSMA作为低成本、高能效且可扩展的6G+网络解决方案的潜力。