The emerging reflecting intelligent surface (RIS) technology promises to enhance the capacity of wireless communication systems via passive reflect beamforming. However, the product path loss limits its performance gains. Fully-connected (FC) active RIS, which integrates reflect-type power amplifiers into the RIS elements, has been recently introduced in response to this issue. Also, sub-connected (SC) active RIS and hybrid FC-active/passive RIS variants, which employ a limited number of reflect-type power amplifiers, have been proposed to provide energy savings. Nevertheless, their flexibility in balancing diverse capacity requirements and power consumption constraints is limited. In this direction, this study introduces novel hybrid RIS structures, wherein at least one reflecting sub-surface (RS) adopts the SC-active RIS design. The asymptotic signal-to-noise-ratio of the FC-active/passive and the proposed hybrid RIS variants is analyzed in a single-user single-input single-output setup. Furthermore, the transmit and RIS beamforming weights are jointly optimized in each scenario to maximize the energy efficiency of a hybrid RIS-aided multi-user multiple-input single-output downlink system subject to the power consumption constraints of the base station and the active RSs. Numerical simulation and analytic results highlight the performance gains of the proposed RIS designs over benchmarks, unveil non-trivial trade-offs, and provide valuable insights.

翻译：新兴的反射智能表面（RIS）技术通过无源反射波束赋形有望提升无线通信系统的容量。然而，乘积路径损耗限制了其性能增益。为应对此问题，近期引入了全连接（FC）有源RIS技术，该技术将反射型功率放大器集成至RIS单元中。此外，为降低能耗，学界还提出了子连接（SC）有源RIS以及混合FC有源/无源RIS变体方案，这些方案仅使用有限数量的反射型功率放大器。然而，它们在平衡不同容量需求与功耗约束方面的灵活性仍存在局限。针对这一方向，本研究提出新型混合RIS结构，其中至少有一个反射子表面（RS）采用SC有源RIS设计。在单用户单输入单输出场景下，分析了FC有源/无源RIS及所提混合RIS变体的渐近信噪比。此外，针对混合RIS辅助的多用户多输入单输出下行链路系统，联合优化了各场景下的发射与RIS波束赋形权值，以在基站与有源RS功耗约束下实现能效最大化。数值仿真与分析结果揭示了所提RIS设计方案相较于基准方案的性能增益，展示了非平凡的折衷关系，并提供了有价值的见解。