Hybrid RIS With Sub-Connected Active Partitions: Performance Analysis and Transmission Design

The emerging reconfigurable intelligent surface (RIS) technology promises to enhance network capacity via passive reflect beamforming. However, product path loss limits its performance gains. Fully-connected (FC) active RIS integrates a reflect-type power amplifier into each element to address this issue, while sub-connected (SC) active RIS, which employs partitioning and power amplifier sharing per partition, and hybrid FC-active/passive RIS utilize fewer power amplifiers to tackle the resulting cost and energy consumption challenges. This study introduces novel hybrid RIS structures with at least one SC-active reflecting sub-surface (RS) to achieve a better capacity/energy efficiency (EE) balance. A system model considering power amplifier sharing and resulting in a proper diagonal SC-active RIS/RS beamforming matrix is derived for the first time. The asymptotic signal-to-noise-ratio of active/passive and active/active RIS with a single shared power amplifier per active RS in a single-user single-input single-output setup is analyzed. The transmit and RIS beamforming weights are jointly optimized to maximize the EE of a hybrid RIS-aided multi-user multiple-input single-output downlink system under the power consumption constraints of the base station and the RIS. Numerical results highlight the substantial performance gains of the proposed designs over benchmarks and provide insights.