Uplink Multiuser SIMO Empowered by Extremely Large STAR-RIS

Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have emerged as a promising technology for shaping wireless environments favorable to their supporting systems while providing full-space coverage. However, the path loss product of passive STAR-RISs necessitates the deployment of extremely large (XL)-STAR-RIS elements. We integrate an XL-STAR-RIS operating in energy-splitting mode into an uplink multiuser single-input multiple-output (SIMO) system to improve spectral efficiency. Our primary focus is to investigate the impact of the near-field channels between the XL-STAR-RIS and users on the sum rate. To this end, we propose a sum rate maximization algorithm that alternately optimizes XL-STAR-RIS beamforming and power allocation, leveraging closed-form power allocation through La-grangian dual and quadratic transforms using the optimal receive beamforming. Performance evaluation demonstrates the sum rate gains attributed to near-field effects by comparing the optimized sum rates in near-field and far-field channel models.