Irregular STAR-RIS-Aided Wireless Systems with a Limited Number of Passive Elements

Simultaneous transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have recently gained a great deal of attention due to their ability of providing full-space service coverage and supporting stable transmission for ultra reliable and low latency communications (URLLC). In this paper, a novel concept called irregular STAR-RIS is proposed with a small portion of activated transmitting and reflecting elements, where users in opposite sides of the surface are served simultaneously. In this proposed system, a tabu search-based sparse deployment and neighbor extraction-based cross-entropy method for the beamforming optimization of the proposed irregular STAR-RIS are considered. For the sake of fair comparison and to highlight the achievable gains, the weighted sum-rate (WSR) performance of a regular STAR-RIS-aided wireless system and a traditional system without STAR-RIS is also evaluated. The results show that the proposed irregular STAR-RIS with a small portion of active elements is capable of significantly improving the WSR performance compared to the conventional systems without STAR-RIS. It is also shown that the position of the users has an impact on the WSR performance, and that the near users have better performance than the far ones.