Simultaneously Transmitting and Reflecting (STAR) RIS Aided Wireless Communications

The novel concept of simultaneously transmitting and reflecting (STAR) reconfigurable intelligent surfaces (RISs) is investigated, where the incident wireless signal divided into transmitted reflected signals passing both sides space surrounding surface, thus facilitating a full-space manipulation propagation. Based on introduced basic model `STAR', three practical operating protocols for STAR-RISs are proposed, namely energy splitting (ES), mode switching (MS), time (TS). Moreover, STAR-RIS aided downlink communication system considered unicast multicast transmission, multi-antenna base station (BS) sends information to two users, i.e., one each side STAR-RIS. A power consumption minimization problem joint optimization active beamforming at BS passive transmission reflection formulated proposed protocols, subject rate constraints users. For ES, resulting highly-coupled non-convex solved by an iterative algorithm, which exploits penalty method successive convex approximation. Then, penalty-based algorithm extended solve mixed-integer MS. TS, decomposed subproblems, can be consecutively using state-of-the-art algorithms techniques. Finally, our numerical results reveal that: 1) TS ES generally preferable respectively; 2) required scenarios significantly reduced employing instead conventional reflecting/transmiting-only RISs.