Simultaneously Transmitting And Reflecting (STAR) RIS Aided Wireless\n Communications

Abstract

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