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