Generalized Reconfigurable Intelligent Surfaces: From Transmitting and Reflecting Modes to Single-, Group-, and Fully-Connected Architectures

Reconfigurable intelligent surfaces (RISs) are envisioned as a promising technology for future wireless communications. With various hardware realizations, RISs can work under different modes (reflective/transmissive/hybrid) or have different architectures (single-/group-/fully-connected). However, most existing research focused on either reflective RISs or single-connected hybrid RISs while there is lack of a comprehensive study for RISs unifying different modes/architectures. In this paper, we solve this issue by analyzing and proposing a general RIS-aided communication model which unifies the reflective/transmissive/hybrid modes and single-/group-/fullyconnected architectures. With the proposed model, we consider jointly designing the transmit precoder and RIS beamformer to maximize the sum-rate for RIS-aided systems. Leveraging fractional programming theory, the original sum-rate maximization problem is equivalently transformed into a multi-block optimization, which can be solved by block coordinate descent methods. We also provide simulation results to compare the performance of RISs with different modes/architectures. Compared with singleconnected hybrid RISs, fully- and group-connected hybrid RISs can increase the sum-rate by around 75% and 37% for Rayleigh fading channels.Comment: 12 pages, 8 figures, 2 tables, submitted to Transactions on Wireless Communication