Reconfigurable Intelligent Surfaces Based on Single, Group, and Fully Connected Discrete-Value Impedance Networks

Reconfigurable intelligent surfaces (RISs) allow to control the propagation environment in wireless networks by properly tuning multiple reflecting elements. Traditionally, RISs have been realized through a single connected architecture, where each RIS element is controlled by an impedance connected to ground. In a recent work, this architecture has been generalized by realizing RISs through group and fully connected impedance networks. However, impedance networks reconfigurable with arbitrary precision are hard to realize in practice. In addition, it is still unexplored how to group together the RIS elements in group connected architectures. These two problems are addressed in this paper. Firstly, we propose a RIS design strategy based on reconfigurable impedance networks with discrete values. Secondly, we present three approaches to design the grouping strategy in group connected RISs. Numerical results show that fewer resolution bits are necessary to achieve the performance upper bound as the group size increases. While four resolution bits are needed in single connected architectures, only a single resolution bit is sufficient in fully connected ones. In addition, we show that by dynamically optimizing the grouping strategy, RISs with group size 4 nearly achieve the same performance as fully connected RISs, with reduced hardware complexity.