Reconfigurable Intelligent Surfaces for Wireless Communications: Principles, Challenges, and Opportunities

Recently there has been a flurry of research on the use of reconfigurable intelligent surfaces (RIS) in wireless networks to create smart radio environments. In a smart radio environment, surfaces are capable of manipulating the propagation of incident electromagnetic waves in a programmable manner to actively alter the channel realization, which turns the wireless channel into a controllable system block that can be optimized to improve overall system performance. In this article, we provide a tutorial overview of reconfigurable intelligent surfaces (RIS) for wireless communications. We describe the working principles of reconfigurable intelligent surfaces (RIS) and elaborate on different candidate implementations using metasurfaces and reflectarrays. We discuss the channel models suitable for both implementations and examine the feasibility of obtaining accurate channel estimates. Furthermore, we discuss the aspects that differentiate RIS optimization from precoding for traditional MIMO arrays highlighting both the arising challenges and the potential opportunities associated with this emerging technology. Finally, we present numerical results to illustrate the power of an RIS in shaping the key properties of a MIMO channel.Comment: to appear in the IEEE Transactions on Cognitive Communications and Networking (TCCN

On Spatial Multiplexing Using Reconfigurable Intelligent Surfaces

We consider an uplink multi-user scenario and investigate the use of reconfigurable intelligent surfaces (RIS) to optimize spatial multiplexing performance when a linear receiver is used. We study two different formulations of the problem, namely maximizing the effective rank and maximizing the minimum singular value of the RIS-augmented channel. We employ gradient-based optimization to solve the two problems and compare the solutions in terms of the sum-rate achievable when a linear receiver is used. Our results show that the proposed criteria can be used to optimize the RIS to obtain effective channels with favorable properties and drastically improve performance even when the propagation through the RIS contributes a small fraction of the received power.Comment: 5 pages, 4 figures, accepted for publication in IEEE Wireless Communications Letter

RIS Optimization on the Complex Circle Manifold for Interference Mitigation in Interference Channels

In this paper, we investigate the use of reconfigurable intelligent surfaces (RIS) to allow multiple user pairs to communicate simultaneously over the same channel. We propose a Riemannian manifold optimization approach to solve the problem of configuring the RIS passive reflection coefficients to minimize the total interference under constant modulus constraints. We compare the proposed approach to the widely-used semidefinite relaxation approach (SDR) for dealing with the constant modulus constraints. We investigate, using extensive numerical simulations, the effects of various system parameters, such as the number of users, the number of RIS elements, and the fraction of power received through the RIS. Our results demonstrate that RISs can substantially minimize interference allowing multiple user pairs to simultaneously communicate over the same channel and that the proposed approach vastly outperforms the semidefinite relaxation-based approach, which fails to find satisfactory solutions