Relay-Reconfigurable Intelligent Surface Cooperation for Energy-Efficient Multiuser Systems

TL;DR

This paper proposes a cooperative system combining reconfigurable intelligent surfaces and relays to enhance energy efficiency in multi-user communications, optimizing phase shifts and beamforming for power minimization.

Contribution

It introduces a novel cooperative framework of RIS and relays, with two efficient algorithms for joint optimization of phase shifts and beamforming in multi-user systems.

Findings

Proposed solutions outperform systems with only RIS or relay.

Performance improves when RIS and relay are closer to users.

Joint optimization reduces total transmit power significantly.

Abstract

Reconfigurable intelligent surfaces (RIS) have drawn considerable attention recently due to their controllable scattering elements that are able to direct electromagnetic waves into desirable directions. Although RISs share some similarities with relays, the two have fundamental differences impacting their performance. To harness the benefits of both relaying and RISs, a multi-user communication system is proposed in this paper wherein a relay and an RIS cooperate to improve performance in terms of energy efficiency. To utilize the RIS efficiently, the discrete phase shifts of the RIS elements are optimized along with the beamforming matrices at the transmitter and the relay, targeting the minimization of the total transmit power subject to a quality-of-service (QoS) constraint. Then, two suboptimal efficient solutions are proposed for the resulting discrete and non-convex problem, one based on singular value decomposition (SVD) and uplink-downlink duality and the other is based on SVD combined with zero-forcing. Simulations show that the proposed solutions outperform a system with either a relay or an RIS only, especially when both are closer to the users than to the base-station.