Joint Beamforming Design for Multiuser MISO Downlink Aided by a Reconfigurable Intelligent Surface and a Relay

Mohanad Obeed, Anas Chaaban

Research output: Contribution to journalArticlepeer-review

14 Scopus citations


Reconfigurable intelligent surfaces (RISs) have drawn considerable attention due to their ability 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, we propose a downlink system wherein a relay and an RIS improve performance in terms of energy-efficiency. Using singular value decomposition (SVD), semidefinite programming (SDP), and function approximations, we propose different solutions for optimizing the beamforming matrices at the base-station (BS), the relay, and the phase shifts at the RIS to minimize the total power under quality-of-service (QoS) constraints. The problem is solved when the relay operates in half-duplex and full-duplex modes and when the reflecting elements have continuous and discrete phase shifts. Simulation results compare the performance of the system with and without the RIS or the relay, under different optimization solutions. The results show that the system with full-duplex relay and RIS outperforms the other scenarios, and the contribution of full-duplex relay is higher than that of the RIS. However, an RIS outperforms a half-duplex relay when the required QoS is high. The results also show that increasing the number of reflecting elements improves the performance better in the presence of a relay than in its absence.
Original languageEnglish (US)
Pages (from-to)8216-8229
Number of pages14
Issue number10
StatePublished - Apr 12 2022
Externally publishedYes

Bibliographical note

KAUST Repository Item: Exported on 2022-10-25
Acknowledged KAUST grant number(s): OSR2018-CRG7-3734
Acknowledgements: This work was supported by the King Abdullah University of Science and Technology (KAUST) under Award OSR2018-CRG7-3734.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.


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