Hybrid Active-Passive Reconfigurable Intelligent Surface-Assisted Multi-User MISO Systems

TL;DR

This paper proposes a hybrid active-passive reconfigurable intelligent surface (RIS) to enhance multi-user MISO system performance, demonstrating significant improvements over passive RIS through an efficient optimization algorithm.

Contribution

It introduces a hybrid RIS with active elements and develops a low-complexity algorithm for joint beamforming and RIS coefficient optimization.

Findings

Hybrid RIS with 4 active elements improves system performance by up to 80%.

Passive RIS achieves only 27% improvement under similar conditions.

The proposed method effectively maximizes the minimum user rate.

Abstract

We consider a multi-user multiple-input single-output (MISO) communications system which is assisted by a hybrid active-passive reconfigurable intelligent surface (RIS). Unlike conventional passive RISs, hybrid RIS is equipped with a few active elements with the ability to reflect and amplify incident signals to significantly improve the system performance. Towards a fairness-oriented design, we maximize the minimum rate among all users through jointly optimizing the transmit beamforming vectors and RIS reflecting/amplifying coefficients. Combining tools from block coordinate ascent and successive convex approximation, the challenging nonconvex problem is efficiently solved by a low-complexity iterative algorithm. The numerical results show that a hybrid RIS with 4 active elements out of a total of 50 elements with a power budget of -1 dBm offers an improvement of up to 80% to the considered system, while that achieved by a fully passive RIS is only 27%.