Active RIS vs. Passive RIS: Which Will Prevail in 6G?

TL;DR

This paper introduces active RISs with amplification capabilities to overcome the capacity limitations of passive RISs caused by multiplicative fading, demonstrating significant performance improvements in 6G wireless systems.

Contribution

The paper proposes and models active RISs with signal amplification, experimentally verifies their performance, and develops a joint beamforming scheme to maximize sum-rate in multi-user systems.

Findings

Active RISs achieve a 130% sum-rate gain in simulations.

Passive RISs only achieve a 22% sum-rate gain.

Active RISs overcome the multiplicative fading effect in wireless channels.

Abstract

As a revolutionary paradigm for controlling wireless channels, reconfigurable intelligent surfaces (RISs) have emerged as a candidate technology for future 6G networks. However, due to the "multiplicative fading" effect, the existing passive RISs only achieve limited capacity gains in many scenarios with strong direct links. In this paper, the concept of active RISs is proposed to overcome this fundamental limitation. Unlike passive RISs that reflect signals without amplification, active RISs can amplify the reflected signals via amplifiers integrated into their elements. To characterize the signal amplification and incorporate the noise introduced by the active components, we develop and verify the signal model of active RISs through the experimental measurements based on a fabricated active RIS element. Based on the verified signal model, we further analyze the asymptotic performance of active RISs to reveal the substantial capacity gain they provide for wireless communications. Finally, we formulate the sum-rate maximization problem for an active RIS aided multi-user multiple-input single-output (MU-MISO) system and a joint transmit beamforming and reflect precoding scheme is proposed to solve this problem. Simulation results show that, in a typical wireless system, passive RISs can realize only a limited sum-rate gain of 22%, while active RISs can achieve a significant sum-rate gain of 130%, thus overcoming the "multiplicative fading" effect.