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

As a revolutionary paradigm for controlling wireless channels, reconfigurable intelligent surface (RIS) has emerged as a candidate technology for future 6G networks. However, due to the multiplicative fading effect, the existing passive RISs only achieve a negligible capacity gain in many scenarios with strong direct links. In this paper, the concept of active RISs is proposed to overcome this fundamental limitation. Unlike the existing passive RISs that reflect signals without amplification, active RISs can amplify the reflected signals actively through integrating amplifiers into their elements. To characterize the signal amplification and incorporate the noise introduced by active components, we develop a signal model for active RISs, which is validated through the experimental measurements on a fabricated active RIS element. Based on the developed signal model, we further analyze the asymptotic performance of active RISs to reveal its notable capacity gain for wireless communications. Finally, we formulate the sum-rate maximization problem for an active RIS aided multiple-input multiple-output (MIMO) system and a joint transmit beamforming and reflect precoding algorithm is proposed to solve this problem. Simulation results show that, in a typical wireless system, the existing passive RISs can realize only a negligible sum-rate gain of 3%, while the proposed active RISs can achieve a significant sum-rate gain of 108%, thus overcoming the multiplicative fading effect.