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, 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. We develop a signal model for active RISs, which is validated through experimental measurements. Based on this model, we formulate the sum-rate maximization problem for active RIS aided multiple-input multiple-output (MIMO) systems and a precoding algorithm is proposed to solve this problem. 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.
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