  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.
