  Reconfigurable intelligent surfaces (RISs) are promising candidate for the 6G
communication. Recently, active RIS has been proposed to compensate the
multiplicative fading effect inherent in passive RISs. However, conventional
distributed active RISs, with at least one amplifier per element, are costly,
complex, and power-intensive. To address these challenges, this paper proposes
a novel architecture of active RIS: the centralized active RIS (CA-RIS), which
amplifies the energy using a centralized amplifying reflector to reduce the
number of amplifiers. Under this architecture, only as low as one amplifier is
needed for power amplification of the entire array, which can eliminate the
mutual-coupling effect among amplifiers, and significantly reduce the cost,
noise level, and power consumption. We evaluate the performance of CA-RIS,
specifically its path loss, and compare it with conventional passive RISs,
revealing a moderate amplification gain. Furthermore, the proposed CA-RIS and
the path loss model are experimentally verified, achieving a 9.6 dB net gain
over passive RIS at 4 GHz. The CA-RIS offers a substantial simplification of
active RIS architecture while preserving performance, striking an optimal
balance between system complexity and the performance, which is competitive in
various scenarios.
