  Reconfigurable intelligent surfaces (RISs) have attracted extensive attention
from industry and academia. In RIS-assisted wireless communication systems,
practical channel measurements and modeling serve as the foundation for system
design, network optimization, and performance evaluation. In this paper, a RIS
time-domain channel measurement system, based on a software defined radio
platform, is developed for the first time to investigate the small-scale fading
characteristics of RIS-assisted channels. We present RIS channel measurements
in corridor and laboratory scenarios and compare the power delay profile of the
channel without RIS, with RIS specular reflection, and with RIS intelligent
reflection. The multipath component parameters and cluster parameters based on
the Saleh-Valenzuela model are extracted. We find that the power delay profiles
(PDPs) of the RIS-assisted channel fit the power-law decay model better than
the common exponential decay model and approximate the law of square decay.
Through intelligent reflection, the RIS can decrease the delay and concentrate
the energy of the virtual line-of-sight (VLoS) path, thereby reducing the delay
spread and mitigating multipath fading. Furthermore, the cluster
characteristics of RIS-assisted channels are highly dependent on the
measurement environment. In the laboratory scenario, a single cluster dominated
by the VLoS path with smooth envelope is observed. On the other hand, in the
corridor scenario, some additional clusters introduced by the RIS reflection
are created.
