  The reconfigurable intelligent surface (RIS) with low hardware cost and
energy consumption has been recognized as a potential technique for future 6G
communications to enhance coverage and capacity. To achieve this goal, accurate
channel state information (CSI) in RIS assisted wireless communication system
is essential for the joint beamforming at the base station (BS) and the RIS.
However, channel estimation is challenging, since a large number of passive RIS
elements cannot transmit, receive, or process signals. In the first part of
this invited paper, we provide an overview of the fundamentals, solutions, and
future opportunities of channel estimation in the RIS assisted wireless
communication system. It is noted that a new channel estimation scheme with low
pilot overhead will be provided in the second part of this paper.
