  Digital twin channel (DTC) is the real-time mapping of a wireless channel
from the physical world to the digital world, which is expected to provide
significant performance enhancements for the sixth-generation (6G)
air-interface design. In this work, we first define five evolution levels of
channel twins with the progression of wireless communication. The fifth level,
autonomous DTC, is elaborated with multi-dimensional factors such as
methodology, characterization precision, and data category. Then, we provide
detailed insights into the requirements and architecture of a complete DTC for
6G. Subsequently, a sensing-enhanced real-time channel prediction platform and
experimental validations are exhibited. Finally, drawing from the vision of the
6G network, we explore the potential applications and the open issues in future
DTC research.
