  Reliable and efficient validation technologies are critical for the recent
development of multi-vehicle cooperation and vehicle-road-cloud integration. In
this paper, we introduce our miniature experimental platform, Mixed Cloud
Control Testbed (MCCT), developed based on a new notion of Mixed Digital Twin
(mixedDT). Combining Mixed Reality with Digital Twin, mixedDT integrates the
virtual and physical spaces into a mixed one, where physical entities coexist
and interact with virtual entities via their digital counterparts. Under the
framework of mixedDT, MCCT contains three major experimental platforms in the
physical, virtual and mixed spaces respectively, and provides a unified access
for various human-machine interfaces and external devices such as driving
simulators. A cloud unit, where the mixed experimental platform is deployed, is
responsible for fusing multi-platform information and assigning control
instructions, contributing to synchronous operation and real-time
cross-platform interaction. Particularly, MCCT allows for multi-vehicle
coordination composed of different multi-source vehicles (\eg, physical
vehicles, virtual vehicles and human-driven vehicles). Validations on vehicle
platooning demonstrate the flexibility and scalability of MCCT.
