  For the realization of the future urban air mobility, reliable information
exchange based on robust and efficient communication between all airspace
participants will be one of the key factors to ensure safe operations.
Especially in dense urban scenarios, the direct and fast information exchange
between drones based on Drone-to-Drone communications is a promising technology
for enabling reliable collision avoidance systems. However, to mitigate
collisions and to increase overall reliability, unmanned aircraft still lack a
redundant, higher-level safety net to coordinate and monitor traffic, as is
common in today's civil aviation. In addition, direct and fast information
exchange based on ad hoc communication is needed to cope with the very short
reaction times required to avoid collisions and to cope with the the high
traffic densities. Therefore, we are developing a \ac{d2d} communication and
surveillance system, called DroneCAST, which is specifically tailored to the
requirements of a future urban airspace and will be part of a multi-link
approach. In this work we discuss challenges and expected safety-critical
applications that will have to rely on communications for \ac{uam} and present
our communication concept and necessary steps towards DroneCAST. As a first
step towards an implementation, we equipped two drones with hardware prototypes
of the experimental communication system and performed several flights around
the model city to evaluate the performance of the hardware and to demonstrate
different applications that will rely on robust and efficient communications.
