  Decentralized multi-robot systems typically perform coordinated motion
planning by constantly broadcasting their intentions as a means to cope with
the lack of a central system coordinating the efforts of all robots. Especially
in complex dynamic environments, the coordination boost allowed by
communication is critical to avoid collisions between cooperating robots.
However, the risk of collision between a pair of robots fluctuates through
their motion and communication is not always needed. Additionally, constant
communication makes much of the still valuable information shared in previous
time steps redundant. This paper presents an efficient communication method
that solves the problem of "when" and with "whom" to communicate in multi-robot
collision avoidance scenarios. In this approach, every robot learns to reason
about other robots' states and considers the risk of future collisions before
asking for the trajectory plans of other robots. We evaluate and verify the
proposed communication strategy in simulation with four quadrotors and compare
it with three baseline strategies: non-communicating, broadcasting and a
distance-based method broadcasting information with quadrotors within a
predefined distance.
