  Unmanned aerial vehicles (UAVs) have gained a lot of popularity in diverse
wireless communication fields. They can act as high-altitude flying relays to
support communications between ground nodes due to their ability to provide
line-of-sight links. With the flourishing Internet of Things, several types of
new applications are emerging. In this paper, we focus on bandwidth hungry and
delay-tolerant applications where multiple pairs of transceivers require the
support of UAVs to complete their transmissions. To do so, the UAVs have the
possibility to employ two different bands namely the typical microwave and the
high-rate millimeter wave bands. In this paper, we develop a generic framework
to assign UAVs to supported transceivers and optimize their trajectories such
that a weighted function of the total service time is minimized. Taking into
account both the communication time needed to relay the message and the flying
time of the UAVs, a mixed non-linear programming problem aiming at finding the
stops at which the UAVs hover to forward the data to the receivers is
formulated. An iterative approach is then developed to solve the problem.
First, a mixed linear programming problem is optimally solved to determine the
path of each available UAV. Then, a hierarchical iterative search is executed
to enhance the UAV stops' locations and reduce the service time. The behavior
of the UAVs and the benefits of the proposed framework are showcased for
selected scenarios.
