  The use of the unmanned aerial vehicle (UAV) has been foreseen as a promising
technology for the next generation communication networks. Since there are no
regulations for UAVs deployment yet, most likely they form a network in
coexistence with an already existed network. In this work, we consider a
transmission mechanism that aims to improve the data rate between a terrestrial
base station (BS) and user equipment (UE) through deploying multiple UAVs
relaying the desired data flow. Considering the coexistence of this network
with other established communication networks, we take into account the effect
of interference, which is incurred by the existing nodes. Our primary goal is
to optimize the three-dimensional (3D) trajectories and power allocation for
the relaying UAVs to maximize the data flow while keeping the interference to
existing nodes below a predefined threshold. An alternating-maximization
strategy is proposed to solve the joint 3D trajectory design and power
allocation for the relaying UAVs. To this end, we handle the information
exchange within the network by resorting to spectral graph theory and
subsequently address the power allocation through convex optimization
techniques. Simulation results show that our approach can considerably improve
the information flow while the interference threshold constraint is met.
