  We consider unmanned aerial vehicle (UAV)-assisted wireless communication
employing UAVs as relay nodes to increase the throughput between a pair of
transmitter and receiver. We focus on developing effective methods to position
the UAV(s) in the sky in the presence of interference in the environment, the
existence of which makes the problem non-trivial and our methodology different
from the current art. We study the optimal position planning, which aims to
maximize the (average) signal-to-interference-ratio (SIR) of the system, in the
presence of: i) one major source of interference, ii) stochastic interference.
For each scenario, we first consider utilizing a single UAV in the dual-hop
relay mode and determine its optimal position. Afterward, multiple UAVs in the
multi-hop relay mode are considered, for which we investigate two novel
problems concerned with determining the optimal number of required UAVs and
developing an optimal fully distributed position alignment method.
Subsequently, we propose a cost-effective method that simultaneously minimizes
the number of UAVs and determines their optimal positions to guarantee a
certain (average) SIR of the system. Alternatively, for a given number of UAVs,
we develop a fully distributed placement algorithm along with its performance
guarantee. Numerical simulations are provided to evaluate the performance of
our proposed methods.
