  A promising application of unmanned aerial vehicles (UAVs) to the future
communication networks is to address emergency communications. This paper
considers such a scenario where a UAV is employed to a malfunction area
(modeled as a circular disc) in which all ground base stations (BSs) break
down. The ground BSs outside the malfunction area are modelled as a homogeneous
Poisson point process (HPPP). Particularly, a user-centric cooperative scheme
is proposed to serve the UEs in the malfunction area. According to the user
equipment's (UE's) connections to the UAV and the nearest ground BS, the
malfunction area is divided into three regions, namely the UAV region, the
cooperation region and the nearest ground BS region, in which the UEs are
served by the UAV only, both the UAV and the nearest ground BS, and the nearest
ground BS, respectively. The region size of each type can be adjusted by a
cooperation parameter $\delta$. Through rigorous derivations, an expression for
the coverage probability achieved by the UE in the malfunction area is
obtained. In order to provide a fair comparison, the normalized spectral
efficiency (NSE) which is defined by taking both system throughput and the
number of serving BSs into consideration, is used as a criterion for the
performance evaluation. Numerical results are presented to verify the accuracy
of the analytical results and also to demonstrate the superior performance of
the proposed scheme.
