  In this work, we study the trade-off between the reliability and the
investment cost of an unmanned aerial system (UAS) consisting of a set of
unmanned aerial vehicles (UAVs) carrying radio access nodes, called portable
access points (PAPs)), deployed to serve a set of ground nodes (GNs). Using the
proposed algorithm, a given geographical region is equivalently represented as
a set of circular regions, where each circle represents the coverage region of
a PAP. Then, the steady-state availability of the UAS is analytically derived
by modelling it as a continuous time birth-death Markov decision process (MDP).
Numerical evaluations show that the investment cost to guarantee a given
steady-state availability to a set of GNs can be reduced by considering the
traffic demand and distribution of GNs.
