In this paper, we consider the use of UAVs to provide wireless connectivity
services, for example after failures of wireless network components or to
simply provide additional bandwidth on demand, and introduce the concept of
UAVs as a service (UaaS). To facilitate UaaS, we introduce a novel framework,
dubbed D3S, which consists of four phases: demand, decision, deployment, and
service. The main objective of this framework is to develop efficient and
realistic solutions to implement these four phases. The technical problems
include determining the type and number of UAVs to be deployed, and also their
final locations (e.g., hovering or on-ground), which is important for serving
certain applications. These questions will be part of the decision phase. They
also include trajectory planning of UAVs when they have to travel between
charging stations and deployment locations and may have to do this several
times. These questions will be part of the deployment phase. The service phase
includes the implementation of the backbone communication and data routing
between UAVs and between UAVs and ground control stations
