  Deploying unmanned aerial vehicle (UAV) networks to provide coverage for
outdoor users has attracted great attention during the last decade. However,
outdoor coverage is challenging due to the high mobility of crowds and the
diverse terrain configurations causing building blockage. Most studies use
stochastic channel models to characterize the impact of building blockage on
user performance and do not take into account terrain information. On the other
hand, real-time search methods use terrain information, but they are only
practical when a single UAV serves a single user.In this paper, we put forward
two methods to avoid building blockage in a multi-user system by collecting
prior terrain information and using real-time search.We proposed four
algorithms related to the combinations of the above methods and their
performances are evaluated and compared in different scenarios.By adjusting the
height of the UAV based on terrain information collected before networking, the
performance is significantly enhanced compared to the one when no terrain
information is available.The algorithm based on real-time search further
improves the coverage performance by avoiding the shadow of buildings. During
the execution of the real-time search algorithm, the search distance is reduced
using the collected terrain information.
