  In this paper, the deployment of drone base stations to provide higher
performance in the cellular networks is analyzed. In particular, we investigate
a new mobility model for drone base stations where they can move freely in the
network, ignoring the cell boundaries. Free movement model for drones bring out
new challenges such as user association and physical collision among drones. We
consider two user association schemes and evaluate their performance through
simulation. We show that by deploying a smart user association scheme in the
free movement model, the obtained results are greatly better than those
restricting each drone base station to fly over a certain small cell area, and
serving local users. Additionally, the impact of drones' movement on the load
balance, signal strength and interference are studied. Moreover, we show that
our proposed algorithm can maintain a comfortable distance among the drones to
avoid physical collision.
