# A Service-Constrained Positioning Strategy for an Autonomous Fleet of Airborne Base Stations
## 1 Introduction  
The development of new strategies to enhance the capacity of cellular subscribers has been an important research field for many years. While many systems have already been developed, aerial communications based on unmanned aerial vehicles (UAVs) that are used as airborne base stations (ABBSs), are gaining interest. UAVs appeared some years ago in tactical applications, mainly to reduce pilot losses in hostile territories. Since then, their cost, size and autonomy have been continuously improved until becoming viable for commercial purposes. Nowadays, many types of UAVs with different capabilities and features are available in the market and allow a huge variety of applications. In the wireless communication field, UAVs can be used as ABBSs to create non-fixed cellular networks in an ad-hoc way. A new type of network can be designed, in which each base station (BS) is moving continuously depending on the time-varying users spatial distribution and traffic requirements. A possible scenario is, for example, an outdoor sporting event, where a large amount of people is gathered in a small area, whereas the users density remains low outside the venue. Other relevant scenarios for ABBSs are those where the fixed ground network is unreachable (e.g., remote regions in emerging countries) or temporally unavailable (e.g., due to natural disasters). ABBSs have the potential to improve the versatility and availability of wireless services and, as such, they have motivated recent research work. Many existing works have focused on proposing several air-to-ground channel models [1; 2; 3; 4; 5; 6; 7], which are analyzed in Section 2. More recently, deeper research has been carried out by includingdifferent distributions of users in the scenario. According to this, the scenarios can be classified depending on the number of UAVs deployed, as explained next. Previous works [8; 9; 10; 11] analyze a single UAV scenario. In [8], the authors developed an algorithm that finds the 3-D placement of the UAV that allows serving the maximum number of users. The other three works consider a continuously moving UAV. In [9], beam division multiple access is proposed using millimeter-wave communication and the UAV is moved to combat the blockage problem and improve the user discovery. In [10], three different multiple access strategies are studied, where the UAV moves in response to the users' activities and positions. Finally, in [11], the moving UAV is used as a mobile cloud to offer computation offloading opportunities to users with limited local processing capabilities. Thus, the UAV is moved to minimize the total mobile energy consumption by also optimizing the bit allocation among users for uplink and downlink communications. In previous works [10; 11], several experiments are carried out but without considering the optimization of the altitude of the UAV. In [12; 13; 14; 15; 16], the case of multiple UAVs is considered. Specifically, in [12], an algorithm that aims to serve all users in an specific region with the minimum number of UAVs is developed. The altitudes of UAVs are constant and the signal level received per user is not considered. Thus, the deployment of UAVs is formulated as a geometric problem where a given area has to be served by the minimum number of coverage regions of the same radius with possible overlapping among them. In [13], a scenario consisting of UAVs belonging to different operators is addressed. The authors developed a non-cooperative game theory strategy where rational agents compete to maximize their own individual payoff. Finally, the other three works [14; 15; 16] focus on heterogeneous networks, where multiple UAVs assist the terrestrial network. In [14], the UAVs are deployed randomly to evaluate their impact on the terrestrial network. In [15], to optimally place the UAVs, the service provided to users is considered. However, the optimal placement is obtained through exhaustive search. In [16], a more sophisticated approach is developed.