# Cellular and Virtualization Technologies for UAVs:
## 2 Related Work and Background  
### Communications  
Conventional research related to UAVs has typically been focused on navigation, optimal trajectories, and control challenges. Unfortunately, many articles underestimate the difficulties of maintaining stable and reliable communications between the aircrafts and the GCS, where connectivity might not be stable due to the unreliable nature of wireless communications. However, there has been a sharp increase of interest in this particular topic, as it can be seen in some examples that focus on UAS communications. On the one hand, UAVs beyond their traditional usage as standalone mission platforms are more and more being proposed as flying wireless base stations [6; 7] in 5G networks and beyond (also known as UAV-assisted cellular communications) to support network connectivity in multi-UAV missions. UAVs include different application scenarios such as coverage and capacity enhancement. UAVs can provide on-demand connectivity in events with vast amounts of people, like sport events, or concerts [8]. The authors in [9] explore the utilization of low-altitude UAVs that are provided with Base Stations (BS) to complement the terrestrial network. Aerial BSs can also assist terrestrial networks such as vehicular networks for information dissemination and connectivity improvement, thanks to their inherent mobility [10]. In [11], the authors present a UAV-assisted cellular network with optimized spectrum sharing and cyclical multiple access, which significantly improves spatial throughput over the conventional network, although the interference control complexity was higher. Other representative application is to use UAVs as BSs for public safety applications [12; 13] such as natural disasters (e.g., earthquakes, typhoons, floods) management, where the conventional cellular network might be unavailable, damaged, or insufficient. Through different simulations, these articles show that the optimized deployment of aerial BSs at different areas may improve throughput coverage and efficiency. Another example is SARDO [14], a UAV-based cellular search and rescue solution (i.e., a UAV equipped with a light-weight BS) to localize missing victims through mobile phones. On the other hand, UAVs are also being proposed as aerial user equipment (known as cellular-connected UAVs) [7; 15]. Cellular-connected UAVs make use of existing cellular infrastructures and should co-exist with existing ground-based user terminals (i.e., normal users of the cellular network). This paradigm has obtained significant interest to establish reliable communications among UAVs and different devices that may be used during a complete mission (e.g., UAV-controller, UTM). Besides, considering the recent appearance of 5G networks, short term bandwidth and latency improvements are expected to be introduced once these networks have properly been deployed into the cellular landscape, which may be beneficial for multi-UAV systems for efficient fleet control [16; 17]. However, the existing infrastructure is designed and optimized for terrestrial users needs, and there are still several challenges to be considered before relying on cellular-connected UAVs as a valid solution. For example, cellular-connected UAVs require high-speed uplink connectivity, while most applications for ground users demand high-speed downlink connectivity instead. Although the downlink is critical since the UAV trajectories are transmitted in this direction, most UAV applications generate information from the UAV upon the UAV-controller (e.g., video streaming, sensors), requiring higher data rates in the uplink. Moreover, UAVs might deal with significant interference introduced by the neighboring BSs because of the nonexistence of obstacles between the multiple BSs and the UAVs [18]. Finally, a pretty well established UAV paradigm for multi-UAV missions are Flying Ad hoc Networks (FANETs) [19; 20]. These networks are scalable due to their facility to integrate new UAVs to the system with small deployment and maintenance costs.