# NFV/SDN as an Enabler for Dynamic Placement Method of mmWave Embedded UAV Access Base Stations
## 1 Introduction  
In recent years, the total traffic in mobile networks has been increasing at an average annual rate of 46% due to the explosive growth of various applications such as video streaming services [1]. To cope with this problem, 5G (5 th Generation Mobile Communication System) services were launched worldwide in 2019. In this 5G era, not only human beings but also objects have been connected to the Internet as a social infrastructure. This trend is expected to continue in the future, with the mobile communication system expected to function as a fundamental infrastructure of society in the Beyond 5G/6G era [2]. As a result, it has become more important than ever to be \"connected\" in any place and under any circumstances. For this reason, UAVs (Unmanned Aerial Vehicles) have, in recent years, become a familiar part of our lives and are expected to be utilized in various fields such as infrastructure monitoring, delivery, and disaster investigation. In this paper, we focus on the use of access UAVs as temporary base stations for deploying wireless networks (UAV networks) [3], knowing that our investigation on backhaul UAVs was studied in [4, 5]. UAV networks have several advantages because of their ability to place base stations in the sky. The first is that they can move regardless of the constraints on the ground. This allows for rapid deployment of base stations when needed and allows for optimal placement of base stations for a particular distribution of users. The other point is the ability to provide data from the sky. This reduces the probability of being blocked by buildings, etc., and increases the probability that the propagation path between the UAV and the user is in line-of-sight condition. These advantages make UAV networks attractive applications for many use cases but, in this study, we mainly consider disaster-stricken areas as our target use case. For example, the Great East Japan Earthquake in 2011 caused great damage to the Pacific coast of Japan. As a result of the earthquake, about 29,000 cell phonebase stations and PHS (Personal Handy-phone System) base stations were shut down [6], disabling the communication of life-threatening information such as safety information and evacuation information to/from cell phone terminals. In such situations, it necessitates the establishment of a temporary wireless network that can be constructed quickly and stably. To this end, these requirements can be realized by exploiting the mobility of UAVs. The placement of access UAVs in UAV networks has been studied from various viewpoints. In [7], a method to place UAVs to cover users of degraded data rate located at the edge of the service area was studied. In [8], the placement was optimized to reduce the total transmission power of UAVs while ensuring service requirement constraints of each user. In [9], the placement of UAVs was obtained by solving a packing problem on the service area to enhance the system's coverage. In [10], UAV placement was further optimized by solving a particle swarm optimization problem to maximize the area coverage. In these conventional studies, the microwave bands of limited bandwidth were assumed since these lower frequency bands are expected to attain higher connectivity. However, for future applications including digital twins to support rescue activities using remotely controlled equipment in disaster areas, higher data rate transmission will be become indispensable. Furthermore, the data volume of web browsing is increasing yearly along with the increase of communication speed [11], so large-capacity transmission will also be necessitated for information collection in disaster areas [4]. The bandwidth of the microwave band is not sufficient to support such use cases. In other words, it necessitates the introduction of higher frequency bands such as millimeter-wave to guarantee a larger system bandwidth [12].