# Multi-IRS-Assisted mmWave UAV-BS Network for Coverage Extension
## 1 Introduction  
Recently, Industry 5.0 has been envisioned by the European Commission as a road map for the future, thereby aiming to cultivate a prosperous society through the strategic use of advanced technologies such as artificial intelligence (AI), robotics, big data, and the Internet of Things (IoT) [1]. These technologies find practical applications in digital twins (DTs), which serve as the conduit between the real and virtual worlds [2]. On the other hand, in recent years, with the improvement of hovering position accuracy and the update of control functions considering safety [3], unmanned aerial vehicles (UAVs) are planned to be deployed in various fields such as agriculture, defense, delivery, and surveying, which will facilitate the realization of Industry 5.0 [4, 5, 6, 7]. Furthermore, current research has shed light on the capabilities of nonterrestrial networks (NTNs) to equip UAVs and low Earth orbit satellites with robust communication systems. These networks are particularly valuable when terrestrial infrastructure compromises natural disasters such as earthquakes, lightning, and typhoons [8]. Among the promising solutions in this arena are UAV base stations (UAV-BS) [9]. Thanks to this feature, we can build temporary networks as wide as required and when we need them, thus making them not costly. These stations are increasingly seen as vital for enhancing communication performance in areas experiencing sudden spikes in data traffic or regions affected by disasters [10]. Research on UAV-BS began in the mid-2010s, and nowadays, UAV relay and base station demonstrations are being conducted by companies, albeit in limited scenarios. However, it has not yet reached the stage of practical operation. The main reason is that we are still exploring unique issues and solutions for airborne base stations, which differ from existing base station designs. In addition, telecommunication organizations should standardize dissemination to many people, and governments should establish laws. As in previous UAV network studies, [PERSON] et al. [11] proposed a centralized algorithm for positioning UAVs to maximize the throughput. [PERSON] et al. [12] optimized the UAV's moving trajectory for a UAV-enabled mobile relaying system. [PERSON] et al. [13] improved the energy efficiency of an entire intelligent reflective surface (IRS)-mounted UAV network. However, many studies on UAV networks employ microwaves, which limit the range of possible applications due to bandwidth constraints. There is also a pressing concern about frequency resource depletion, thereby necessitating new exploration into frequency sharing and developing higher frequency bands. To further the capabilities of UAV networks, especially for applications requiring high-speed and high-definition video transmission, we propose the utilization of the millimeter wave (mmWave) [14]. Using mmWaves also allows array antennas to be deployed on a UAV by making individual antenna sizes smaller. Therefore, the beam shape can be changed to suit the situation. Unlike traditional ground base stations, UAV-BS can be flexibly positioned in three dimensions, thus ensuring line-of-sight (LoS) communication, which is crucial for mmWave frequencies. This flexibility reduces signal attenuation caused by obstacles such as buildings. However, maintaining LoS paths to all user equipment (UE) for high-speed communication remains challenging in densely constructed urban environments. On the other hand, NTT DOCOMO released the white paper for the 6 th generation (6G) mobile communication system and stated a new radio network topology that increases the number of connection paths to the network [15]. It says that the IRS is an important technology. According to [16; 17], the IRS is composed of a large number of elements that scatter electromagnetic waves, and it often uses metamaterial/metasurface technology that enables the design and control of the distribution of scattering properties over an area.