# Three-Dimensional Drone Cell Placement: Drone Placement for Optimal Coverage
## 1 Introduction  
Due to the ever-growing demand for mobile and communication networks, future Radio Access Networks (RANs) will be put under unprecedented strain to fulfill the needs for all connected users and devices. Providing pervasive connectivity to users and connected devices will have to be the main objective of future RAN architectures, as guaranteed Quality of Service (QoS) is expected by users. High-rise buildings, natural formations or lack of infrastructure can cause network blockages in certain areas, creating Coverage Holes (CHs) or Demanding Areas (DAs). These problems, included with highly dynamic and unevenly distributed traffic, present challenges in providing pervasive connectivity in modern day scenarios [1, 2, 3]. Deploying network towers or Base Stations (BSs) en masse in DAs or CHs is impractical since they are ineffective and increasingly expensive for construction. Moreover, due to their static nature, BSs cannot be redeployed in required DAs to deal with peak hour traffic or in areas further away from the range of the BS. To tackle these problems, drones or Unmanned Aerial Vehicles (UAVs) with onboard communication modules are leveraged as drone cells (DCs) to assist in network connectivity, creating the Drone-Assisted Radio Access Network (DA-RAN) [2]. DCs can enhance the performance of future RANs by using better Drone-to-Ground (D2G) wireless communication models, and on-board computational capabilities for dynamic and controllable deployment. Drones fitted with communication and relay modules can provide on-demand network access in both temporal and spatial domains. DCs act as relay stations between theconnected users and their respective BSs, extending the wireless communication range for users in CHs or DAs. Due to their flexibility, DCs can be deployed in emergency scenarios such as any temporary occurring event, natural disasters or ground BS failure to enhance network throughput [4]. DCs with their on-demand wireless coverage capabilities are also more economical in terms of operational expenditure [5]. Advances in drone development and related technology have propelled drones to perform complex tasks in diverse scenarios of human life. Equipped with a plethora of equipment and communication modules, drones can tackle a number of tasks such as traffic monitoring and surveillance [6], wildfire monitoring and control [7], security monitoring [8], logistic application as in Amazon Prime Air [9], DHL Parcelcopter [10], communication assistance [11; 12], etc. The market share of drones was valued at USD 35.28 billion in 2024 by Modor Intelligence [13] with a market projection of the Compound Annual Growth Rate (CAGR) of 13.90% during the period of 2024-2029. The utility of DCs assisting wireless communication has attracted much attention from both industry and researchers alike. Significant steps were taken by Google and Facebook to create their own wireless drone communication network projects, known as Project Loon [14] and Aquila [15], respectively. Nokia put forward the UAV Traffic Management (UTM) architecture for UAV-assisted control [16], a UTM server for automated UAV missions, 3D navigation and collision avoidance. The world's first long-term evolution LTE in association with drones was demonstrated by Intel and AT&T at the 2016 Mobile World Congress [17]. The first 5G-enabled drone prototype was field tested in Wuxi, China, in association with China Mobile and Ericsson [18]. Other companies such as Huawei and Qualcom also implemented DA-RANs in their own way to bolster and improve future 5G networks. In 2017, Huawei launched Digital Sky Initiative as their first low airspace digital network with 24-7 flying capabilities to support a low airspace digitized economy [19]. Qualcom introduced their vision for cellular drone communication where drones can fly up to altitudes of 400 ft to optimize cellular connectivity. These drones are capable of seamless handover between different BSs with zero link failures [20].