# Drone-Based Parcel Delivery Using the Rooftops of City Buildings: Model and Solution
## 1 Introduction  
Cities all over the world are leveraging state-of-the-art digital technologies to build open and intelligent control systems for their smart city initiatives using real-time information for public traffic, emergency management, and public security. Empowered by new tools and recently developed technologies, cities are now far more designable than ever before. Among many new technologies, drones and small unmanned aerial vehicles (UAVs) can play vital roles in the smart city environment from the viewpoint of parcel delivery. Drones can help to deliver small parcels to destinations without any traffic congestion, which would otherwise delay delivery and affect the customer service level. Logistical drones (i.e., drones designed for parcel delivery services) are typically small battery-powered rotary-blade copters with a payload capacity of approximately 2 kg. Amazon, DHL, UPS, and Google have already announced various plans to use drones to deliver parcels from their distribution centers (warehouses) directly to end-customers in urban areas [1; 2; 3; 4]. Domino's Pizza delivered its pizza to actual customers using drones in the town of Whangaparaoa in New Zealand [5]. Other successful applications include hybrid truck-drone delivery by UPS in Florida, USA [6] and drone-based package delivery to islands by the Chinese e-commerce company Alibaba [7]. Drone delivery could offer several advantages over traditional parcel delivery, such as shorter delivery times, lower maintenance costs, and environmental friendliness [8; 9]. Industrial and commercial applications of drones have strongly increased in the last few years, which we will review in Section 2. Especially with the help of the recent growth of e-commerce, the importance of the rapid delivery of ordered products (i.e., last-mile delivery) to consumers has increased. Increasing delivery speed has become a major challenge in urban areas. Thus, as a new element of urban logistics, we need to determine the feasibility of drone-based delivery. In addition, proper places for unloading theparcels should be prepared to implement drone-based parcel delivery, and the rooftops of buildings can be a suitable candidate. In this research, we first propose a mixed-integer linear programming (MILP) model for generating a schedule for drone-based parcel delivery using the rooftops of buildings in cities. Even if there are several regulatory and technological barriers to overcome before the widespread adoption of drones for urban logistics, it appears that drone-based delivery will be viable in the near future. Drone-based delivery could overcome any accessibility restrictions associated with residential/commercial locations in dense urban areas. Cities are already packed with buildings and people, and any increase in the number of delivery vehicles in urban areas results in more traffic congestion, air and noise pollution, and traffic accidents [10]. However, drones are powered by electric batteries, which means that they are more eco-friendly, and they move efficiently and quickly because they are not constrained by road conditions and traffic congestion. Next is the validation of the proposed model by conducting numerical experiments with random data settings that mimic plausible situations in the cities. In addition, we develop a heuristic that can generate solutions (i.e., schedules) very quickly regardless of problem size. Although any commercial optimization software can generate solutions using a sophisticated algorithm for small- and medium-sized MILP problems, it is very difficult to provide solutions for large problems that have many decision variables and constraints in a reasonable amount of time [11]. This issue will be discussed again when we illustrate the experimental results in Section 4. The remainder of this paper is organized as follows. Section 2 introduces the related literature. The mathematical model for our problem is proposed in Section 3.