## 1 Introduction  
With the popularity of the Internet, more and more people choose online shopping. A large number of online orders have brought great pressure to the express industry. Therefore, the transformation of the production pattern needs to be carried out for traditional delivery companies. As one of the autonomous things, UAVs are considered to be the top strategic technology for logistics industry [1]. Both governments and enterprises have been heavily investing in the development of Unmanned Aerial Vehicles (UAVs) [2]. Recently, with the maturity of UAV technology, it has been widely used in various fields such as communication platforms, precision agriculture, surveillance and monitoring, and cargo delivery; the UAV-assisted logistics systems have drawn significant research interests [3]. It is worth noting that in different application scenarios, the number of UAVs included in the UAV-assisted system is different (see Figure 1 for an illustration). Teams of UAVs can be dispatched, for instance, as providing service to disaster-affected areas, detecting environment conditions or as an aerial sensor network, collecting data in large areas. UAV logistics is also an emerging application scenario, and most commercial UAV logistics auxiliary systems are exactly multi UAV cooperative systems, whose execution efficiency depends on the number of UAVs. It uses UAVs to improve the efficiency of the logistics distribution and schedule, reduce operating costs, and optimize the link of terminal distribution. In this paper, we mainly present an automatic delivery system for logistics UAVs in urban environment. The algorithm frameworks based on Variable Neighborhood Search (VNS) are proposed to solve the small scale scheduling problem of this system, and we consider several important optimization objectives. In the traditional logistics industry, all packages that need to be delivered are stored in the trans-shipment depot. These packages in the trans-shipment depot are classified by their destinations. Multiple couriers deliver these packages, and each courier is responsible for the delivery of packages within a certain area. In the above system, a group of UAVs are used to replace a Courier. Some autonomous express cabinets are set up near the package's destinations. These UAVs drop some packages from the trans-shipment depot into express cabinets according to the task scheduling results. The aforementioned UAVs and express cabinets are all connected to the Internet. The execution status of the system is obtained by the dispatch center in real time. The dispatch center performs the task scheduling at intervals and reports the status of the system to the relevant staff. The execution process of the system is shown in Figure 2: the dispatch center uploads the data to the Internet. The trans-shipment depot S_0, express cabinets from S_1 to S_9, and UAVs from u_1 to u_5 all connected with the Internet. Packages are sorted in S_0 according to instructions downloaded from the Internet; UAVs and express cabinets can realize reasonable dispatching through the interaction data from the dispatch center. The scheduling problem of logistics UAVs is a special kind of Unmanned Aerial Vehicle routing problem (UAVRP). Similar to the traditional vehicle routing problem (VRP), the UAVRP adds some new features such as limitations of flight time and load capacity. VRP is well known as an NP-hard problem. As a variant of VRP, the scheduling problem of logistics UAVs is also NP-hard. The VNS algorithm  
Figure 1: Application areas over a range of distance vs. number of Unmanned Aerial Vehicles (UAVs). Figure 2: The execution process of the delivery system. framework is used to address the computational intractability and search for the approximate optimal solution of the proposed problem. The rest of the paper is organized as follows. Related works are reviewed in Section 2. Section 3 describes and formalizes the problem under study. An algorithm framework is proposed for the considered problem in Section 4.