# UWB Localization with Battery-Powered Wireless Backbone for Drone-Based Inventory Management
## 1 Introduction  
The constantly increasing success of e-commerce requires new supply chain solutions at every stage of operations, including (continuous) inventory management [1]. Industry 4.0 digitization (Also known as the next industrial (or data) revolution: using smart technologies in industry to collect data ) with goods being produced more flexibly, impacts inventory management. Also for rental companies, the trend towards \"X-as-a-service\" (XaaS) (XaaS refers to the growing diversity of services available over the Internet via cloud computing as opposed to being provided locally, or on premises.) requires a continuous stock management. To realize this flexibility, powerful warehouse management software [2] has been introduced. On the warehouse floor, we have RFID or barcode based solutions and even robots with RFID readers. However, although these warehouse management systems provide some automation, most traditional inventory checking approaches remain very labor-intensive and time-consuming in vertical warehouses, involving reach trucks, forklifts, man-cages or scissor-lifts and taking up hundreds of man-hours every counting cycle. In addition, human errors occur frequently, thereby impacting the quality of the data and resulting in additional lost man-hours. Finally, as the racks can be stacked quite high (up to 16 m), manual inventory tracking can result in serious accidents. To overcome these limitations, automated inventory management solutions are needed, especially in larger-sized warehouses with high racks. For this purpose, recent years have witnessed the surge of research interest on micro unmanned aerial vehicles (UAVs) for stock counting and localization [3; 4; 5]. As the logical heir of ground based mobile robots, UAVs have the aerial manoeuvring ability to easily avoid obstacles and provide an excellent bird's-eye view, which is beneficial for mapping and monitoring tasks. In an outdoor environment, space-based satellite navigation systems such as Global Positioning System (GPS) are able to provide an absolute position for the UAV with an approximate error of 2-5 m in civilian applications, allowing automated outdoor missions. Since GPS units are unable to work reliable in an indoor environment, this approach is not feasible indoors. Moreover, a more precise positioning accuracy is required to fly between narrow racks, thereby achieving autonomous navigation of micro UAVs. Instead, such systems can replace the GPS by anchor-based localization systems. Typically, anchors are placed at points with known coordinates, and the distance from an anchor to the moving UAV can be estimated from RF signals for the localization purpose. Different kinds of ranging techniques have been proposed for the distance estimation: Time of Flight (ToF), Time of Arrival (ToA) and Received Signal Strength Indication (RSSI). One emerging RF technology for accurate localization is Ultra-Wideband (UWB) [6] ranging technology, which is robust to multipath and non-line-of-sight (NLOS) effects [7], and can achieve a cm-level ranging error [8]. The UWB radio modules can estimate inter-module distance by measuring the transmission and reception time of UWB pulses. In contrast with other RF systems, UWB pulses can be transmitted between 3 GHz and 5 GHz and have an RF bandwidth of 500 MHz or 1.4 GHz. The increased bandwidth not only avoids the interference with other types of RF signals, e.g., remote control and WiFi signals. Moreover, the ultra-short duration pulses permit an easy filtering method to deal with the multipath effect, and provide an accurate timestamp allowing determination of ToF. In large warehouses of multiple square kilometers, such cabling and power infrastructure costs are prohibitive, limiting the adoption of large-scale UWB systems. Most UWB solutions from scientific literature are not well-suited due to one or more of the following reasons.