On the benefits of cooperation for dependable wireless communications

The emerging Industrial Internet-of-Things (IIoT) improves flexibility, productivity, and costs of industrial processes by connecting sensors, actuators, and controllers to each other and the Internet. On the factory floor, such interconnections increasingly rely on wireless communications, reducing deployment and maintenance costs while supporting the mobility of communication partners. The industrial domain, however, is mainly characterized by safety- and mission-critical Machine-to-Machine communication. Therefore, state-of-the-art wireless communication protocols for home and business environments, such as WLAN and Bluetooth, are not suited for the IIoT. Consequently, the IIoT requires dependable wireless communication, achieving both high reliability and low latency. A promising approach for so-called Ultra-Reliable Low-Latency Communication (URLLC) in the IIoT is cooperative diversity, since the participating stations already collaborate toward a common goal, i.e., keeping the industrial process running. A sending station exploits multiple independent transmission paths via cooperating relays to reliably convey a packet to its destination. In contrast to spatial diversity, this approach also works with single-input single-output transceivers. However, when considering relaying for URLLC, it is particularly challenging that all participants have to share the scarce transmission resources. Hence, in this dissertation, we investigate various mechanisms enabling dependable wireless communication, i. e., increasing communication reliability within a bounded low latency, mainly focusing on cooperative diversity benefits. Therefore, we explore different design options for URLLC and evaluate them, leveraging the advantages of distinct methodological approaches. We begin with mathematical analysis to assess the possible benefits of cooperative diversity for URLLC and to develop basic protocol design options. Our analysis shows promising results for cooperative diversity compared to other diversity techniques, especially when all stations are involved in the relaying. Based on these results, we implement a relaying protocol for URLLC and evaluate it in a prototypical deployment. We confirm some of our analytical findings while also revealing a substantial performance gap between analysis and real-world evaluation. Moreover, we identify remaining open issues, which mainly include the evaluation of scalability and mobility effects. Therefore, we propose code-transparent simulation to switch seamlessly from real-world deployment to simulations. Our code-transparent simulator, based on ns-3, thus simulates the same code that we used for the prototypical deployment while achieving accurate simulation results. Subsequently, we find that typical moving velocities in industrial scenarios do not negatively impact the relay selection process. Furthermore, cooperative systems support a higher number of stations than comparable systems based on other diversity techniques. In summary, this dissertation offers valuable insights into designing communication protocols with challenging requirements. Thus, at the example of cooperation, we thoroughly retrace the development process from analysis to prototypical deployment. On the one hand, the achieved results contribute to URLLC for the IIoT; on the other hand, they provide a critical examination of the selected evaluation methodologies