# Wireless Communication Technologies for Safe Cooperative Cyber Physical Systems
## 1 Introduction  
Modern embedded systems, coupled with the advancements of digital communication technologies, have been enabling a new generation of systems, tightly interacting with the physical environment via sensing and actuating actions: Cyber Physical Systems (CPS) [1]. These systems, characterized by an unprecedented level of pervasiveness and ubiquity, have been increasingly relying on wireless communication technologies to provide seamless services for the Internet of Things (IoT) [2]and Industry 4.0 [3], via flexible cooperation. As these Cooperative CPS (Co-CPS) starts approaching safety-critical application domains (e.g., automated vehicles platooning in the automotive and maritime domains, process control in hazardous industries, etc. ), safety shows-up as a crucial topic that must be carefully analyzed because failures and errors might lead to hazardous situations, e.g., death, injuries or environmental damages. All these systems are required to perform specific safety functions to ensure that the risk of system's failure is maintained at an acceptable level. What makes the safety of these systems even more challenging is the fact that they heavily rely on wireless communication to exchange safety-critical information. For example, in automotive applications like vehicular platooning [4; 5; 6], the IEEE 802.11p standard is used as a communication protocol among vehicles, in a closed-loop control system where exchanged messages contribute to maintain the inter-vehicle safety distance. Message losses or delays may lead to serious crashes among the vehicles in the platoon with dramatic consequences. In this case, real-time and reliability are two important aspects for ensuring the safety of operation of the platoon. Furthermore, security is also very relevant as any possible attack on the platoon, such as for example false data injection, spoofing or jamming, would lead to disastrous consequences as well. For several years, topics such as safety in Co-CPS have been mostly ignored to the point that, currently, the absence of a de-facto standard on safe and secure Co-CPS is becoming an impediment to their adoption. Security, on the other hand, has been investigated to some extent in several protocols like IEEE 802.11p, IEEE 802.15.4 [7; 8] and its variants [9]. However, there are still several challenges in what concerns the impact analysis of several attacks upon cooperating functions, or the integration of security mechanisms with remaining Quality of Service (QoS) properties for safety assurance. In this line, the ECSEL SafeCOP EU research project addresses these issues regarding the design of wireless safe and secure Co-CPS [10]. In fact, the SafeCOP project deals with safe cooperation of complex CPS that relies on wireless communications. The main objective is to provide a _safety assurance methodology_ for these systems with emphasis on applications in the healthcare, maritime, automotive domains. In this paper, we provide an overview on the safety requirements, challenges and solutions of Co-CPSs relying on wireless communications. We review the state-of-the-art of standard protocols used in Co-CPSs and assess their compliance to the requirements of safety, security and real-time for Co-CPSs. The main result provided by this survey is a collection of general recommendations for decision-making about the use of wireless technologies in Co-CPSs, which are illustrated through the application in relevant real-world use cases. In addition, given that currently 5G constitutes one of the most important areas of research in Europe towards enabling the interoperability and cooperation of heterogeneous radio technologies for the provision of innovative powerful services, we discuss how the current efforts in 5G standardization contribute to addressing the Co-CPSs challenges.