# Wireless Communication Technologies for Safe Cooperative Cyber Physical Systems
## 4 Safety and Security for SafeCOP Co-CPSs
### Vehicle Control Loss Warning  
In the vehicle platooning scenarios, one vehicle may influence the behavior of other vehicles, and, since the consequences of a failure can harm human life, these systems are considered safety-criticaland must be designed according to relevant methodologies to ensure safety, also from a communication perspective. The main goal in vehicle platooning is finding the best trade-off between performance (i.e., maximize speed and minimize vehicles reciprocal distance) and safety (i.e., avoid collision). The largest part of the literature focuses on advanced control schemes, without modeling the communication medium properly. Delay of communication is typically considered as a fixed delay or through probabilistic models. This allows the analytical derivation of string stability models [64] under some hypotheses of the dynamical system, but it may be unreliable under realistic conditions. Two branches are evident from the literature in this respect: the derivation of simple models of the delay bound that guarantees safety (see, e.g., section IV.C of [65]) and brute force simulation with visualization of safety regions under a reduced set of parameters [66; 67]. Ongoing research addresses formal verification to extract evidence of safety conditions [68]. Authors in [69] apply machine learning for sensitivity analysis of safety conditions in platooning, under the constraint of no false negative, i.e., avoiding to predict safety (no collision) while collision happens in reality. Regarding communications, and in particular safety, we identified several concerns. An unexpected interruption of the communication between two nodes can have an extremely negative impact in the safety of the platooning. A proper defense mechanism against erasure must be in place. Furthermore, communication must support response to violations on time (before a deadline), thus delivery time, must be ensured to bound the delay. Finally, these systems must also cope with RF interference without loss of main functionality. Although generic, this requirement aims at guaranteeing that the communication system is resilient to wireless interference on one hand (accidental or not), and can be robust to cope with an eventual attack, such as a DoS attack. We believe that, by relying on the EN 50159 black-channel approach, we will be able to address most of the safety issues for inter-vehicular communication, while escaping the need to certify the whole communication protocol, easing the use of COTS communication stacks. While this approach can mitigate several (if not all) of these safety constraints, there are still others requirements that must be addressed, particularly in what concerns QoS performance, to support the expected behavior of the platoon. In a vehicular platoon, a lead vehicle that is responsible for managing the platoon's moving directions and velocity, periodically disseminates control commands to following vehicles based on vehicle-to-vehicle communications. Inevitably, pushing vehicles to drive in close formation as the platoon requires low latency driving command transmission from the lead vehicle to the tail. Two critical challenges arise in the inter-vehicle wireless communication. The first challenge is that signal fading induces dynamic wireless channels, which causes command loss at the receiver. This command loss is especially crucial in vehicular platoons since command reception at each vehicle highly depends on the reception of its preceding vehicle. Moreover, command loss at preceding vehicles can impact the command dissemination due to retransmissions, which may lead to accidents due to lack of timely updates. The second challenge is the possibility of assigning the exact transmit rate to each vehicle in the platoon. Although a high transmit rate achieves low transmission latency for each vehicle, increasing the transmit rate results in increasing the receiver's bit error rate (BER) at a given Signal-to-Noise Ratio (SNR).