###### Abstract
For example, a mobile edge computing (MEC) infrastructure was deployed in Germany [10] along 30 km of the A9 motorway, where it was demonstrated that the vehicles can exchange information with an end-to-end latency below 20 ms using LTE. Similarly, but in a different context, the on-going 5G Cross-Border Control (5 GcroCo) project [11] is conducting trials of C-V2X based vehicular use cases that are deployed in cross-border scenarios. One of the use cases deployed in 5 GcroCo is the anticipated cooperative collision avoidance (ACCA) service [12]. The aim of the ACCA service is to anticipate impending collisions ahead of time and, subsequently, inform approaching vehicles to avoid any dangerous event, e.g., stationary vehicle, an accident, a vehicle with emergency brake lights, or a traffic jam, among others. As such, each vehicle shares the status information and detected roadside hazardous events with the ACCA service deployed on the MEC infrastructure, which, in turn, creates a distributed dynamic map with all of the collected information and informs those vehicles that are required to include all the relevant information within their geographical region of interest (ROI). One of the challenges in the deployment of the ACCA service is to facilitate seamless interoperability among OBUs from different original equipment manufacturers (OEMs), since they typically need to support different V2X protocols to communicate with heterogeneous backend applications that are allocated on the MEC/cloud servers. In the cross-border field trials and development of ACCA in the 5 GcroCo project, two OEMs are involved: Peugeot Citroen Automobiles (PSA) and Renault, S.A. (RSA). As a result, the testing of the ACCA functionalities and the interoperability of V2X communication interfaces can be a complex and expensive task, as this requires the use of several commercial OBUs from each OEM. Being motivated by this need, we present in this paper an experimental OBU that supports two V2X communication protocols to exchange information with applications running on the MEC/cloud infrastructure: UDP/IP protocol, carrying ETSI ITS ASN.1 encoded messages; and, the MQTT protocol on top of TCP/IP, carrying ETSI ITS ASN.1 encoded messages in JSON format. The OBU can readily be integrated into any vehicle and it can also be used as a vehicle emulator in case no real vehicles are in-place. The OBU assists the driver by displaying warnings on a human-machine interface (HMI) upon the detection of hazards on the road as well as adapting own driving intentions upon reception of warnings based on the detected events by other vehicles. It includes an interface to the in-vehicle sensors and actuators network for the acquisition of measurements (e.g., speed of the vehicle) and notifications of detected hazards in real-time. Furthermore, it supports the easy integration of different types of 4G/5G cellular modems for V2N communications. Note that this paper is an extension of our preliminary work presented as a poster in [PERSON] et al. [13]. In this paper, we have specifically demonstrated the OBU's operations in a real-world deployment of the ACCA service components on the MEC and cloud servers [14]. In the experimental evaluation of the OBU's operations, we have considered three types of V2X information flows (as detailed in Section 5.2) that can occur in a cross-border deployment of the ACCA service. The OBUs established the connectivity with the ACCA service components over LTE cellular technology, and the traffic from the OBUs was generated by emulating vehicle movements in a lab setup (as detailed in Section 5.1). Moreover, we have measured end-to-end application-level latencies for each of those three types of V2X information flows as the key performance indicator, while demonstrating the effectiveness of interoperability in V2X communication between different vehicle OEMs. The remainder of this paper is organized as follows.