# Networked Roadside Perception Units for Autonomous Driving
## 1 Introduction  
Road transport infrastructure is an essential part of modern human life. However, it currently poses many issues, such as accidents, high energy consumption, time loss, and CO\({}_{2}\) emission. Intelligent Transportation System (ITS) objectives include road traffic optimization in terms of safety, efficiency, and comfort. In recent years, among the many technologies associate with the domain of ITS, autonomous vehicles have attracted the most attention of researchers. Autonomous vehicles use a set of sensors such as LiDAR systems, cameras, and global navigation system satellite (GNSS)/inertial measurement units to perceive the surrounding environment. Using the sensor data, the autonomous driving system accelerates, steers, and brakes without the help of human input. However, at a more abstract level, standalone autonomous vehicles are not significantly different from conventional vehicles in that they merely replace human eyes, brain, arms, and legs with sensors, processors, and actuators. Hence, both human drivers and autonomous vehicles share some limitations in terms of their capability of perception, planning, and control. Especially with regard to perception capability, both eyes and sensors have very similar fields of view because they are both positioned on the vehicle, and heavy vehicles and buildings affect both in the same way by obstructing their line of sight (LOS). These shared limitations prevent autonomous vehicles from a safer or more efficient driving operation than human drivers. For example, both need to stop in front of an unsignalized intersection and approach it slowly. In theory, it would be safer and more efficient to move in the intersection without stopping by shortening the time in the intersection. Vehicle-to-everything (V2X) communication connects vehicles, roadside infrastructure, pedestrians, and networks, and has the potential to overcome some of the issues described above through better perception of the surrounding environment. Two types of wireless technologies have been under research and development for V2X: Dedicated short-range communication (DSRC) and cellular V2X. Cooperative ITS refers to transport systems, where the cooperation between two or more ITS sub-systems using these V2X technologies. Therefore, cooperative ITS needs the same architecture, technologies, and protocols for interoperability among the products and software in the market. The common architecture, known as the ITS station architecture [1; 2], was developed by European Telecommunications Standards Institute (ETSI) and International Organization for Standardization (ISO). According to this standard, a cooperative awareness message (CAM) [3] is the most basic safety-related V2V message, using which, a vehicle can intimate the surrounding vehicles regarding its presence in real-time. A local database, known as the local dynamic map (LDM) [4], stores the received information and provides support for various ITS applications. Traditional cooperative ITS largely relies on a network of on-board units (OBUs) and roadside units (RSUs). Therefore, cooperative ITS has not been able to accommodate non-connected road users, such as vulnerable road users (VRUs; e.g., pedestrians and bicycles) and legacy vehicles--neither of which have V2X devices. These non-connected users are, however, detected in the sensors equipped in the vehicle or the infrastructure environment. Cooperative perception, also known as collective or collaborative perception, integrates these non-connected users to the cooperative ITS by sharing sensor information. Cooperative perception plays a significant role in the deployment phase of V2X technologies when legacy vehicles and connected autonomous vehicles would coincide on the roads. Collective perception messages (CPMs) [5; 6] is currently being developed in ETSI. The sensors can be installed either in vehicles or in the infrastructure environment, and cooperative perception works in either configuration.