1.

Introduction

The idea of vehicles sharing information and working together to make transportation safer, greener, and more enjoyable, is truly compelling. The technologies associated with this concept, collectively known as Cooperative Intelligent Transportation Systems (C-ITS), promise to reduce traffic congestion, lessen the environmental impact of transportation, and significantly reduce the number of lethal traffic accidents. The impact on safety alone makes C-ITS worth considering, since, according to the World Health Organization (WHO), roughly 1.25 million people died in 2015 due to traffic accidents, and with an associated governmental cost of about 3% of GDP [1].



Figure 1 Comparison between IEEE 802.11p and cellular connectivity pipes to the car. The key difference is the direct communications among 802.11p equipped devices. Cellular based services rely on the presence of the network.



A key enabling technology of C-ITS is wireless communication, covering vehicle-to-vehicle (V2V) communication, vehicle-to-infrastructure (V2I) communication, and infrastructure-to-vehicle (I2V) communication (Figure 2). Collectively, these wireless transactions are referred to as V2x communication.

V2x has to support the many safety-related and non-safety-related use-cases of ITS systems. Tables I and II, in the Appendix, list the primary use-cases. Table I gives safety-related use-cases, such as the ability to transmit and receive the message “emergency electronic brake lights,” a message that is transmitted by a vehicle in broadcast mode every tenth of a second to signal the event of emergency breaking. Table II gives non-safety-related use-cases, such as the message “traffic light optimal speed advisory,” which is designed to improve traffic flow by using periodic broadcasts to recommend the best speed.



Figure 2 Artist's view of communicating vehicles and infrastructure. Vehicles can be cars, airplanes, trains and ships. The Central traffic Management System (CMS) manages the multiple aspects of the Cooperative Intelligent Transport System (C-ITS)



To support safety-related and non-safety-related messages, the wireless technologies used in V2x communication need to do several things. They need to operate in a very dynamic