Scalable beaconing for cooperative adaptive cruise control

Abstract

Over the past two hundred years, automotive technology has evolved from mechanised horse carriage to high-tech systems which pack more computing power than the entire space program that put Neil Armstrong on the moon. Hand-in-hand with this evolution came a proliferation of ownership and use of cars. This enormous success causes one of modern society’s largest problems: where many vehicles accumulate, traffic congestion occurs. To a large degree, the cause of traffic congestion lies in the poor ability of the human driver to control the (longitudinal) motion of the vehicle under congested traffic circumstances. This leads to so-called string instabilities or shock waves, traveling against the flow of traffic. The traffic flow performance can be improved if the control of acceleration and deceleration is automated. Presently available solutions use radar or lidar to detect and measure the distance to the vehicle in front, and a cruise controller automatically reacts by adjusting the vehicle speed. However, the performance of these systems is not sufficient to prevent shock waves, predominantly due to the delay introduced by the sensors. The Cooperative Adaptive Cruise Control (CACC) is a system which circumvents this by using wireless communication to exchange information about vehicle dynamics using the periodic transmission of so-called beacon messages. The technology proposed for this wireless communication is IEEE 802.11p, a modified version of the IEEE 802.11a designed for Wireless LAN applications. However, the wireless medium succumbs to a congested state in a similar fashion as the traffic on the road in response to an increase of the traffic density. This dissertation focusses on the beaconing communication, used to generate a cooperative awareness in each vehicle. Given the real-time nature of the CACC system, it is important that the information in the cooperative awareness is accurate and fresh, even under an increasing number of communicating nodes in near vicinity. To this end, beaconing is evaluated through analytical modelling, discrete-event simulation and proof-of-concept implementations. The purpose is to determine the scalability limits of the IEEE 802.11p Medium Access Control mechanism when used for beaconing, and find and address bottlenecks. In this disseration, detailed analytical models of the Distributed Coordination Function (DCF) and the Enhanced Distributed Channel Access (EDCA) are proposed, validated, and compared. Various mechanisms which impact the scalability of a beaconing system are described and evaluated using both these analytical and simulation models. In particular, an extensive comparison between the DCF and EDCA access mechanism variants of IEEE 802.11 is performed based on their performance in the face of increasing traffic density. The conclusion is that, although IEEE 802.11p defines the EDCA to be used, the DCF is a more favourable access mechanism for the broadcast transmission of beacon messages. Under both access methods, the use of the EIFS is found to be redundant because the beacon channel is broadcast-only. Furthermore, the periodic channel switching defined by IEEE 1609.4, which defines a way to use single-radio IEEE 802.11p on multiple channels in a time-division fashion, has a detrimental effect on beaconing performance. In addition, the way beacon messages are buffered and scheduled for transmission is evaluated. We conclude that it is beneficial to use a dropping policy which drops the oldest information in the queue, as opposed to the most recent arrival as is often implemented. This method is coined the Oldest Packet Drop (OPD) mechanism and is described and evaluated in detail. The outlook of a CACC application operating on beacons transmitted using IEEE 802.11p is good. However, care has to be taken that the system does not become congested. This dissertation provides a set of tools to estimate when the channel becomes congested, and to evaluate the impact of various design choices on communication performance.