F. Schmidt-Eisenlohr

Vehicle-to-X (V2X) communication, i.e. the direct communication between vehicles and/or the communication between vehicles and stationary road side units, is an envisioned technology that should enable improved cooperative safety systems, advances in traffic efficiency as well as the support of infotainment applications. In order to enable cooperative safety systems a direct, reliable and immediate communication directly between vehicles is mandatory, so called Vehicle-to-Vehicle (V2V) communication. Wireless transmission technologies are used and, due to the unreliable characteristics of the wireless medium, performance guarantees concerning the "quality" of the communication cannot be easily given. In order to design traffic safety applications on top ofV2V communication the system's characteristics have to be known and understood. The operation conditions of the communication system are challenging due to the combination of specific properties that emerge from the scenario under investigation: applications require timely and reliable information delivery, the available bandwidth has to be shared decentralized and without central coordination, data is mainly transmitted as broadcast, and the mobile vehicles are exposed to a varying radio environment. The properties lead to a communication network where reliable communication should be guaranteed under challenging conditions. Mutual interferences due to the shared usage of one restricted communication channel are an important factor.

In this thesis we analyze, model and simulate and assess IEEE 802.11p based vehicular communication networks under thementioned challenging conditions. Focus is put on the communication paradigm of periodic distribution of broadcast status messages by all nodes with the intent that all nodes in the geographical surrounding receive thesemessages and thus are aware of the position of vehicles around them. Such type of communication is an essential basis for all cooperative safety systems. We first analyze applications foreseen for V2V communication networks and derive their communication requirements. We also provide an overview of past and current research projects, architectures and standardization efforts, especially IEEE 802.11p. Then, review radio propagation and their characterization and we review different sources and reasons of interference.

In order to evaluate periodic local broadcasts we formalize the dependencies between available data rate, vehicle density, and the area to which information should be provided with a required reliability by the definition of the metric "local broadcasts capacity". We analytically derive the maximum capacity as well as the worst case capacity and thus provide an orientation of the possible range of operation in that real systems are expected to perform. The derivation of the realistic performance of a system with respect to local broadcasts capacity is yet of a level of complexity that can not be covered easily by analytics.

A suitable way to analyze the communication behaviormore deeply is the use of simulation studies that include detailed models for physical and interference effects. We investigate on models that cover the specifics of the physical layer in the V2V domain. In collaboration with Mercedes-Benz Research & Development North America, themodels were integrated into the network simulator NS-2 that experiences a complete overhaul of the lower communication layers. The detailed implementation has become an integral part of today's distribution of the NS-2 and provides detailed simulation capabilities.

We then provide a broad assessment by a simulation study in order to identify the performance of local broadcast communication in V2V networks. All nodes in a scenario periodically transmit status information messages with the intent that nodes in the geographic surrounding receive the messages and thus become aware of the status of the surrounding nodes. The analysis covers a wide range of node densities, system factors and configuration parameters. The evaluation of the simulations is performed with several metrics and a detailed reception and failure analysis. An analysis on the influence and the sensitivity of system performance is provided. Further, we use local broadcasts capacity to analyze the effectiveness of the underlying communication system from an overall systems point of view. We observe that the ratios between results achieved by intensive simulation studies and the theoretical maximum do not vary much over different node densities when one set of basic system factors, like radio propagation, is assumed. The derived observations and numbers allow the prediction of system performance when the penetration rate among vehicles becomes high.

This thesis provides the following main contributions: a formal definition of local broadcasts capacity to evaluate this type of communication, a simulation framework built on validated models and allowing detailed studies of V2V networks, a broad assessment of the effects of interference and of parameters on the performance of V2V networks and, an analysis on the achievable effectiveness of local broadcast communication in V2V networks. Fundamental insights are presented, that can be used for system optimization and the design of effective algorithms in V2V communication networks.

KIT Scientific Publishing, ISBN 978-3-86644-571-0

2010

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