Location-based Forwarding in Vehicular Networks

In this thesis we focus on location-based message forwarding in vehicular networks to support intelligent transportation systems (ITSs). ITSs are transport systems that utilise information and communication technologies to increase their level of automation, in this way levering the performance of such a system beyond the capabilities of the human driver. Such systems provide an increased level of traffic safety, traffic efficiency, and driving comfort, and reduce the environmental impact of traffic. An example ITS application is platoon driving, a form of automated driving in which vehicles cooperatively control their speed using wireless communication. The field of wireless networking that enables such systems is called vehicular networking. In a vehicular network vehicles, or nodes, act both as end-user and as router. Direct vehicle-to-vehicle (V2V) communication is possible using short-range communication technologies such as the IEEE 802.11p standard. Communication with more distant vehicles is supported by multi-hop forwarding protocols, typically using georouting. Georouting is a form of location-based forwarding in which data is addressed to a specific geographic location, and delivered to all nodes that are inside the geographic location at the time of delivery. It has been made possible by means of positioning techniques such as GPS and is used to disseminate location-relevant data. In this thesis we consider two distinct challenges related to location-based forwarding in vehicular networks. The first challenges concerns georouting. Although it is currently the method of choice to disseminate data over multiple hops, georouting has a number of challenges left open. Specifically, its method of addressing does not always fit the requirements of the higher-level ITS application, causing data to be routed in an inefficient manner. This inefficiency stems from the fact that with standard georouting nodes are distinguished based on their current position only, a method which poorly meets the requirements of a typical ITS scenario where a node's trajectory from its current position onward is important. We address this shortcoming by proposing constrained geocast, a novel form of georouting in which destination nodes are addressed based on their conjectured future position rather than their current position. This allows data to be routed in a more selective manner, such that it only reaches those parts of the network where it is needed, i.e., where there are nodes that are headed in the direction of the event location and for that reason have interest in the data. We define a set of generic forwarding rules for constrained geocast and test it by means of simulation of a small-scale scenario, demonstrating the effectiveness of our solution. The second challenge concerns the analytical modelling of multi-hop location-based forwarding inside vehicular networks. Despite the fact that several multi-hop forwarding protocols have already been standardised for use in vehicular networks, a thorough understanding of the performance of these protocols is still lacking. In particular, there is hardly any analytical work available on the subject. To analytically model a multi-hop forwarding protocol in a realistic manner is a challenging task because of the size of the system and the inter-dependencies of successive hops, such that the complexity of such an analysis increases with each following hop. Existing analytical studies are therefore typically based on overly-simplified assumptions and give only limited insights. In this thesis we analytically model three multi-hop forwarding protocols in detail. One of these protocols is also referred to as beacon dissemination, or piggybacking. Another has recently been standardised as the contention-based forwarding (CBF) protocol. We express the behaviour of all three protocols in a number of fast-to-evaluate analytical expressions, with a high level of detail. Our models cover the multi-hop transmission of a single message from source to sink over a straight road with results including the full probability distribution of (i) the length of each hop, (ii) the delay of each hop, (iii) the success probability of each hop, (iv) the position of successive forwarders, (v) the required number of hops to have the message delivered, and (vi) the end-to-end delay to have the message delivered. Extensive verification of our analyses by detailed simulation showed the analytical results to be very accurate. Our analytical models allow for easy and fast evaluation of the performance of the considered multi-hop forwarding protocols. In addition, they provide useful insights regarding the behaviour of the respective protocols, such as the way they are influenced by the various network and protocol parameters. Because of their high evaluation speed compared to existing simulation models, our analytical models can also speed up and, hence, considerably improve the usability of ITS application level simulations by emulating the communication layer. The results of this thesis improve the efficiency and understanding of location-based forwarding in vehicular networks. The insights provided by our work can be used to increase the effectiveness of vehicular communication protocols and in this way advance the overall performance of ITS applications.