PhD Defence Mozhdeh Gholibeigi

reliable broadcasting in vehicular networks

Mozhdeh Gholibeigi is a PhD student in the research group Design and Analysis of Communication Systems. Her supervisors are G.J. Heijenk and prof.dr. J.L. van den Berg from the Faculty of Electrical Engineering, Mathematics and Computer Science.

Vehicular transportation is an integral part of today's life. In this respect, Intelligent Transport Systems (ITS) applications relying on vehicular communications provide means for increased efficiency and safety of vehicular transportation. ITS utilize advanced information and communication technologies in order to serve many novel application types, targeting improvement of traffic situations on the roads. By increasing the level of automation and assisting human drivers, they contribute to higher traffic safety and reduced congestion and environmental impact. Wireless communication among vehicles, the so-called vehicular networking, is the main enabler for ITS applications. Vehicular broadcasting refers to dissemination of data from a single node to all other nodes within the scope of a vehicular network and is a common communication type that many ITS applications rely on.

Assuring reliable delivery of broadcast data is of paramount importance for many and in particular safety-critical ITS applications, built upon this type of communication. That is, delivery of broadcast data within reasonable time to all intended nodes of a vehicular network. Direct Short Range Communication (DSRC) based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11p standard is considered as the main wireless communication technology to enable vehicular communication, including broadcasting. However, IEEE 802.11p-based broadcast is defined as a best-effort service, lacking an acknowledgment technique. Targeting reliability improvement of vehicular broadcasting may lead to inefficiency. For instance, redundant retransmissions or acknowledgments overload the limited wireless medium with excess traffic. Hence, to be considered in this regard, is the cost at which broadcast reliability is achieved. Motivated by this, in this thesis, we focus on the reliability requirement of vehicular broadcasting. After providing a background to the scope of this thesis, we first propose an End-to-End (E2E) reliability assurance mechanism based on a sequence checking module where receivers can detect missing packets and explicitly request them. Such an approach provides broadcast reliability, while at the same time not overloading the network with redundant traffic. Using absorbing Markov chains, we analytically model the functionality of the proposed mechanism and analyze its performance in the context of a single-hop vehicular communication scenario based on the IEEE 802.11p standard. We further validate our analytical analysis via simulations. At the next step, we extend our earlier work by considering application of the proposed E2E reliability assurance mechanism in the context of a multi-hop vehicular communication scenario based on the IEEE 802.11p standard. For this, we first analytically model multi-hop data dissemination throughout the network by means of developing a closed form recursive function, quantifying the probability of network nodes having obtained broadcast data. Accordingly, we assess the error recovery performance of the E2E reliability assurance mechanism, applied upon multi-hop broadcasting, by means of analytical analysis, based on Markov chains and Bayesian networks. Using simulations, we further validate our analytical modeling. The results of our analysis show that the proposed reliability assurance mechanism performs efficiently, in both single-hop and multi-hop scenarios, by imposing little burden of error recovery even for high number of receivers.

Besides lacking a built-in acknowledgment technique, the IEEE 802.11p standard has other shortcomings,

namely in terms of scalability under rather high network loads, with respect to fully suiting performance requirements of ITS applications. This has been the driver for the next stage of our work in this thesis. The 3rd Generation Partnership Project (3GPP) cellular communication system is a promising alternative for the IEEE 802.11p standard with all its potential to support vehicular communication, including large-scale deployment and infrastructure-based resource management capabilities. Specifically, Device-to-Device (D2D) communication technology has been introduced and further evolved in the recent releases of the 4th generation of 3GPP mobile networking system (i.e., Long Term Evolution (LTE)) towards supporting high-performance vehicular communications and accordingly ITS applications. D2D refers to direct communication between users in close vicinity, by utilizing the cellular radio spectrum and without traversing the infrastructure, as opposed to conventional cellular communications. Accordingly, it can result in proximity gain, resource reuse gain and hop gain. Ultra Reliable Low Latency Communication (URLLC) is one of the three main services of the next generation (i.e., 5G) mobile networking system, targeting ITS use cases, which can serve D2D-based broadcast. In our work, we focus on the resource allocation aspect of the D2D communication technology and its utilization for vehicular broadcasting. Radio resource reuse, aiming efficient utilization of the scarce spectrum, is an important aspect of D2D resource allocation and in our work we propose a reuse-based resource allocation approach being adaptive to the network load and topology. That is, by taking into account the number of users seeking for D2D broadcast and their geographical distribution in the network, resources are allocated in the most efficient manner, aiming spectrum efficiency and collision avoidance, due to reuse. We model our proposed resource allocation approach and extensively evaluate its performance in comparison with a baseline resource allocation approach, in the context of single- and multi-cell scenarios. The results verify spectrum efficiency and reliability of the proposed resource allocation approach, in comparison with the baseline approach.

Our analytical models provide means for analysis of vehicular broadcasting and our proposed approaches of improving its reliability. This allows for efficient study of impact of various factors on the performance of reliable broadcasting. The results of this thesis provide insight into the behavior of broadcasting in vehicular networks and further solutions towards its performance and efficiency. Such results cannot only be used in the design of reliable vehicular communication mechanisms and accordingly high-performance ITS applications, but also can serve as a basis for future research in this direction.