Tristan Gaugel

Feasibility of virtual traffic lights in non-line-of-sight environments

Motivated by the idea to reduce deployment costs and to dynamically regulate vehicular traffic flows at intersections, inter-vehicle communications based virtual traffic lights are envisioned to replace traditional infrastructure based traffic lights. According to recent studies, virtual traffic lights are expected to increase traffic flow by up to 60%. Yet, those studies were based on the assumption of a perfectly reliable communication, i.e., notification messages which signal a traffic light were always received by vehicles located within a certain distance to the sender. Hence, effects such as signal fading or non-line-of-sight conditions due to buildings were neglected. Such effects, however, can have a negative impact on the dissemination of the notification messages. This poster paper therefore studies whether these effects lead to significantly larger dissemination delays or not, and whether this increase is crucial for the feasibility of virtual traffic lights. According to the results of this study, the delay is not significantly larger, and virtual traffic lights seem to be feasible under such challenging conditions.

Accurate Simulation of Wireless Vehicular Networks Based on Ray Tracing and Physical Layer Simulation

Vehicle-to-vehicle (V2V) and vehicle-to-roadside (V2R) communications is required for numerous applications that aim at improving traffic safety and traffic efficiency. As recent studies have shown, communications in this context is significantly influenced by radio propagation characteristics of the environment and the signal processing algorithms that are executed on the physical layer of the communications stack. Whereas a shadowing of the transmitted signal, e.g. due to buildings, determines the ability to communicate “around corners”, channel estimation, channel equalizing, and advanced coding schemes determine whether a receiver can decode a received signal successfully or not. Consequently, a proper assessment and evaluation of V2V and V2R communications, especially when traffic safety applications are considered, requires an accurate simulation of the wireless channel as well as the physical layer of the protocol stack. To enable a proper assessment, we integrated a physical layer simulator into the popular NS-3 network simulator, validated our implementation against commercial off the shelf transceiver chipsets, and employed ray tracing as a method to accurately simulate the radio propagation characteristics of the Karlsruhe Oststadt. Since the simulation of signal processing details and ray tracing are computationally expensive modeling methods, we based our work on the HP XC4000 to speedup the computation of both aspects.

A change in perspective: information-centric modeling of inter-vehicle communication

Inter-vehicle communication with the objective to increase safety and efficiency of our transportation network has been studied extensively in the last few years. However, we are still struggling in answering the fundamental question of to which extent can inter-vehicle communication improve our wide-area transportation network that consists of more than several cities?. In this position paper we discuss from a methodological point of view and by analyzing recent trends how the VANET community could move forward with respect to evaluation methodologies. Based on our observations, we argue that the vehicular networking community might need to extend its perspective -- from bits and packets towards information-centric models -- when assessing efficiency gains in large scale scenarios, thereby providing a multi-scale approach as it is done in various disciplines. We further believe that the community should join forces to convert existing results as well as current and upcoming measurements to provide information-centric models of inter-vehicular communication. We finally propose a modeling approach that could serve as a foundation for future joint efforts and which might take us one step closer to the answer of the fundamental question stated above.

Enabling traffic safety assessment of VANETs by means of accident simulations

Vehicular ad hoc networks are conceived as a means to increase traffic safety. Primary means to demonstrate their beneficial impact before deployment are computer simulations that comprise communication models, mobility models of vehicles and models of VANET applications. In particular, the movement behavior of vehicles poses questions when evaluating traffic safety since driver models typically do not allow accidents to happen. In this paper we address modifications to the popular Wiedemann model and present an integration of the modified driving behavior to the traffic simulator VISSIM which thus enables simulations containing accidents. Finally, we show in a simulative study how locally available information on the current traffic situation can contribute to safer road traffic. Our contributions represent a first step on how to assess VANETs with respect to traffic safety.

Understanding differences in MAC performance

The suitability and performance of medium access protocols in vehicular environments is already being investigated over a long period of time. Carrier Sense Multiple Access (CSMA) has been shown to perform sufficiently well in most situations and being able to support safety and efficiency vehicular applications. Recently, Self-organizing Time-Division Multiple Access (STDMA) is being considered as an alternative and has been shown to coordinate the channel slightly better under certain situations. However, when comparing both protocols the precise details of radio and network conditions and parametrization of the protocols are decisive on which protocol takes a slight lead. Consequently, scenarios can be constructed quite easily in which one protocol is superior over the other one. The focus of this work is thus not to absolutely compare both protocols, but rather to understand the strengths and weaknesses of both protocols in certain situations. In particular, we consider i) to which degree hidden nodes influence the coordination ability, ii) how an extended carrier sensing range is beneficial and iii) how temporary fading influences the performance of both MAC protocols. Our results show that while an extended carrier sensing range is only beneficial for CSMA, the existence and severity of fading is far less detrimental for STDMA than for CSMA.

Appropriate selection of urban vehicle-to-vehicle radio propagation models

Urban vehicle-to-vehicle radio propagation is commonly modeled using simplified path loss models which are either based on empirical measurements or analytical considerations. These models have in common that their parameterability is rather limited and it is thus only possible to consider, e.g., the distance between transmitter and receiver, antenna heights or street widths. Aspects, such as the shape of an intersection or to which degree vehicles or vegetation are present cannot explicitly be considered using these models. In contrast, recent work suggests that even effects, such as obstructed line-of-sight by vehicles on the road or destructive multipath propagation can drastically influence simulation results. Accordingly, adjusted models are suggested which, however, incorporate only one additional aspect at a time. In this paper, we pursue a different approach and take a first step in evaluating which aspects influence radio conditions in urban areas to which degree and how these individual aspects affect each other. We consequently consolidate existing works and evaluate in which situations a more detailed radio propagation modeling might yield substantially different results with respect to the requirements of IEEE 802.11p networks. In order to achieve comparable results and being able to model several aspects, we employed ray tracing with an accurate geographic scenario and individually modeled obstacles, such as post boxes or trees at the roadside. Our results show and explain that the presence of radio obstacles can indeed make a difference within certain ranges if considered individually, the overall path loss in a heterogeneous and crowded urban scenario, however, is still approximated well with currently employed path loss models.

Multiscale Modeling of Inter-Vehicle Communication

Within this thesis, different modeling approaches at different scales in the domains of urban radio propagation, decentralized channel coordination, and information dissemination in inter-vehicle communication networks are investigated. The contributions reveal the suitability of existing models for network-oriented research, propose a novel information-centric modeling approach, and identify characteristics of inter-vehicle communication systems which determine key dependability aspects.

On Spatial Measures of Geographic Relevance for Geotagged Social Media Content

Recently, geotagged social media contents became increasingly available to researchers and were subject to more and more studies. Different spatial measures such as Focus, Entropy and Spread have been applied to describe geospatial characteristics of social media contents. In this paper, we draw the attention to the fact that these popular measures do not necessarily show the geographic relevance or dependence of social content, but mix up geographic relevance, the distribution of the user population, and sample size. Therefore, results based on these measures cannot be interpreted as geographic effects alone. By means of an assessment, based on Twitter data collected over a time span of six weeks, we highlight potential misinterpretations and we furthermore propose normalized measures which show less dependency on the underlying user population and are able to mitigate the effect of outliers.

On spatial measures of geographic relevance for geotagged social media content

Recently, geotagged social media contents became increasingly available to researchers and were subject to more and more studies. Different spatial measures such as Focus, Entropy and Spread have been applied to describe geospatial characteristics of social media contents. In this paper, we draw the attention to the fact that these popular measures do not necessarily show the geographic relevance or dependence of social content, but mix up geographic relevance, the distribution of the user population, and sample size. Therefore, results based on these measures cannot be interpreted as geographic effects alone. By means of an assessment, based on Twitter data collected over a time span of six weeks, we highlight potential misinterpretations and we furthermore propose normalized measures which show less dependency on the underlying user population and are able to mitigate the effect of outliers.

Characterization and modeling of dissemination delays in inter-vehicle communication networks

It is most likely that upcoming vehicles will be equipped with inter-vehicle communication technologies that enable neighboring (and remote) vehicles to exchange information directly (or indirectly over multiple hops) as needed. Although those technologies obviously compete against cellular approaches, inter-vehicle communication will most likely contribute to applications such as cooperative sensing or traffic information systems. In order to evaluate the effectiveness and benefit of such applications - most importantly in very large network scenarios - it is essential to understand the time-delay characteristics of data dissemination processes and to model them efficiently. In this paper, we explore an information-centric modeling approach to describe the time delay of beaconing based data dissemination. Our approach is based on the analogy of a construction kit with elementary, connectable road shapes (or “blocks”, e.g. a straight road, an intersection, or similar). The proposed concept supports arbitrary road network layouts, is extensible, and allows to improve the accuracy (and statistical correctness) of blocks over time. We further provide two elementary blocks of the construction kit: a straight road of arbitrary length for highway, rural and city scenarios, as well as an intersection. Compared to previous work, our underlying model of these blocks addresses not only application and communication related aspects, but also mobility related aspects.