Future safety-related vehicular applications require reliable information exchange provided by cooperative Vehicular Ad-hoc NETworks (VANETs). Although the vehicular WLAN standard IEEE 802.11p has been adapted to the challenging vehicular environment, it has not been adapted to the stringent communication requirements imposed by vehicular applications. In particular, broadcast transmissions are mostly periodic and initiated at common TX powers. This makes potential interferences recurring instead of spurious and lowers the performance of medium access for vehicular applications.In this paper, we propose to leverage recurring interferences by randomly selecting each TX power following a given probability distribution. Such randomization reduces the chances of recurring interferences, and the probability distribution provides control to the applications regarding the required Awareness Quality, in particular by providing a higher Awareness Quality at close range. This concept also reduces congestions by transmitting less at high distances. It is transparent to the applications, and manages to improve the Awareness Quality in a dense highway by a factor 2 to 20, yet at a factor 2 to 3 lower channel load.

Cooperative safety applications require Dedicated Short-Range Communications (DSRC) to provide position-awareness of neighboring vehicles at a specific level of reliability, i.e. awareness-quality, up to a given distance, i.e. awareness-range. However, heavy communication loads negatively impact such awareness requirements due to communication impairments, ranging from strict capacity limitations of DSRC channels to correlated packet collisions due to periodic communication patterns. Transmission control strategies may adapt power or rate to control such impairments but risk missing the requirements of cooperative safety applications. In this paper, we design a new awareness control strategy by implementing a spatial awareness framework. Specifically, we adapt the distribution of the awareness-quality as a function of the awareness-range. Therefore, we first propose Random Transmit Power Control (RTPC), which manages to provide different levels of awareness-quality at different ranges, while mitigating correlated packet collisions by randomizing them in space. As RTPC is able to reduce the channel load, we secondly propose to combine RTPC with Transmit Rate Control (TRC) and to benefit from the gained channel resources by subsequently increasing the update-rate and by implication, the quality of position-awareness. The spatial awareness control capability of RTPC+TRC has been evaluated through simulations. We discuss the influence of RTPC+TRC on cooperative safety applications exemplarily for the Forward Collision Warning (FCW) application.

Forward collision warning systems, lane change assistants or cooperative adaptive cruise control are examples of safety relevant applications that rely on accurate relative positioning between vehicles. Current solutions estimate the position of surrounding vehicles by measuring the distance with a RADAR sensor or a camera system. The perception range of these sensors can be extended by the exchange of GNSS information between the vehicles using an inter-vehicle communication link. In this paper we analyze two competing strategies against each other: the subtraction of the absolute positions estimated in each vehicle and the differentiation of GNSS pseudoranges. The aim of the later approach is to cancel out correlated errors in both receivers and, thus, achieve a better relative position estimate. The theoretical analysis is backed with Monte-Carlo simulations and empirical measurements in real world scenarios. Further on, two Bayesian approaches that make use of pseudorange differences are proposed. In a Kalman Filter pseudorange and Doppler measurements are used to estimate the baseline between two vehicles. This is extended in a second filter using on-board inertial and speed sensors following a multisensor fusion approach. The performance is evaluated in both, a highway and an urban scenario. The multisensor fusion approach proves to be able to stabilize the baseline estimate in GNSS challenging environments, like urban canyons and tunnels.

Abstract-ETSI ITS-G5 is the current vehicle-to-vehicle communication technology in Europe, which will be standardized by ETSI TC ITS 1 . It is based on IEEE 802.11p and therefore uses a CSMA/CA scheme for Media Access Control (MAC). In this paper we analyze the performance of Cooperative Awareness Message (CAM) based safety applications using the ETSI ITS-G5 MAC technology in a challenging scenario with respect to MAC issues: A suitable freeway segment with 6 lanes in each direction. The freeway scenario is thoroughly modeled and implemented in the well known ns-3 simulation environment. Based on this model, the paper shows the performance of CAM based safety applications under MAC challenging conditions. We provide a set of simulation results resting upon a particular performance metric which incorporates the key requirements of safety applications. Finally we analyze two concrete example scenarios to determine how reliable CAM based safety applications are in high dense traffic scenarios with respect to MAC issues.

Abstract. High channel load in vehicle-to-vehicle communication leads to a degradation of the vehicles' communication range, due to interference and hence packet loss at larger distances. Packet loss results from two or more concurrent transmissions, colliding at receivers located inbetween, which is also known as the hidden station problem. In previous works, our simulation study has shown that this packet loss leads to a degradation of 90% of the communication range. In this paper, we confirm the simulation results by real-world measurements. We present a methodology for transferring the simulation scenario to a real-world measurement scenario, able to evaluate the problem of hidden stations. With three radios applying the IEEE 802.11p standard, we measure the degradation of the communication range under interference. In the measurement, we find a degradation of 50 to 70%. On the one hand, there are less collisions due to only one hidden station. On the other hand, we identify that the receiving vehicle as a shadowing object itself is an additional origin for hiding the other station which slightly increases the number of collisions even at close distances.

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This paper addresses the problem of efficient data dissemination in Vehicular Ad Hoc Networks (VANETs), which particularly suffer from changing densities in the network topology due to congested and sparse traffic on the roads. We present a new network layer protocol in the family of geographic network protocols, which makes use of distance and time information following a dissemination strategy to efficiently distribute messages adapting to the varying densities in VANETs. We have evaluated the protocol in different road density scenarios and its performance has been proved in comparison to two other recent protocols of the art.