Marc Torrent-Moreno

Vehicle-to-Vehicle Communication: Fair Transmit Power Control for Safety-Critical Information

Direct radio-based vehicle-to-vehicle communication can help prevent accidents by providing accurate and up-to-date local status and hazard information to the driver. In this paper, we assume that two types of messages are used for traffic safety-related communication: 1) Periodic messages (ldquobeaconsrdquo) that are sent by all vehicles to inform their neighbors about their current status (i.e., position) and 2) event-driven messages that are sent whenever a hazard has been detected. In IEEE 802.11 distributed-coordination-function-based vehicular networks, interferences and packet collisions can lead to the failure of the reception of safety-critical information, in particular when the beaconing load leads to an almost-saturated channel, as it could easily happen in many critical vehicular traffic conditions. In this paper, we demonstrate the importance of transmit power control to avoid saturated channel conditions and ensure the best use of the channel for safety-related purposes. We propose a distributed transmit power control method based on a strict fairness criterion, i.e., distributed fair power adjustment for vehicular environments (D-FPAV), to control the load of periodic messages on the channel. The benefits are twofold: 1) The bandwidth is made available for higher priority data like dissemination of warnings, and 2) beacons from different vehicles are treated with ldquoequal rights,rdquo and therefore, the best possible reception under the available bandwidth constraints is ensured. We formally prove the fairness of the proposed approach. Then, we make use of the ns-2 simulator that was significantly enhanced by realistic highway mobility patterns, improved radio propagation, receiver models, and the IEEE 802.11p specifications to show the beneficial impact of D-FPAV for safety-related communications. We finally put forward a method, i.e., emergency message dissemination for vehicular environments (EMDV), for fast and effective multihop information dissemination of event-driven messages and show that EMDV benefits of the beaconing load control provided by D-FPAV with respect to both probability of reception and latency.

Broadcast reception rates and effects of priority access in 802.11-based vehicular ad-hoc networks

One key usage of VANET is to support vehicle safety applications. This use case is characterized by the prominence of broadcasts in scaled settings. In this context, we try to answer the following questions: i) what is the probability of reception of a broadcast message by another car depending on its distance to the sender, ii) how to give priority access and an improved reception rate for important warnings, e.g., sent out in an emergency situation, and iii) how are the above two results affected by signal strength fluctuations caused by radio channel fading? We quantify via simulation the probability of reception for the two-ray-ground propagation model as well as for the Nakagami distribution in saturated environments. By making use of some IEEE 802.11e EDCA mechanisms for priority access, we do not only quantify how channel access times can be reduced but also demonstrate how improved reception rates can be achieved. Our results show that the mechanisms for priority access are successful under the two-way-ground model. However, with a non-deterministic radio propagation model like Nakagamis distribution the benefit is still obvious but the general level of probability of reception is much smaller compared to two-ray-ground model. The results indicate that -- particularly for safety-critical and sensor network type of applications -- the proper design of repetition or multi-hop retransmission strategies represents an important aspect of future work for robustness and network stability of vehicular ad hoc networks.

Overhaul of ieee 802.11 modeling and simulation in ns-2

NS-2, with its IEEE 802.11 support, is a widely utilized simulation tool for wireless communications researchers. However, the current NS-2 distribution code has some significant shortcomings both in the overall architecture and the modeling details of the IEEE 802.11 MAC and PHY modules. This paper presents a completely revised architecture and design for these two modules. The resulting PHY is a full featured generic module able to support any single channel frame-based communications (i.e. it is also able to support non-IEEE 802.11 based MAC). The key features include cumulative SINR computation, preamble and PLCP header processing and capture, and frame body capture. The MAC accurately models the basic IEEE 802.11 CSMA/CA mechanism, as required for credible simulation studies. The newly designed MAC models transmission and reception coordination, backoff management and channel state monitoring in a structured and modular manner. In turn, the contributions of this paper make extending the MAC for protocol researches much easier and provide for a significantly higher level of simulation accuracy.

Fair sharing of bandwidth in VANETs

We address the challenge of how to share the limited wireless channel capacity for the exchange of safety-related information in a fully deployed vehicular ad hoc network (VANET). In particular, we study the situation that arises when the number of nodes sending periodic safety messages is too high in a specific area. In order to achieve a good performance of safety-related protocols, we propose to limit the load sent to the channel using a strict fairness criterion among the nodes. A formal definition of this problem is presented in terms of a max-min optimization problem with an extra condition on per-node maximality. Furthermore, we propose FPAV, a power control algorithm which finds the optimum transmission range of every node, and formally prove its validity under idealistic conditions. Simulations are performed to visualize the result of FPAV in a couple of road situations. Finally, we discuss the issues that must be taken into account when implementing FPAV.

Inter-vehicle communications: assessing information dissemination under safety constraints

The main goal of inter-vehicle communication technologies is to provide each vehicle with the required information about its surrounding in order to assist the driver avoiding potential dangers. The required information level, or awareness, can be achieved by the exchange of periodic status messages (beacons) among neighboring vehicles together with the quick dissemination of information about potential hazards. In previous work, we proposed an algorithm (D-FPAV) to control the beaconing load on the medium by adjusting the transmission power in a fair and distributed fashion. In this paper, we adapt a promising position-based message forwarding strategy in order to disseminate time-critical safety information. Moreover, we evaluate its performance when making use of D-FPAV, i.e., limiting the beaconing load on the medium, in a realistic highway environment with probabilistic radio propagation characteristics

Effects of a realistic channel model on packet forwarding in vehicular ad hoc networks

The discrepancy between real-world radio channel behavior and its standard modeling in simulations (unit disk graph) is a major reason for protocols to perform differently - often worse - than predicted when deployed in a real-world setup. As researchers having to deal with real ad hoc networks are aware of, assuming a fixed border for a nodes communication range might not only lead to inaccurate results but also to a wrong judgment on the comparison between different protocols. We have set up a simulation study to investigate the effects of realistic channel characteristics on packet forwarding strategies for vehicular ad hoc networks. The contributions of this paper are threefold: i) we provide a performance evaluation of various routing/forwarding strategies under the realistic non-deterministic Nakagami radio propagation model and compare the results with the ones obtained using the standard two-ray-ground model. Validated German highway movement patterns are used to model node mobility. ii) We demonstrate that realistic channel conditions present an opportunity and not only a drawback for some forwarding strategies. More specifically, we show that for contention-based forwarding (CBF) techniques, realistic channel characteristics provide a positive impact in terms of an increased average hop distance. iii) We provide an analytical derivation of the expected hop distance for CBF that provides a basis to optimally adjust CBF parameters

Simulation platform for inter-vehicle communications and analysis of periodic information exchange

Due to the large amount of nodes that could comprise a vehicular network, simulation is a valuable tool used for the design and analysis of inter-vehicle communication protocols. In this paper we first describe all the components of a simulation framework that has ns-2 as core building block. Our main contributions are an accurate implementation of the MAC and PHY modules adjusted to vehicular environments together with a well defined set of metrics to evaluate communication protocols for vehicular networks. Second, we make use of the simulation framework to perform a detailed analysis of periodic exchange of broadcast messages when adjusting transmission power and packet generation rate for different propagation models

A simulation study on the performance of hierarchical mobile IPv6

We performed a simulative evaluation of Hierarchical MIPv6 in comparison with standard MIPv6 using the network simulator ns-2 for a ‘hot spot deployment’ scenario. The simulation scenario comprises four access routers and up to 50 mobile nodes that move randomly and communicate in accordance with the IEEE 802.11 wireless LAN standard. The study provides quantitative results of the improvements provided by HMIPv6 with respect to handoff latency, packet loss, signaling load and bandwidth per station. The simulation environment allowed us also to investigate the behavior of the protocol in extreme cases, e.g., under channel saturation conditions, and considering different traffic sources: CBR, Video, VoIP and TCP.

Adjusting transmission power and packet generation rate of periodic status information messages in VANETs

In Vehicular Ad Hoc Networks (VANETs), each node can benefit of the periodic transmission of one-hop broadcast messages containing status information (e.g., position, speed) by all other nodes in a common wireless channel. Thus, safety applications can make use of collected information in order to assist drivers and avoid dangerous road situations, detecting them before they become critical. Clearly, the semantic requirements of safety applications necessary to ‘achieve safety’ must be mapped into communication protocols’ configuration parameters. In this study we extend the work in [1] in order to provide an answer to two essential questions required by anyone intending to build an automotive safety system based on wireless communications: i) how many status information updates should be sent every second to the wireless medium?, and ii) which transmission power should be used?. Due to technology limitations (see [2]), the existing tradeoffs need to be appropriately balanced in order to find the optimal point of operation. While higher packet generation rates can increase the information accuracy with frequent updates, an uncontrolled strategy can lead to a saturated medium with a high rate of message collisions. Likewise, although a message sent with higher transmission power can reach further distances, it also increases the level of interferences on other transmissions. Additionally, a higher transmission power could increase the robustness of a specific message transmission. Indeed, in real mobile environments the attenuation of the signal amplitude can vary over time and in different vehicular environments due to radio propagation phenomena. Our purpose is to first understand the causes of reception success and failure of one-hop broadcast messages in realistic vehicular scenarios. For this reason, we classify the packet arrival states in six disjoint categories with respect to the signal and interference power level and study them under different vehicular traffic, channel load, and radio wave propagation conditions. Second, we propose a set of performance metrics together with our methodology to complete an appropriate framework that, once safety requirements are ‘well defined’, can assist in the process of mapping applications’ semantic requirements into optimal configuration values of the communication system.

Accurate knowledge of radio channel and network conditions — When does it matter?

One of the main motivations of wireless communication between vehicles is to establish a mutual awareness among vehicles. Such awareness is achieved through the periodic exchange of broadcast messages with information about the current position, heading or speed of a vehicle. The probability that neighboring vehicles receive such messages successfully depends on the current radio channel and network conditions. However, those conditions are not constant but varying due to the wide range of propagation environments, the number of vehicles in the surroundings and the transmission behavior of each vehicle. In order to cope with the current conditions and to achieve a desired communication reliability, communication protocols will have to be dynamically adjusted, e.g. by adjusting transmit power and transmit rate. The questions are therefore: how accurate do vehicles need to know the conditions and in which situations do they need to know them? This paper gives clarifications to those questions by means of a simulation-based sensitivity analysis, and in addition, surveys different methods of obtaining the required knowledge about radio channel and network conditions.