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Abstract-Improving the safety of drivers and passengers by wirelessly exchanging information between vehicles represents a major driving force for the design of vehicular ad hoc networks. In a heavy loaded 802.11-based network, however, safety-related packets might collide frequently and cannot be decoded by a receiver, thus they might not be effective in increasing the safety level on the roads. In this paper, we propose to use transmit power control in order to reduce packet collisions, while taking into account the major design goal of vehicular ad hoc networks, i.e. increasing safety. While previous work has addressed the issue of power control primarily for optimizing network capacity and/or connectivity, the optimization criterion for improving safety has to be built upon the concept of fairness: a higher transmit power of a sender should not be selected at the expense of preventing other vehicles to send/receive their required amount of safety information. In this paper, we propose a fully distributed and localized algorithm called D-FPAV (Distributed Fair Power Adjustment for Vehicular networks) for adaptive transmit power adjustment which is formally proven to achieve max-min fairness. Furthermore, we investigate the effectiveness and robustness of D-FPAV through extensive simulations based on a realistic highway scenario and different radio propagation models. I. INTRODUCTIONThe number of fatalities on public roads is a main concern for both public opinion and country's governments. Several initiatives [1] have been started with the objective of significantly decreasing both the number of accidents and their resulting damage. These efforts do not only consider a better consciousness of drivers and an adequate road system, but also the use of new technologies capable of assisting drivers in order to improve safety conditions. Among the new technologies considered, vehicular ad hoc networks (VANETs) play an important role, since the use of wireless communications offer the beneficial capability of directly exchanging safety-related information between vehicles. Various efforts (projects such as VII [2], C2CCC [3], InternetITS [4], etc., and standard bodies such as IEEE [5]) are currently developing a technology that combines 802.11-based wireless communications with on-board sensors (e.g., GPS, speedometers) in order to improve the driver's awareness of the surrounding environment, making available information which he/she or other on-board sensors (e.g., radar) might not be able to 'see'.The exchange of safety-related information comes into two flavors: i) by detecting potentially dangerous situations through

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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 of 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.

Mobile IP, the current IETF proposal for IP mobility support, represents a key element for future All-IP wireless networks to provide service continuity while on the move within a multi-access environment. We conducted a performance evaluation of Mobile IPv6 and its proposed enhancements, i.e., Fast Handovers for Mobile IPv6, Hierarchical Mobile IPv6 and our proposed combination of them, using the network simulator ns-2 for the case of a 'hot spot' deployment scenario. The simulation scenario comprises four access routers and up to 50 mobile nodes that communicate in accordance with the IEEE 802.11 wireless LAN standard. The study provides quantitative results of the performance improvements obtained by the proposed enhancements as observed by a single mobile user with respect to handoff latency, packet loss rate and achieved bandwidth per station. As a complementary part of the study, the signaling load costs associated with the performance improvements provided by the enhancements has been analyzed. 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, VoIP, Video and TCP transfers. While some simulation results corroborate the intention of the protocols specifications, other results give insights not easily gained without performing simulations. This study provides a deep understanding of the overall performance of the various protocols and supports the design process of a Mobile IPv6-based network when a decision of whether it is appropriate to implement any of the proposed Mobile IPv6 enhancements has to be made.

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