Hector Agustin Cozzetti

Mobile Slotted Aloha for Vanets

Vanet literature is mainly focused on CSMA and CSMA-like access protocols. Actually a deep study on how a slotted approach could be adopted in Vanet is missing. This is mainly due to the lack of a distributed MAC able to guarantee a scalable resource reservation without involving a central poller. In this paper we propose a novel protocol, MS-Aloha, which is based on recent RR-Aloha extensions, in order to overcome its most critical limitations. The protocol is studied theoretically and validated by simulations. Even if an extensive analysis is far from being achieved, some relevant results are discussed as important enabling steps. Index Terms—Vanet, Slotted-ALOHA, Distributed Protocol, Resource Optimisation, Collision Avoidance I. INTRODUCTION

Augmenting Vehicle-to-Roadside connectivity in multi-channel vehicular Ad Hoc Networks

Abstract Vehicle-to-Roadside (V2R) wireless communication is a cornerstone for providing a wide plethora of intelligent transportation system (ITS) applications in the near future. Initial investment costs could discourage the deployment of a ubiquitous roadside infrastructure to support on-the-road networks; this would imply discontinuous coverage and short-lived connectivity. The purpose of this paper is to design techniques that make the best of sparse road-side unit (RSU) placement by supporting the spreading of network initialization advertisements from RSUs, when considering the multichannel features of the recently published IEEE 802.11p/IEEE 1609.4 standards for wireless access in vehicular environment (WAVE). The proposed techniques leverage time, space and channel diversity to improve efficiency and robustness of the network advertisement procedure in a urban scenario where obstructions to signal propagation due to buildings and traffic jam could hinder successful message spreading. Simulation under different RSU density, vehicular networking technology penetration rate, data rate, and packet size, aims at assessing effectiveness and efficiency of the proposed solutions.

Signal Shadowing in Simulation of Urban Vehicular Communications

Vehicular communications have been demonstrated to strongly depend on the general setting of the scenario: a heavy impact on bit-error-rate and packet-error-rate may come from physical layer phenomena which are typically simulated on software platforms different from those used for protocol validation. As a result, current protocol simulations do not take into account environmental constraints: some effects are considered by statistical models (such as Nakagami model), other ones – such as shadowing – remain uncovered phenomena. This paper proposes a simple but effective model for the shadowing raised by large buildings and blocks in urban areas: we demonstrate that the proposed solution improves the realism of simulations and significantly improves the results.

RR-Aloha+: A slotted and distributed MAC protocol for vehicular communications

The state of the art of vehicular communications includes extensive investigations on distributed-contention approaches (mostly focused on WiFi CSMA/CA optimal and/or custom configurations), while scalable solutions for a distributed but contention-free wireless architecture remain unexplored. In this paper, starting from an existing protocol called RR-Aloha, we propose some novel extensions to it, in order to solve some technical open issues, improve its scalability and overcome some criticalities concerning the dynamic setting of vehicular mobility. The study is based both on conceptual tests and on PC simulations on NS-2 (Network Simulator). The proposed scenarios are meant to validate the extensions and confirm the feasibility; the preliminary results show that the enhanced RR-Aloha+ 1 may actually exert a potentially relevant role in the vehicular communications, with a deterministic, reservation-based, QoS-capable and distributed access channel.

Urban VANETs and hidden terminals: Evaluation through a realistic urban grid propagation model

Despite the large number of active field-trials, network simulations still play a vital role in the development and validation of new protocols for vehicular networks. This role still motivates the improvement of simulation tools, especially in terms of realism, so to yield accurate results. One of the major gaps which still needs to be filled is urban propagation: an adequate modeling of obstructions is still missing and has come to light as a critical feature, potentially leading to arguable results. Here a new urban model (called Reliable Urban Grid - RUG) is proposed as an extension of an already validated propagation model of urban corners to more complex grid topologies. Achieved results show the higher impact of collisions by hidden terminals on standard contention-based medium access control protocols when compared to slotted solutions.

GNSS Synchronization in Vanets

The literature about GNSS in vehicular networks is usually restricted to the mutual interaction between positioning and routing (respectively with georouting and services of GNSS assistance). However, despite the apparent contradiction, also asynchronous Vehicular Ad-hoc networks (Vanets) built on the top of IEEE 802.11p require an absolute synchronization: this could be easily provided by the time information embedded in GNSS systems. This position paper analyses the issue of synchronization in Vanet both for the emerging asynchronous standard and for alternative synchronous solutions. The technical discussion adds also new elements useful for the comparative analysis among protocols for Vanets.

Comparative analysis of IEEE 802.11p and MS-Aloha in Vanet scenarios

This paper presents a comparative analysis, set in Vanet scenarios and based on simulations, between the well-known IEEE 802.11p standard and an efficient slotted protocol based on RR-Aloha (MS-Aloha). Notably the chosen slotted protocol is completely distributed and based on reservation, and has been specifically designed to face issues typical of mobility, while preserving quality of service guarantee. The simulations are carried out using the same settings for CSMA/CA and MS-Aloha: separate analyses are meant to discuss the specific characteristics of each of them while a common final discussion is aimed at highlighting the weaknesses and points of strength of the proposed MAC protocols.

Evaluation of Time-Space Efficiency in CSMA/CA and Slotted Vanets

Recently slotted protocols have been demonstrated to work also in a distributed way and in a vehicular scenario. In particular one of them, MS-Aloha, has shown interesting scalability features. Despite a comparative analysis between CSMA/CA (IEEE 802.11p) and MS-Aloha has already been proposed in a simplified topology, a deep analysis on the efficiency of the two protocols has not been achieved: in this paper such analysis is split into the two separate domains of space and time. Results confirm that the slotted approach is more efficient and deterministic thanks to a joint time-space coordination driven by spatial slot assignment and reuse.

Overhauling ns-2 PHY/MAC simulations for IEEE 802.11p/WAVE vehicular networks

The IEEE 802.11p/WAVE family of standards has been designed to provide multi-channel communications for vehicular use. Despite the increasing interest on vehicular networking, further efforts are still required to improve realism of simulations, in order to account for the latest standards and for physical-related phenomena. In this paper, we aim at overhauling the physical and MAC layers of ns-2 to include the last recommendations on frequency allocation, the effects of channel impairments (fading, obstructions, in-channel and adjacent channel interference), an enhanced receiver design, and the WAVE multi-channel related effects.

Advances in the analysis of urban VANETs: Scalable integration of RADII in a network simulator

During the last years, network simulators have reached a certain maturity and have permitted the validation of protocols in several scenarios, also involving a large number of nodes, and the prediction of possible misbehaviors. This holds particularly for vehicular ad hoc networks (VANETs), which are being extensively validated in order to identify possible threats to the safety services which they are aimed at.