Controlo de acesso ao meio em comunicações veiculares de tempo-real

Despite several preventive measures, the number of roadway accidents is still very high, being considered even a problem of public health by some entities. This thesis has as global purpose of contributing to the reduction of that number of accidents, and consequent fatalities, by using safety-related applications that use communication among vehicles. In particular, the primary goal is guaranteeing that communication between users in vehicular environments is done with appropriate time bounds to transfer safety-critical information. In detail, it is studied how to manage the scheduling of message’s transmissions (medium access control - MAC), in order to define precisely who will communicate and when is the appropriate instant. The preferable situation where a communication infrastructure is present with full coverage (RSUs) is also studied, from which medium access control is defined precisely, and vehicles (OBUs) become aware of medium utilization. Also, sporadic situations (e.g., absence of RSUs) are studied in which the communication network is “ad hoc” and solely formed by the current vehicles. It is used the recently WAVE / IEEE 802.11p standard, specific for vehicular communications, and it is proposed a TDMA based solution, with appropriate coordination between RSUs in order to effectively disseminate a critical safety event. It is taken into account two different ways of choosing the instant for the initial broadcast, and both cases are compared. In case there is no infrastructure available, methods are derived to minimize communication medium access collisions, and to maximize the available bandwidth. The results reflect the total end-to-end delay, and show that adequate times are attained, and meet with the requisites for the type of applications being considered. Also, enhancements are obtained when using the alternate choice for the initial broadcast instant.Apesar de diversas medidas preventivas, o número de acidentes rodoviários continua a ser muito elevado, sendo mesmo considerado uma questão de saúde pública por algumas entidades. Esta tese tem como objetivo geral contribuir para a redução desse número de acidentes, e consequentes fatalidades, através da utilização de aplicações de segurança que envolvem comunicação entre veículos. Em particular, o objetivo principal é garantir que a comunicação entre utentes, em ambientes veiculares, seja efetuada com limites temporais apropriados à transferência de informações críticas. De forma mais detalhada, é estudada a gestão do escalonamento das transmissões (controlo de acesso ao meio – MAC) que irá definir quem vai comunicar e quando o pode fazer. São estudadas situações (desejadas) onde há uma infra-estrutura de comunicações com cobertura integral (RSUs), a partir da qual se faz a coordenação do acesso ao meio pelos veículos (OBUs), e situações (esporádicas, por ausência de RSU) em que a rede de comunicação é “ad hoc” e apenas constituída pelos veículos presentes. Utiliza-se a recente norma WAVE / IEEE 802.11p, específica para comunicações veiculares, e propõe-se uma solução baseada em TDMA, com coordenação apropriada entre RSUs para disseminação efetiva de um evento crítico de segurança. A escolha do instante para o broadcast inicial do evento de segurança também é tida em conta, e são comparados dois casos distintos. No caso da ausência de infraestrutura, derivam-se métodos para minimizar colisões no acesso ao meio de comunicação, e maximizar a largura de banda disponível. Os resultados refletem o atraso total end-to-end, mostrando tempos apropriados para os requisitos das aplicações em causa, e evidenciando melhorias aquando da escolha alternativa para o instante do broadcast inicial.Programa Doutoral em Engenharia Eletrotécnic

V2X Content Distribution Based on Batched Network Coding with Distributed Scheduling

Content distribution is an application in intelligent transportation system to assist vehicles in acquiring information such as digital maps and entertainment materials. In this paper, we consider content distribution from a single roadside infrastructure unit to a group of vehicles passing by it. To combat the short connection time and the lossy channel quality, the downloaded contents need to be further shared among vehicles after the initial broadcasting phase. To this end, we propose a joint infrastructure-to-vehicle (I2V) and vehicle-to-vehicle (V2V) communication scheme based on batched sparse (BATS) coding to minimize the traffic overhead and reduce the total transmission delay. In the I2V phase, the roadside unit (RSU) encodes the original large-size file into a number of batches in a rateless manner, each containing a fixed number of coded packets, and sequentially broadcasts them during the I2V connection time. In the V2V phase, vehicles perform the network coded cooperative sharing by re-encoding the received packets. We propose a utility-based distributed algorithm to efficiently schedule the V2V cooperative transmissions, hence reducing the transmission delay. A closed-form expression for the expected rank distribution of the proposed content distribution scheme is derived, which is used to design the optimal BATS code. The performance of the proposed content distribution scheme is evaluated by extensive simulations that consider multi-lane road and realistic vehicular traffic settings, and shown to significantly outperform the existing content distribution protocols.Comment: 12 pages and 9 figure

IEEE 802.15.7-Compliant Ultra-low Latency Relaying VLC System for Safety-Critical ITS

The integration of Visible-Light Communications technology (VLC) in Intelligent Transportation Systems (ITS) is a very promising platform for a cost-effective implementation of revolutionary ITS and cooperative ITS protocols. In this paper, we propose an infrastructure-to-vehicle-to-vehicle (I2V2V) VLC system for ITS, implementing it through a regular LED traffic light serving as a transmitter and a digital Active Decodeand- Relay (ADR) stage for decoding and relaying the received information towards further incoming units. The proposed VLC system targets the challenging and important case of ultra-low latency ADR transmission of short packets, as this is needed for emerging applications of automatic braking, car platooning and other critical automatic and/or assisted driving applications. The experimental validation of the ADR VLC chain, as well as a thorough statistical analysis of errors distribution in the transmission, has been performed for short to medium distances, up to 50 meters. The performances of the designed system are evaluated by measuring the packet error rate (PER) and latency in the whole ADR transmission chain. Our analysis shows that our system attains ultra-low, sub-ms latencies at 99.9% confidence level for PER as high as 5 x 10^-3, yet granting a latency below 10 ms even for distances of 50 m. The demonstrated system prototype is compatible with IEEE 802.15.7 standard

Comunicações sem fios confiáveis para aplicações veiculares

Doutoramento em Engenharia ElectrotécnicaIn the last decades the number of vehicles travelling in European road has raised significantly. Unfortunately, this brought a very high number of road accidents and consequently various injuries and fatalities. Even after the introduction of passive safety systems, such as seat belts, airbags, and some active safety systems, such as electronic brake system (ABS) and electronic stabilization (ESP), the number of accidents is still too high. Approximately eight per cent of the fatal accidents occur in motorways, in the Portuguese case, the number of fatalities has remained constant in the first decade of the 21st century. The evolution of wireless communications, along with the north-American and European policies that reserve spectrum near the 5,9GHz band for safety applications in the vehicular environment, has lead to the development of several standards. Many of these applications are based on the possibility of using a wireless communication system to warn drivers and passengers of events occurring on the road that can put at risk their own safety. Some examples of safety applications are the hard-brake warning, the wrong-way warning and the accident warning. This work aims to contribute in defining a communication protocol that guarantees the timely dissemination of safety critical events, occurring in scenarios with a high number of vehicles or in the neighbourhood of so called motorway “blackspots”, to all vehicles in the zone of interest. To ensure information integrity and user trust, the proposed system is based on the motorway infrastructure, which will validate all events reported by the vehicles with the usage of several means, such as video surveillance or other sensors. The usage of motorway infrastructure that has full motorway coverage using fixed stations also known as road side units, allows to have a global vision of the interest zone, avoiding the problems associated to networks that depend solely on vehicle to vehicle communication, generally total ad-hoc networks. By using the infrastructure, it is possible to control medium access, avoiding possible badly intended intrusions and also avoiding the phenomenon known as alarm showers or broadcast storm that occur when all vehicles want to simultaneously access the medium to warn others of a safety event. The thesis presented in this document is that it is possible to guarantee in time information about safety events, using an architecture where the road side units are coordinated among themselves, and communicate with on board units (in vehicles) that dynamically register and deregister from the system. An exhaustive and systematic state of the art of safety applications and related research projects is done, followed by a study on the available wireless communications standards that are able to support them. The set of standards IEEE802.11p and ETSI-G5 was created for this purpose and is found to be the more adequate, but care is taken to define a scenario where WAVE enabled and non-enabled vehicles can coexist. The WAVE medium access control protocol suffers from collision problems that do not guarantee a bounded delay, therefore a new protocol (V-FTT) is proposed, based on the adaptation of the Flexible Time Triggered protocol to the vehicular field. A theoretical analysis of the V-FTT applied to WAVE and ETSI-G5 is done, including quantifying a real scenario based on the A5 motorway from Lisbon to Cascais, one of the busiest Portuguese motorways. We conclude the V-FTT protocol is feasible and guarantees a bounded delay.Nas últimas décadas tem-se assistido a um aumento do número de veículos a circular nas vias rodoviárias europeias, trazendo consigo um elevado número de acidentes e como consequência muitos feridos e vítimas mortais. Apesar da introdução de sistemas de segurança passivos, tais como cintos de segurança, airbags e de alguns sistemas de segurança activos, tais como o sistema electrónico de travagem (ABS) e o sistema electrónico de estabilidade (ESP), o número de acidentes continua a ser demasiado elevado. Aproximadamente oito por cento dos acidentes fatais na Europa ocorrem em auto-estradas, no caso Português, o número de vítimas mortais tem-se mantido constante ao longo da primeira década do século XXI. A evolução das comunicações sem fios, acompanhada de políticas europeias e norte-americanas no sentido de reservar frequências próximas dos 5,9GHz para aplicações de segurança no ambiente veicular, levou à especificação de várias normas. A maior parte destas aplicações baseiam-se na possibilidade de usar um sistema confiável de comunicação sem fios para alertar os condutores e passageiros de veículos para eventos ocorridos nas estradas que possam colocar em risco a sua segurança. Exemplos de aplicações de segurança crítica são o aviso de travagem brusca, o aviso de veículo em contra mão e o aviso de acidente na estrada. Este trabalho contribui para a definição de protocolos de comunicação capazes de garantir que a informação sobre eventos relacionados com situações de segurança crítica, que ocorram em cenários com um elevado número de veículos em zonas urbanas ou na vizinhança dos chamados “pontos negros” das auto-estradas, é disseminada com pontualidade por todos os veículos localizados na zona de interesse Por uma questão da integridade das comunicações e confiança dos condutores, o sistema proposto baseia-se na infra-estrutura do concessionário da auto-estrada, que validará os eventos reportados pelos veículos usando vários meios à sua disposição, como por exemplo sistemas de videovigilância e outros sensores. O uso de uma infra-estrutura de comunicações, que dispõe de cobertura integral a partir de estações fixas, permite uma visão global da zona coberta, evitando os problemas associados a redes baseadas apenas na comunicação entre veículos, que são em geral totalmente ad-hoc. O uso da infra-estrutura permite, entre outras vantagens, controlar o acesso ao meio, evitando simultaneamente intrusões de estranhos ao sistema e o fenómeno conhecido como “chuva de alarmes” desencadeado quando todos os veículos querem aceder simultaneamente ao meio para avisar os restantes da existência dum evento de segurança crítica. A tese apresentada neste documento defende que é possível garantir informação atempada sobre eventos que põem em risco a segurança dos veículos a partir de uma arquitectura de interligação entre as estações de comunicações fixas, coordenadas entre si, e unidades móveis (veículos) que se registam e se desligam dinamicamente do sistema. Nesta tese faz-se um levantamento exaustivo e sistemático das aplicações de segurança abordando projectos de investigação relacionados, estudam-se as tecnologias de comunicação sem fios disponíveis e a sua possibilidade de suportar aplicações de segurança rodoviária. Desta análise, conclui-se que a norma norte americana WAVE/IEEE802.11p e a europeia ETSI-G5, criadas especificamente para o efeito são as que mais se adequam à finalidade desejada. Considera-se que o cenário de utilização é evolutivo, podendo coexistirem veículos que não dispõem de sistemas de comunicação com outros que suportam a norma WAVE. Dado que o protocolo de acesso ao meio proposto pela norma WAVE não garante um acesso determinístico ao meio partilhado, propõe-se um novo protocolo, o Vehicular Flexible Time-Triggered protocol (VFTT). Faz-se a análise teórica da viabilidade do protocolo proposto para a norma WAVE e respectiva norma europeia (ETSI-G5). Quantifica-se o protocolo VFTT para um cenário real: a auto-estrada A5 Lisboa-Cascais, uma das autoestradas portuguesas mais movimentadas. Conclui-se que o protocolo é viável e garante um atraso restringido temporalmente

Detecting Non-Line of Sight to Prevent Accidents in Vehicular Ad hoc Networks

There are still many challenges in the field of VANETs that encouraged researchers to conduct further investigation in this field to meet these challenges. The issue pertaining to routing protocols such as delivering the warning messages to the vehicles facing Non-Line of Sight (NLOS) situations without causing the storm problem and channel contention, is regarded as a serious dilemma which is required to be tackled in VANET, especially in congested environments. This requires the designing of an efficient mechanism of routing protocol that can broadcast the warning messages from the emergency vehicles to the vehicles under NLOS, reducing the overhead and increasing the packet delivery ratio with a reduced time delay and channel utilisation. The main aim of this work is to develop the novel routing protocol for a high-density environment in VANET through utilisation of its high mobility features, aid of the sensors such as Global Positioning System (GPS) and Navigation System (NS). In this work, the cooperative approach has been used to develop the routing protocol called the Co-operative Volunteer Protocol (CVP), which uses volunteer vehicles to disseminate the warning message from the source to the target vehicle under NLOS issue; this also increases the packet delivery ratio, detection of NLOS and resolution of NLOS by delivering the warning message successfully to the vehicle under NLOS, thereby causing a direct impact on the reduction of collisions between vehicles in normal mode and emergency mode on the road near intersections or on highways. The cooperative approach adopted for warning message dissemination reduced the rebroadcast rate of messages, thereby decreasing significantly the storm issue and the channel contention. A novel architecture has been developed by utilising the concept of a Context-Aware System (CAS), which clarifies the OBU components and their interaction with each other in order to collect data and take the decisions based on the sensed circumstances. The proposed architecture has been divided into three main phases: sensing, processing and acting. The results obtained from the validation of the proposed CVP protocol using the simulator EstiNet under specific conditions and parameters showed that performance of the proposed protocol is better than that of the GRANT protocol with regard to several metrics such as packet delivery ratio, neighbourhood awareness, channel utilisation, overhead and latency. It is also successfully shown that the proposed CVP could detect the NLOS situation and solves it effectively and efficiently for both the intersection scenario in urban areas and the highway scenario