DOI: 10.14457/tu.the.2018.871
title: Receiver-based prioritization scheme for safety messages in high density vehicular ad hoc networks

description: One of the main goals of vehicular ad hoc networks (VANETs) is to increase road safety and traffic efficiency, by using information which is shared among vehicles in a wide range of Intelligent Transportation Systems (ITS) applications such as crash warning, sudden-brake warning, and lane-change warning. Safety messages are transmitted from each vehicle at a fixed rate such as ten messages per second. In dense roads with many lanes or with many vehicles close to each other, there could be many safety messages received by a vehicle. Each safety message must be verified by a time-consuming cryptographic operation before its information can be reliably utilized. This leads to a problem since the rate that messages are received can be much higher than the verification rate. This problem could be serious with densely occupied roads and when ITS applications require high transmission rate of safety messages. To solve this problem, some solutions have been proposed so that the safety messages are verified before other non-safety messages and, furthermore, safety messages from closer vehicles are given higher priority. The solutions can be categorized into two approaches. The first approach, called transmitter-based schemes, uses Enhanced Distributed Channel Access protocol from IEEE802.11 to give higher priority to the safety messages to access the wireless channel. The second approach, called receiver-based schemes, which actually complement the first approach, uses a priority scheduler at the receiving side, to give a higher priority for more important safety messages so that they can be verified first.In this thesis, we follow the receiver-based schemes. We present solutions to two problems: how to prioritize the most relevant safety messages at the receiving vehicle in dense traffic conditions, and how to increase the awareness about nearby vehicles. To solve the problem of too many messages to be verified, the general approach at the receiver is message prioritization. In the first part of our thesis, we propose the Relative-Time Zone (RTZ) scheme which relies on three prioritization mechanisms: i) filtering irrelevant safety messages based on positions and directions of neighboring vehicles (represented as quadrant of the transmitter) with respect to the receiver, ii) use of close proximity to prioritize clusters of vehicles from near to far, and iii) use of relative-time to identify dangerous vehicles. Our proposed method provides higher awareness of nearby vehicles and opposite-direction vehicles than the existing methods. We also consider human reaction time (the time to react to a sudden event such as the sudden braking of the vehicle in front). To accommodate for this reaction time, the proposed method gives the highest priority to messages sent from very near vehicles, independent of their speed or acceleration, since unexpected events can happen with them and hence the receiving vehicle must be fully aware of them. The area containing these vehicles is called the danger-zone. Since vehicles travel at varying speeds, the density of nearby vehicles is also varied. The simulation results show that RTZ achieves the highest awareness with on average 88% for vehicles in danger-zone of receiving vehicle. This is the result of high message verification rate for nearby vehicles (almost zero packet loss for vehicles in the danger-zone). The verification delay, RTZ achieves an average delay less than 40 ms for vehicles within 100 m distance and 100 ms delay for distant vehicles, while the inter-message delay is less than 700 ms across all distances.In the second part of the thesis, we improve RTZ to achieve higher awareness of nearby vehicles and allow the protocol to work in different road configurations. We enhance RTZ with history-based verification (HRTZ) based on the use of priority buffering mechanism. To increase awareness of neighboring vehicles in vicinity, real-time average velocity of neighboring vehicles in the range of communication is used to determine the range of danger-zone and other zones. The messages are ranked based on zone of transmitting vehicle, road configuration (with/without barrier) and transmitting vehicle location and direction, and relative-time. Higher ranked safety messages have higher probability to be verified. Only the most up-to-date message from each vehicle is kept in the buffer. As a result, each neighboring vehicle has only the most recent safety message in the buffer at any time. The simulation results shows that HRTZ achieves 2% to 8% higher awareness for nearby vehicles in the vicinity compare to RTZ. The delay of verified safety messages is 20 ms on average for vehicles within distance less than 50 m and for distant vehicles the delay is less than 100 ms on average while the inter-message delay is less than 600 ms across all distances. Hence, HRTZ is a good candidate for message prioritization scheme for cooperative driving safety applications.