An Accurate Positioning Geocast Protocol for Safety Message Dissemination in VANETs.

Safety applications in vehicular ad hoc networks (VANETs) require vehicles to transmit frequent updates to neighboring nodes about their current position, speed and heading. These messages can also be called beacons, and they are essential to satisfy an application's positioning accuracy requirements. However, in urban scenarios where vehicular density can be high, beacons and other safety messages may lead to a network congestion due to the limited bandwidth available. When this event occurs, the operation of safety applications is hindered, thus putting the life of drivers, passengers and pedestrians at risk. Several works have been proposed to alleviate network congestion, however most do not account for the loss of positioning accuracy that the traditional strategies may generate. In this paper, the Accurate Positioning Geocast Protocol (APGP) is proposed as a novel solution to control network congestion and positioning error. APGP employs a spatial division technique to create geocast groups, where vehicles receive updates at different times according to their position and a prediction error measured locally. The results of simulations conducted in a real-world environment demonstrated that the protocol achieved a reduction in delivery delay of up to four times, as well as improvements in packet delivery ratio of around 43% and 23% when compared to other alternatives. Even in the most unfavorable scenario, channel busy ratio remained below 60%, exhibiting a reduction of 47% and 15% in comparison to other options. In contrast to other protocols, APGP successfully achieved the prescribed levels of positioning accuracy for vehicles at shorter distances under all conditions.