Comparing different GPS solutions for the Cooperative Awareness Messages Distribution.

Cooperative awareness messages (CAM) are a family standardized by ETSI to distribute relevant information to all the nodes in vehicle coverage, such as time, position, speed, direction, etc. The parameters needed to trigger these messages, as well as the information contained in the messages themself, can be obtained with a GPS receiver. Our paper aims to increase the efficiency of CAM generation and dissemination in a less-is-more style, considering different GPS devices of different qualities. We achieve this goal by comparing the effectiveness of two GPS, belonging to two different market segments, with an order of magnitude of a price gap between them. We compared them in two different scenarios, similar in layout and traffic behavior but different in terms of GPS coverage conditions. Field trials with DSRC IEEE 802.11p have been carried out in an urban environment where different GPS coverage conditions widely emerge, with the possible presence of large buildings that emphasize all the GPS challenges: reducing the number of visible GPS satellites, introducing multi-path effects, fading, and reducing the signal strength. While having some notable differences in the two scenarios, we found the triggers' distributions to be similar between both the receiver devices. Instead, the average time between consecutive awareness messages has been found to be significantly lower in the case of bad coverage compared with the open-sky scenario, something that may reflect the presence of unwanted messages triggered by the GPS errors rather than the real vehicle motion. Therefore, we designed a new CAM generation algorithm that also considers the GPS error to reduce the messages transmitted on the radio medium.