Mitigating Broadcast Storm Problem in VANET when Parked Cars being awaken as Relays

In the urban environment, buildings, moving obstacles usually attenuate signals of safety messages, so that the connectivity and the reliability are reduced, which is known as Obstacle Shadowing problem. A typical solution to address this problem is to deploy Road-Side Units (RSUs), making them relay the Basic Safety Message (BSM) from driving vehicles. However, due to the high cost of deploying RSUs, the large numbers of parked cars in urban areas can be temporally used as RSUs for the financial aspect. Unfortunately, the conventional IEEE 802.11p protocol suffers from a so-called Broadcast Storm problem, where a high rate of contentions and packet collisions occur. In this paper, we show how even a small number of parked cars as relay could cause over 65% loss in Packet Reception Ratio (PRR). Therefore, some broadcast mechanisms are proposed to solve this problem. However, in this paper, we show the limitations of the conventional schemes, and we proposed revised and new schemes for selecting parked cars as relays/RSUs according to their geographic information and the surrounding traffic condition. The benefits are the great reduction in the deployment of RSUs, and significant improvement in the connectivity (up to 60%) and the reliability (up to 70%).