Experimental analysis of multi-hop vehicle node CCA threshold selection for EWM transmission

Emergency warning message (EWM) transmission insurance is one of the most interesting topics for Vehicular Cyber-physical System (V-CPS) application. A relatively low transmission requirement should improve EWM's real-time delivery, and however, decrease reliability of packet delivery by its corresponding burst converge-cast, especially under the multi-hop condition. In this paper, a clear channel assessment (CCA) threshold selection method is proposed to ensure both real-time and reliability for EWM transmission. A series of experiments under several scenarios on the sensor network testbed evaluate its effectiveness. The experimental results show that the threshold adjusting method has a positive effect on the delivery rate. With an appropriately diminishing clear channel assessment threshold related to the packet hops, the global dissemination successful rate can be up to 73.5%, and more than 90% of the 1-hop nodes can receive the correct packet. These findings provide a practicable method to improve the dissemination performance.