Safety message dissemination in NLOS environments of intersection using TV white space

Safety message dissemination is a key mechanism for preventing intersection collisions. Normally, the dissemination is based on vehicular communications, of which the de-facto standard is Dedicated Short Range Communications (DSRC). However, due to DSRC high operation frequency, a signal suffers from serious attenuation when being propagated in Non Light-Of-Sight (NLOS) conditions. Previous works have leveraged the use of relay units (e.g., repeaters installed on traffic lights) that enabled a safety message to detour large obstacles. However, one cannot guarantee that vehicles find the relay units at every intersection. To solve this problem, in this paper we propose a novel scheme that enables reliable dissemination in NLOS conditions by exploiting a TVWS band. Specifically, the proposed scheme leverages the excellent propagation characteristics of a TVWS signal even without relays. To increase the benefit of a TVWS band, the proposed scheme employs an efficient retransmission protocol, the Repetitive Retransmission via Spatial Diversity (RRSD). The above method ensures the safe message dissemination even in very harsh NLOS conditions. Moreover, the proposed scheme reduces network congestion via a cluster approach that uses both DSRC and TVWS bands. To the best of our knowledge, this is the first attempt to propose a reliable dissemination scheme that supports intersection safety services without special relaying units in NLOS conditions. Simulation studies show that the proposed scheme outperforms the previous scheme in the delivery ratio of a safety message by 25%.