RTOB: A TDMA-based MAC protocol to achieve high reliability of one-hop broadcast in VANET

Vehicular Ad Hoc Network (VANET) is considered promising for ubiquitous communication on roadways and one-hop broadcast plays a leading basic role in VANET. Thus, there is a strong need of high reliability related to packet transmission and reception especially when VANET is used for life-safety applications. Though IEEE 802.11p has been defined as an international standard for VANET, IEEE 802.11p has in practice some limitations in terms of reliability. Therefore, a new MAC protocol named Mobile Slotted Aloha (MS-Aloha) has been proposed and developed by ISMB in Italy to achieve higher reliability and MS-Aloha has become recommended by ETSI after evaluation. However, because of inefficient use of radio channels, the reliability of MS-Aloha is still not satisfying especially under very congested traffic conditions in urban area. In this paper, we propose a new MAC protocol named Reliable TDMA-based One-hop Broadcast (RTOB). RTOB is based on MS-Aloha, inheriting all its advantages, and RTOB can achieve much higher reliability by making efficient use of radio channels. We also propose a novel metric named Cover Ratio (CR) which is more appropriate than the conventional Packet Delivery Ratio (PDR) to evaluate reliability from the viewpoint of life-safety applications. This paper describes in detail the main principles and the techniques of RTOB and demonstrates quantitatively its sufficiently high reliability in terms of both CR and PDR by clarifying efficient use of radio channels even under very congested traffic conditions.