  In this paper, we consider safety message transmission in a dense vehicular
network. With increasing vehicular network density, the collision rate
increases when multiple vehicles transmit safety messages simultaneously. To
address this issue, we propose a request-transmission split time division
multiple access (TDMA) scheme, referred to as RTS-TDMA. In our scheme, we
divide a frame into three phases, i.e., a contention access phase, a broadcast
feedback phase, and a contention-free transmission phase. Each vehicle selects
a repetition rate according to a given probability distribution and repeats the
transmission of its request packet to improve the reliability of the request.
In addition, a roadside unit acts as the coordinator and uses a successive
interference cancellation technique to resolve request collisions. RTS-TDMA
also reduces the request time percentage by containing only the vehicle
identity in each request packet. Both theoretical analysis and numerical
results verify that the RTS-TDMA scheme can provide higher throughput than the
coded slotted ALOHA scheme.
