A Study of Congestion Control Scheme in Vehicle to Vehicle Safety Communications

In vehicular safety communication services, every vehicle periodically broadcasts basic safety messages to inform neighbor vehicles of host vehicle information. However, this can cause network congestion in a region that is crowded with vehicles resulting in a reduction in the message delivery ratio and an increase in the end-to-end delay. Such congestion results in destabilization of a vehicular safety service system. To mitigate the congestion problem, this dissertation introduces the HYbrid Congestion CONtrol (HYCCON) scheme combined with transmission power control, data rate control, and time slot based transmission control algorithms. The HYCCON is a scheme using the congestion level estimation of the entire network. The performance of this HYCCON is evaluated using the Qualnet network simulator. The HYCCON mitigates network congestion in heavy traffic cases and enhances network capacity in light traffic cases. Thus, the simulation results show that the HYCCON has the good performance that the packet error rate is below 10 %, and the entire network load level is maintained below 60???70 %. Moreover, field operational tests for vehicular safety communication applications can consume a lot of time and effort in real vehicular environments. For this reason, performance evaluation for systems under development is dominantly performed through simulation experiments, rather than field operational tests. This dissertation presents V2X Real-time Emulation Framework (V2XREF), where network and traffic simulators are coupled together in setting up a V2X communication test-bed. V2XREF is a federated emulator of two independent simulators running over multiple networked computers. A distinctive feature of V2XREF is to handle vehicular safety communication messages in real packets. It can verify the performance and operation of vehicular safety communication applications without field operational tests. It can also be used as a development platform for vehicular safety communication applications.Docto