合作式碰撞預防（Cooperative Collision Avoidance, CCA）是一項智慧型運輸系統（Intelligent Transportation System, ITS）裡的應用發展，是用來增加行車安全並減少事故的發生，此應用是一旦只要有緊急事故發生在車輛的前面，車輛將會週期性的廣播警告信息去通知其它鄰近的車輛，立即做停止的動作來避免碰撞事故發生。為了改善合作式碰撞預防的效能，在車載隨意網路（Vehicular Ad Hoc Networks, VANETs）的許多先前研究，主要是會去設計媒介存取控制（Medium Access Control, MAC）協定去縮小警告信息的傳送延遲，然而，大部分在先前的設計是假定系統裡的所有車輛都裝載有CCA系統，而在本論文當中，我們是以一個混合的環境作為考量，在系統裡會有一些車輛沒有裝載CCA系統，這個情況也許會發生在ITS廣泛部署之前的最初階段，或者是在中間階段，我們提出了兩個方法是用來改善在一個混合環境的CCA的效能，在最後的模擬結果證明了它們的有效性。

Cooperative Collision Avoidance (CCA) is one of Intelligent Transportation System (ITS) applications developed for enhancing vehicle safety. Once an emergency event occurs in front of a vehicle, the vehicle will periodically broadcast warning messages instructing other vehicles to slow down for avoiding collisions. In order to improve CCA performance, several previous works have been focused on designing Medium Access Control (MAC) protocols to minimize transmission delay of warning messages in Vehicular Ad Hoc Networks (VANETs). However, most previous designs are evaluated under the assumption that all vehicles in the system are equipped with CCA. In this paper a mixed environment is instead considered, where some vehicles may not be CCA capable. This situation may happen during initial or intermediate phases before the widespread deployment of ITS. Two schemes are proposed to improve the CCA performance in a mixed environment, and extensive simulations are performed to examine their effectiveness.

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