在車載網路 (Vehicular Ad Hoc Network, VANET) 中，緊急訊息 (Emergency Message) 能夠透過廣播的運用，使得道路上的駕駛人提前得知附近的危險區域，進而改道避免前往該區域。然而，緊急訊息需要即時 (Real-Time) 地讓周圍道路使用者知道，因此，該如何選擇最佳的中繼車輛 (Relay Vehicle)，讓訊息能夠快速地通知駕駛人，且花費最少的網路資源成了一個值得探討的問題。於是，本碩士論文設計了一種挑選中繼車輛的演算法，其根據單跳 (One-Hop) 鄰居與訊息源車輛 (Source Vehicle) 間的距離進行篩選，再將過濾出的候選車輛進行詳細的分類，最後，依照其方向性與區域性挑選出中繼車輛。而為了防止選出的中繼車輛數量不符合當前道路的密度所需，本碩士論文還提出了一種根據兩跳 (Two-Hop) 內車輛間的空間關係以進一步的挑選中繼車輛。透過模擬的方式，本碩士論文所提出的方法與相關的文獻進行比較，其不僅在覆蓋率 (Coverage Ratio) 方面有所提升，在相同接收程度下的重複廣播次數 (Retransmission Times) 和重複封包接收次數 (Duplicate Packets) 都能有所下降。

In the vehicular ad hoc network (VANET), the emergency message can be used through broadcasting so that drivers on the road can know the nearby dangerous areas in advance, then diverting to avoid going to these areas. However, emergency messages need to be known by surrounding users in real-time manner. Therefore, how to choose the best relay vehicle so that the message can be quickly forwarded to the driver and consume the least amount of network resources becomes an issue worth discussing. Towards this goal, this thesis will design an algorithm for selecting relay vehicles through screening via the distance between the one-hop neighbors and the source vehicle with detailed analysis on the filtered candidate vehicles. Accordingly, relay vehicles are selected based on their directionality and regionality. In order to prevent the number of selected relay vehicles from mismatching the current road density requirement, this thesis also incorporate the spatial relationship between two-hop vehicles to our selection of relay vehicles. Via simulations, we successfully demonstrate that our proposed method not only improves the coverage ratio but also improves the number of retransmission times and the number of times of duplicate packets as compared to the closely related methods in the literature.

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