車載隨意網路中緊急訊息之自適性廣播調控機制

Abstract

因各國交通事故頻繁，智慧型運輸系統 (Intelligent Transport Systems, ITS) 的研究日益受到重視，本研究就是針對車載隨意網路 (Vehicular Ad Hoc Networks, VANET)傳遞緊急訊息(emergency message)之議題，來解決訊息無法成功且即時地傳送到危險區域 (Warning Zone) 周圍街道上之車輛的問題，我們提出一套緊急訊息之自適性廣播調控機制。本篇論文分為兩部分： (1) 訊息廣播頻率與傳送可靠度關係的推導：利用組合數學計算車輛可能的分布情形，推測出車輛無法成功接收訊息之機率，進而求出最少所需的重複傳送次數與傳送間隔時間，方可使傳送可靠度 (reliability) 達到目標值； (2) 訊息廣播頻率調控機制設計：讓事故發源車輛 (Source Vehicle) 利用竊聽 (overhearing) 機制推測危險區域之街道中車輛密度變化，求出當下最佳的訊息傳送頻率，並定期做更新。模擬結果顯示，自適性廣播調控機制之可靠度推測確實與模擬數據相當符合，並適用於車流量多變化的現實市區街道環境中，能讓危險區域中車輛即時接收緊急訊息之可靠度達到目標值，並有效地降低網路負載 (overhead)。

The problem considered in this thesis is how to broadcast the emergency message instantly and successfully to the streets of the hazard zone in Vehicular Ad Hoc Networks (VANET) environment. We propose an adaptive emergency message broadcast control mechanism to solve it. The proposed mechanism can be divided into two parts: (1) determining the relationship between the reliability and the message broadcast frequency; (2) adjusting message broadcast frequency based on traffic load. The simulation results show that our adaptive emergency message broadcast control mechanism can effectively reduce the network overhead under a given reliability constraint.