| 摘要: | 全球每年因車禍意外死傷人數始終居高不下，因此各大車廠及政府機關團體等努力尋找改善之方案；由於無線網路技術的進步，發展了車載網路的網路架構。 車載網路的特性是能在沒有基地台的協助下與其它車輛進行資料交換，並且能透過基地台與外界取得連絡，上述特性誕生了許多應用，例如車輛碰撞避免機制。目前許多文獻對於車輛碰撞避免機制提出解決方案，雖然改善了頻寬的使用率以及縮減訊息的傳遞延遲時間，卻未曾探討駕駛者的反應及煞車距離等議題。 本論文提出使用Warning Message以及Emergency Message，透過上述兩類訊息，Warning Message提供安全距離預警能力，Emergency Message則確認緊急煞車的產生。另外，本論文利用CTB-to-itself機制，因此可減少碰撞封包數量、避免Hidden node問題及縮減訊息發送前的等待時間。 論文中提出的EWCCA（Early Warning Scheme based on CCA）機制無需其它裝置取得車輛絕對位置，並具有判斷車流及訊息傳播方向能力。EWCCA的轉遞機制能解決廣播風暴問題，利用RSSI以及TOA之測距技術，將量測數據轉換成等待時間，避免所有接收端同時轉遞訊息，造成封包碰撞導致效能降低。在論文最後的模擬測試中，區分成EWCCA for RSSI以及EWCCA for TOA，並與Simple Flooding機制互相比較。透過實驗結果，加入Fading後的實驗數據，End-to-end delay time改善幅度為60%，所產生的Collision packets改善幅度為86%，接收失敗率改善幅度為56%，證明EWCCA在真實環境下，能提供較低延遲時間，較少碰撞封包數量以及較低接收失敗率，達到增加駕駛者反應時間之目地。 There are many people die or hurt because of car accident every year in the world. Therefore, automotive industry, government, and organization are trying to find a solution to improve it. The technique of wireless made grate progress, so the framework of Vehicle Ad-hoc Network (VANET) is developed. It can exchange data between vehicles without the help of base station or it can communicates with outside by base station for VANET. The characteristic produces many applications, such as the vehicular Cooperative Collision Avoidance (CCA). Many proposals provide some solution for CCA. Although they improve the bandwidth of usage and reduce the end to end delay time, they don’t to confer with driver reactive time and braking distance. This thesis proposes the using of Warning Message and Emergency Message; Warning Message provide the early warning ability, and Emergency Message confirm that the emergency brake occurred. This thesis utilizes CTB-to-itself scheme, hence it can reduce the number of collision packets, avoid Hidden node problem and reduce waiting time before sending message. The Early Warning scheme based on CCA (EWCCA) that this thesis proposes does not require vehicular position by other device, and it can determine driving direction and the direction of message propagation. EWCCA can solve broadcast storm problem. It utilizes the ranging technique of RSSI and TOA which transform the measuring data into waiting time. Therefore, it can avoid a solution that all receiving vehicles forward this message at the same time. Finally, the thesis uses QualNet simulation software to simulate, and it differentiates between EWCCA for RSSI form and EWCCA for TOA, and comparing itself with Simple Flooding. By the simulating results, in fading model experiments, it improves 60% for end-to-end delay time, it improves 86% for collision packets and it improves 56% for receiving failure rate. It proves that EWCCA can provide lower delay time, less number of collision packets and lower receiving failure rate. Hence, it can achieve the purpose of increasing reactive time for driver. |
