| 摘要: | 近幾年無線網路快速的成長，使得其應用與服務變得越來越普遍。車載資通訊有多樣應用服務，例如，行車安全、全球定位導航、多媒體、影音串流以及網路存取服務。隨著無線科技的成熟，各種無線網路需求的概念被提出，以智慧運輸系統為例，許多交通設備皆配備車載裝置，以提供多樣化服務，其中通訊功能更是扮演了重要的角色。民生資訊的取得更是和網路密不可分，當駕駛欲知道路況情形、便利資訊、氣候、新聞等等即時資訊時，藉由車用網路與外界（其他車輛或道路網路系統）的資訊交換，不但能方便駕駛了解目前的行車狀況，避開較為壅塞的路段，也提供駕駛者行車建議，使駕駛者做出較佳的判斷。由於資通訊技術的成熟，讓原本處於閉塞而無法與外界進行單向或雙向資訊傳輸互動的汽車環境，增加與外界溝通的通訊能力，也因此Telematics（Telecommunication+Information）車載資通訊系統產生了強烈的市場需求並顯示其重要性與其研究價值。美國材料試驗協會（American Society for Testing and Materials，ASTM）為了發展專用短距通訊（Dedicated Short Range Communication，DSRC）技術，採用IEEE 802.11a作為實體傳輸技術，並促成IEEE 802.11p WAVE（Wireless Access in the Vehicular Environment）與IEEE 1609相關規格的誕生，但這些通訊協定的行為存在許多效能不彰的問題。由於WAVE設備所配置的天線數量有限，欲使用頻道資源，則需要透過競爭，易造成效率降低，又頻道切換機制，使存取網路機會變得更少，因此需要有效使用頻道資源的方法提出。在研讀車用無線網路及相關的規格與文獻後，本論文經由估算網路頻道資源以及系統效率提出精確數學模型與網路模擬器（Network Simulator 2，NS-2）。透過驗證與分析IEEE 802.11p以及IEEE 1609規格之系統效能後，提出車用網路協調式頻道存取方法，包含以分群為基礎之車用頻道切換協定有效使用頻道資源並舒緩使用者競爭情形、以代理者協調排程方法之車用網路通道存取方法，並延伸至代理者的排程與中繼方法來達成有效的頻道資源管理以針對不同車道環境的頻道存取方法安排使用者存取頻道資源，以及記憶性載波偵測多重存取避免碰撞協定提高頻道使用率與減少競爭失敗的機會使得頻道使用率能達到100%且有效改善系統效能。During the last few years, wireless networks have been rapidly growing and becoming more and more popular. In particular,telematics are receiving a lot of attentions due to wide variety ofservice which are safety, GPS navigation, multimedia, streaming, and other Internet applications. With the matured wireless transmission technology, the new demand on wireless applications is toward the concept of deploying wireless devices on transportation systems suchas buses, trains and vehicles. Telematics services and entertainment are also important for users.Dedicated short range communications (DSRC) technique has been formally defined by American Society for Testing and Materials (ASTM) association couple years ago. Subsequently, the DSRC standardization process has been smoothly transferred to the IEEE 802.11p working group for the consideration of world-wide market and regulatory. Moreover, some documents released from IEEE 1609 Workinggroup (e.g., IEEE 1609.3 and IEEE 1609.4), which name the DSRC system as wireless access vehicular environment (WAVE), have defined that the wireless communication technique among vehicles and roadside system mainly inherits from IEEE 802.11a physical (PHY) layer and IEEE 802.11 carrier sense multiple access with collision avoidance (CSMA/CA) medium access control (MAC) protocol. Generally, number of antennas which each vehicle equips with is limited. If vehicles transmit and receive data over wireless medium, they shall contend each other on the same wireless channel. However, it usually suffers from packet collision, which decreases data throughput and increases delay as the number of vehicle increases. Moreover, the mandatory operation of channel switching between service channel and control channel will waste half channel resource. It is desired to have a smart channel access scheme for the vehicular networks.In this dissertation, we propose coordinated channel access schemes including coordinated interleaving access (CIA), agent-based coordination (ABC), vehicular grouping access (VGA) under different roadway environments, and memorized carrier sense multiple access with collision avoidance (MCSMA/CA) so that the channel utilization is increased from 42\% to 100\% as compared to the alternating channel access scheme and system performance is improved significantly. |
