感知型無線網路之網路存取層設計

Abstract

近幾十年來，隨著無線網路的成長以及相關應用服務的開發，市場對於無線頻譜的需求急速增加。但最近已有不少研究報告指出，分配固定頻帶執照給特定使用者的這種傳統分配資源方式，並沒有充分利用有限的頻帶。當頻帶的擁有者沒有使用其頻帶時，這個頻帶就會一直呈現無人傳輸的狀態。在感知型無線網路的概念被提出後，這種浪費頻寬的情形即將獲得改善。它創新的概念，允許第二種使用者在頻帶所有人不使用其頻帶時，得以有效利用這空閒的資源，以提高整體頻譜的效能。這是前所未有的想法，打破了頻帶與頻帶間的界線進行傳輸，因此感知型無線網路需要一個新的媒體存取層協定，使其適應變動快速的環境並進行傳輸。這篇論文提出了一個新興的感知型無線網路媒體存取層協定(Opportunistic Cognitive MAC Protocol)，使感知型無線網路設備可以有效利用空閒的資源，與該頻段和平共存。這項協定藉由預測高優先權使用者的行為和禮讓性的傳輸規則，避免對資源擁有者造成致命的傷害。接著，我們對於這項協定進行傳輸瓶頸以及公平性的分析。最後，在ns2網路模擬器上的實驗結果證明，OC-MAC的預測模組與禮讓機制，使得我們的媒體存取層設計可以有效的提升原本網路8.74%的整體使用率至37.49%，而且對頻帶擁有者只造成了0.52%的碰撞。這效能甚至是我們在2006年提出之協定的九倍。這些數據證明了OC-MAC可以有效地達成感知型無線網路的目標。

In last decades, the demand of wireless spectrum has increased rapidly with the development of mobile communication services. Recent studies recognize that traditional fixed spectrum assignment does not use spectrum efficiently. When licensees are not transmitting, spectrum remains idle. Such a wasting phenomenon could be amended after the present of cognitive radio. Cognitive radio is a new type of technology that enables secondary usage to unlicensed user. Because its innovative idea overthrows the exclusive spectrum policy, cognitive radio requires a new open spectrum access protocol to transmit intelligently in fluctuant environment. This thesis presents a MAC protocol, namely Opportunistic Cognitive MAC Protocol (OC-MAC), for cognitive radios to access unoccupied resource opportunistically and coexist with wireless local area network (WLAN). By a primary traffic predication model and transmission etiquette, OC-MAC avoids to producing fatal damage to licensed users. Then We examine and analyze the design of OC-MAC in the aspects bottleneck and fairness. Finally, a ns2 simulation model is developed to evaluate the performance of OC-MAC in scenarios with coexisting WLAN and cognitive network. The results indicate OC-MAC enhances network utilization from 8.74% to 37.49% and only results in 0.52% collision to legacy users. The efficiency is almost nine times of our former work while with the same level of damage.