感知型無線網路之隨機傳輸方法

Abstract

近年來各種軟硬體的提升，使得無線網路通訊技術得以蓬勃發展。然而，這亦使一個問題逐漸浮上檯面，也就是無線網路資源的限制以及現狀的浪費。根據FCC的報告指出，已發出執照的頻譜利用率在離峰時段近乎浪費，而全天平均也僅有15%~85%的利用率。這說明了一件事，也就是無線網路可使用的資源在現在的環境下是被浪費的。若無線網路的利用持續增加，在可預見的將來，這些低使用率的頻譜將會成為一種資源浪費。於是，一種新的無線網路技術在近年逐漸受到關注，也就是所謂的感知型無線電(Cognitive Radio)技術。此一技術使得無照使用者得以利用有照使用者浪費掉的資源，藉以提升整體的使用率。 然而，此一技術的使用必然對於有照使用者帶來影響。於是，再避免對有照使用者產生影響的前提下如何提升整體頻譜使用率，就是感知型無線電的技術發展重點。有鑑於此，我們針對感知型無線電的使用進行了問題的研究，主要面臨下列挑戰。 在媒體存取控制(MAC)層來說，我們有以下挑戰 (1) 如何進行流量的控制以及頻道的選擇以減低對有照使用者的影響 (2) 如何在複數不同配備的感知型無線電使用者之間達成公平議題 在傳輸控制協定(TCP)層來說，我們有以下挑戰 (3) 感知型無線電如何對TCP造成影響，我們是否有辦法解決 本論文的目的在於發展一套高效能感知型無線電的媒體存取控制和傳輸控制協定方法，以期提升頻譜的使用率。對於問題(1)，我們提出兩個方法來控制:分別為RTS-CTS-CRTS協定和主動式流量控制。前者對於頻道選擇能產生更好的結果，而後者則可以避免對有照使用者造成過大傷害。我們運用了馬可夫鏈(Markov Chain)來進行數學分析，並且用NS2來進行模擬實驗。 對於問題(2)，我們考慮兩種不同的公平性議題：不均勢的主要使用者偵測能力以及不均勢的頻譜單位大小。我們同樣利用馬可夫鏈和NS2來進行分析與驗證。針對此問題，我們提出一個以干擾控制為主的媒體存取控制層解法。主要用意在於藉此避免異質感知型無線電系統下的硬體限制。 對於問題(3)，我們利用NS2模擬與自建模型的分析，將感知型無線電環境對於傳輸控制協定造成的影響定義為下列三個事件：主要使用者干涉丟失事件，輕度壅塞事件RTT變動事件。針對此三事件，我們提出三個對應解法，並設計事件處理器來進行改善。最後結果顯示在感知型無線電網路的環境下，我們可以提升相當的效能。

With the increasing of wireless in this dissertation, Radio spectrum is a kind of limited natural resource as well as water and crude oil. Its use is licensed and assigned by governmental agencies. However, according to the statistics of the Federal Communications Commission (FCC), temporal and geographical variations in the utilization of the assigned spectrum range from 15% to 85%. The temporally unused spectrum is referred to as spectrum hole or white space. A new wireless technology called Cognitive Radio (CR) is introduced to reduce the waste of spectrum resources. CR system dynamically accesses the spectrums to improve overall spectrum usage of frequency bands. However, the access of unlicensed users to licensed spectrums causes interference to the licensed users on the bands. Therefore, the key of CR design is to improving overall spectrum usage in the constraint of avoiding harmful interference to the licensed users. To achieve our CR design, we focus on the following challenges. In the MAC layer, we have: (1) To control CR users’ traffic and channel selection for avoiding interference to licensed users. (2) To achieve fairness between heterogeneous users in different CR systems In the TCP layer, we have: (3) To analysis and solve the impact of CR link to TCP layer In this dissertation, we develop efficient CR MAC and TCP protocols to improve spectrum utilization. To th first problem, we present two functions: RTS-CTS-CRTS handshaking and active traffic control mechanisms. The RTS-CTS-CRTS handshake mechanism can select a channel with better transmission quality. Then, the active traffic control mechanism is used to control the amount of DATA packets transmitted on the selected channel. We analysis the problem and solution by Markov chains and simulate by NS2. To the second problem, we study two fairness issues: uncoordinated of PU-detection ability and uncoordinated of spectrum unit size. We propose several Markov-chain models to study the potential unfairness problems. The proposed solution is a jamming-based MAC-layer approach, called probing function, to enhance PU-detection ability and fairness feature. A MAC protocol, called CCR-MAC, is proposed based on the combination of probing function and our previous work on CR MAC to avoid hardware limitation and protect PU’s traffic at the same time. To problem (3), we verify the ineffectiveness of WTCP in CR environment by NS2 simulation. Afterward, we analysis the cause of throughput decay conditions and concluding them into three events: PU-Interference Loss, Mild-Congestion and RTT-Variance events. To improve TCP throughput, three event handlers are proposed: PU-Interference handler, Faster-Recovery mechanism and RTT-Adjustment function. By applying the three handlers, a cognitive TCP that adapting CR environment, called CR-TCP, is proposed. The results show that CR-TCP improve throughput over 50% in both stable and varying environment while WTCP solutions only improve 10% to TCP-Reno.