本論文在LAA (License Assisted Access)與Wi-Fi的共存網路中提出一個更新式LBT (Modified Listen Before Talk, M-LBT)演算法，透過動態調整與Wi-Fi競爭Unlicensed Bands的通道佔有時間(Channel Occupancy Time, COT), M-LBT演算法可以在變動位元速率(Variable Bit Rate, VBR)的流量下解決eNodeB (Evolved Node B)緩衝器可能發生Overflow或Underflow的問題。我們提出的更新式LBT演算法主要分成兩個部份，當eNodeB緩衝器的佇列長度(Queuing Length, QL)大於或等於一個預設的門檻值時，eNodeB會使用初始的COT與Wi-Fi競爭Unlicensed Band的頻寬，另外，當所得到的平均Data Rate小於或等於封包抵達速率(Arrival Rate)時，M-LBT演算法會根據通道的忙碌次數、前次所得到的Data Rate與Arrival Rate的比值來動態增加或減少COT。最後，我們使用NS-3來模擬並分析所提出更新式LBT演算法的優缺點，從UE (User Equipment)端的Throughput與封包佇列延遲時間的模擬結果中，我們驗證此更新式LBT演算法確實可以降低eNodeB緩衝器的Overflow與Underflow。

In a LAA (License Assisted Access) and Wi-Fi co-existed network, we propose a Modified Listen Before Talk (M-LBT) algorithm to compete with Wi-Fi for unlicensed band through dynamically adjusted Channel Occupancy Time (COT). The proposed M-LBT algorithm can solve the problem of buffer overflow or underflow within an eNodeB caused by Variable-Bit-Rate (VBR) traffic. M-LBT algorithm is divided into two parts. Firstly, when the queuing length (QL) of the eNodeB’s buffer exceeds a predefined threshold, the initial COT is used to compete with Wi-Fi for the bandwidth of unlicensed bands. Secondly, when the obtained average data rate is less than or equal to the mean data arrival rate, the M-LBT algorithm will dynamically increase or decrease the COT according to a channel-busy counter and a ratio between the previous-round average data rate and the mean data arrival rate. Finally, we use NS-3 to simulate and analyze the advantages and disadvantages of the M-LBT algorithm. From the simulation results of the UE throughput and the average packet queuing delay, we validate the proposed M-LBT algorithm can indeed reduce the overflow and the underflow problem in the eNodeB buffer.

論文審定書 i
致謝 ii
摘要 iii
Abstract iv
目錄 v
圖目錄 vii
表目錄 ix
第一章　緒論 1
1.1 研究動機 1
1.2 研究方法 2
1.3章節介紹 3
第二章　LBT與VBR 4
2.1 Wi-Fi的CSMA/CA機制 4
2.2 LTE層級的架構 6
2.3 LAA與Wi-Fi共存的網路架構 10
2.3.1 LAA的LBT演算法 11
2.3.2 LAA的eNodeB與Wi-Fi的AP競爭Unlicensed Band 13
2.4 VBR對LAA的影響 14
2.4.1 多媒體影音串流的VBR 14
2.4.2 VBR對LAA的影響 15
2.5 相關研究 16
2.6 本論文與相關文獻的比較 18
第三章　針對VBR流量的更新式LBT演算法 20
3.1 M-LBT的演算法 20
3.2 動態增加或減少COT 23
第四章　NS-3的模擬與結果討論 25
4.1 LAA與Wi-Fi共存的模擬環境 25
4.2 增加與修改NS-3的模組 26
4.2.1 M-LBT的參數 27
4.2.2 VBR Traffic Generator 28
4.2.3 M-LBT演算法的實作 30
4.2.3.1 量測Unlicensed Band的Data Rate 31
4.2.3.2 COT的增長與縮短 33
4.2.3.3 區別UE不同的優先權 38
4.2.3.4 緩衝器Overflow與Underflow的判斷 39
4.2.3.5 計算不同優先權的佇列延遲時間 42
4.3 模擬參數的設定 44
4.4 模擬結果與討論 45
4.4.1 UE的Throughput 45
4.4.1.1 改變Wi-Fi的CBR 46
4.4.1.2 改變Wi-Fi的通道佔有時間 47
4.4.2 封包的佇列延遲時間 48
4.4.2.1改變Wi-Fi的CBR 49
4.4.2.2改變Wi-Fi的通道佔有時間 52
第五章　結論與未來工作 56
5.1 結論 56
5.2 未來工作 57
Reference 58
Acronyms 65
Index 67

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