以跨層式鏈結調整演算法提升無線區域網路之吞吐量效能

Abstract

在本計畫中，我們研究在IEEE 802.11 無線區域網路(WLAN)中有多個工作站使用 不同傳輸速率時的吞吐量(throughput)效能。無線網路在實體層(PHY)使用不同的調變與 編碼技術(Modulation and Coding Schemes， MCS)支援多重傳輸速率。當某一個工作站 的通道品質變差時，它會使用較低的傳輸速率來獲得較佳的連線品質。然而，IEEE 802.11 媒體存取控制層(Medium Access Control， MAC)所使用的分散協調式功能 (Distributed Coordination Function， DCF)對於每一個工作站提供了相同的傳輸機會， 無論它們所使用的傳輸速率為何。這將導致使用高速率的工作站其資料傳輸率被限制 在某一個使用中的最低速率，使得整體系統的吞吐量下降。這種現象稱為IEEE 802.11 無線區域網路的傳輸效能不規則(performance anomaly)，而這個現象在提供高範圍傳輸 速率的網路例如IEEE 802.11g 網路中將會是更為嚴重的問題。 我們將會以理論推導的方式深入分析效能不規則現象，並且研究既有鏈結調整(link adaptation， LA)演算法的效能。我們也將提出我們包括實體層及媒體存取控制層的跨 層最佳化鏈結調整演算法，並且評估比較我們方法與其他文獻提出方法的效能。我們 提出的演算法考慮實體層及媒體存取控制層之間的交互影響並進行跨層效能最佳化， 預期將能提高系統吞吐量，並且同時維持工作站之間使用通道的公平性。在本計畫中 所提出的理論分析以及跨層最佳化解決方法，希望可作為未來無線區域網路系統設計 之參考。

In this project, we investigate the throughput performance of IEEE 802.11 Wireless Local Area Networks (WLAN) with multiple stations using different data rates. IEEE 802.11 WLAN PHYsical Layer (PHY) provides multiple data rates with different Modulation and Coding Schemes (MCS). If one station experiences bad channel conditions, it would degrade the data rate to achieve a more reliable transmission quality. However, 802.11 Distributed Coordination Function (DCF) protocol at Medium Access Control (MAC) layer essentially provides equal transmission opportunities to each transmitting station. Thus, the throughput of stations with high data rates will be restricted within the lowest rate used by some stations, resulting in the degradation of system throughput. Such the phenomenon is so called “performance anomaly” and can be an extremely severe problem in the network which provides large-scale PHY rates, e.g. 802.11g WLAN with rates ranging from 1 Mbps up to 54 Mbps. We will deeply investigate the impact of “performance anomaly” on WLAN system throughput by theoretical analysis, and assess the performance of the existing Link Adaptation (LA) schemes. Furthermore, we will propose our cross-layer LA scheme which performs the optimization between PHY and MAC layer, and evaluate the performance of the proposed method by comparison with the existing LA schemes. The proposed scheme is expected to improve system throughput and also provide access fairness among stations by taking into account the PHY/MAC dependence and therefore performing the cross-layer optimization. By both the theoretical analysis and the cross-layer solution, we hope to thoroughly provide an in depth view about the design and the implementation of the LA scheme in IEEE 802.11 WLAN environments.