使用鏈路調節與多輸入多輸出編碼改善正交分頻多工無線網路效能之研究

Abstract

日益增多的各式無線服務需求，如語音、資料傳輸及多媒體，促進了高速無線區域網路的蓬勃發展。IEEE 802.11是眾多致力於標準化無線區域網路之媒介存取與實體層的主要力量之一。其中於媒介存取控制層規範了兩種通道存取機制─基本存取機制與請求發送⁄允許發送存取機制。此外，延伸的IEEE 802.11a 透過實體層的正交分頻多工技術可實現6 Mbps 至54 Mbps之高速傳輸。在多傳輸速率的系統中，鏈路調節顯得愈加重要，其作用在於可動態地轉換傳輸速率以配合變動之通道狀態。在本論文中，吾人所提出之鏈路調節演算法可更適當地利用IEEE 802.11a無線區域網路之媒介存取與實體層。此鏈路調節演算法可根據通道狀態、競爭站台之多寡與資料長度選擇存取機制與傳輸速率之最佳組合以提升固有之無線區域網路效能。再者，多輸入多輸出編碼與正交分頻多工技術之結合被視為提昇新世代無線通訊系統傳輸速率之前瞻技術。依據機能之不同，多輸入多輸出編碼可分為兩類。其中空間分集之作用在於改善鏈路之可靠度，另一方面，空間分工則著墨於增加使用頻譜的效率。吾人對所提出之鏈路調節演算法進行修改，使其可選取適當的多輸入多輸出編碼，以適應當下環境提供良好的鏈路品質或提昇傳輸速率，進而改善無線網路之效能。最後，藉由電腦模擬結果驗證上述方法之表現，吾人可證實其在室內無線環境有優異的效能。

The increasing demand for all types of wireless services (e.g. voice, data, and multimedia) promotes fast growing of the high-speed wireless local area networks (WLANs). One of the major efforts in standardizing the medium access control (MAC) and physical (PHY) layers for WLANs is IEEE 802.11. It defines two channel access mechanisms, the basic and RTS/CTS access mechanism, at the MAC layer. Besides, the extended IEEE 802.11a achieves high data rates ranging from 6 Mbps to 54 Mbps by using orthogonal frequency division multiplexing (OFDM) at the PHY layer. With multiple rates, link adaptation (LA), a process to dynamically switch data rates to best match the varying channel condition, becomes increasingly important. In this thesis, we propose an LA algorithm to better exploit the MAC and PHY layer in the IEEE 802.11a WLAN. According to the channel condition, number of contending stations, and data length, the proposed LA algorithm chooses the optimum combination of the access method and data rate to achieve higher throughput in the inherent IEEE 802.11a WLAN. Furthermore, the combining of multiple-input multiple-output (MIMO) coding with OFDM is regarded as a promising technique for enhancing the data rates of next generation wireless communication systems. The MIMO coding can be categorized into two types based on their functionality. Spatial diversity (SD) is used to improve the link reliability while spatial multiplexing (SM) increases the spectral efficiency. For further improvements in the WLAN’s throughput, we modify the proposed LA algorithm so that it can choose the appropriate MIMO coding as well to provide the better link quality or higher data rate according to channel condition. Finally, we evaluate the performance of the proposed algorithms by computer simulations, and confirm that they work well in indoor wireless environments.