在IEEE 802.11無線區域網路規範中，實體層（PHY Layer）提供了多重速率之選擇，但該規範在媒介存取控制層（MAC Layer）中並無明確機制來達到多重速率管理的功能，為有效利用無線通道資源、提昇資料傳輸速率及擴大無線區域網路的服務範圍，可設計一有效可執行的鏈結調適（Link Adaptation）機制。調適機制可依據傳輸過程中無線通道的品質來動態選擇適當之傳輸參數（例如調變方式、傳輸速率與編碼方式），以提高成功傳送機率，進而提昇網路之產能（Throughput）。
無線通訊環境中因路徑損失、遮蔽效應、多重路徑衰落及頻率干擾等因素，使得接收訊號之SNR（Signal to Noise Ratio）及RSL（Received Signal Level）皆隨接收端所在位置及時間產生不等之變化。為迅速有效掌握無線通道之狀況，吾人以接收端接收每一訊框（fame）之SNR及RSL作為無線通道估測及傳輸參數之選擇依據，較能及時反應無線通道之狀態並可機動調整最適合通道狀態之傳輸參數。此一將實體層之SNR或RSL資訊供媒介存取控制層使用的作法，即為”跨層”（Cross-layer）觀念。基於IEEE 802.11無線區域網路之分散式協調功能（Distributed Coordination Function, DCF），本論文提出跨層鏈結調適（Cross- Layer Link Adaptation）的機制。此機制依據媒介存取控制層傳送訊框成功與失敗次數及實體層之通道狀態，動態選擇最適當之媒介存取機制及傳輸參數，使每一個傳送訊框於該通道品質下，均以最適當之媒介存取機制及最高之可用傳輸速率傳送訊框，進而提昇系統之產能。
本論文以模擬方式驗證吾人所提出之跨層鏈結調適機制，並與固定速率及其它鏈結調適機制相比較。結果顯示本論文所提出之跨層鏈結調適機制無論在靜止或移動的環境下，其系統之產能均有較佳的表現。

In order to improve throughput, dynamic link adaptation schemes are designed so that the signal and protocol parameters can be adjusted as the radio link conditions change, according to the quality of a wireless channel. IEEE 802.11 standards, such as 802.11g and 802.11b, support multiple data transfer rate capability in the physical (PHY) layer, which allow medium access control (MAC) management entity to perform dynamic rate switching with the objective of improving the performance. Although the PHY layer supports multi-rate capability, MAC control schemes are required to exploit this capability.
The receiver's signal-to-noise ratio (SNR) and received signal level (RSL) vary with time due to path loss, shadowing effect, multi-path fading and interference. Hence, according to the SNR and RSL of the latest received frame, a link adaptation scheme can quickly respond to the channel variation and suitably adjust parameters for transmissions. This is a cross-layer design because the PHY layer shares the wireless channel information, i.e., SNR and RSL, with the MAC layer. To address the issues of dynamic rate adaptation, improve the data rate and extend the service coverage of an AP, we propose in this thesis a cross-layer link adaptation (CLLA) scheme that is effective and highly executable. The CLLA scheme uses the number of successful transmissions, the number of transmission failures and the channel information from the PHY layer to determine proper transmission parameters for subsequent medium accesses. The CLLA scheme lets frames be transmitted at the highest available data rate using proper medium access methods defined in the scheme, so high throughput can be achieved.
The CLLA scheme is compared with the fixed-rate scheme and other well-known link adaptation schemes, performance comparison is validated against simulations. Simulation results show that the CLLA scheme can achieve higher throughput in stationary or even mobile environment.

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