從網路吞吐量觀點分析並設計合作式通訊的媒介存取策略

Abstract

近幾年來，合作式通訊被提出來成為一種新的通訊模式，利用周遭的鄰近節點來提昇自我的分集增益（Diversity Gain），進而改善點對點之間的通訊品質。從實體層（Physical Layer）的角度上來評論，合作式通訊能有效地降低接收端的位元錯誤率，以達到通訊可信度上的提升。然而，在實作複雜度的限制下，半工傳輸（Half-Duplex）是現今大部分通訊系統的假設，因而，合作式通訊的模式往往需要兩個階段才能完成，導致在傳輸時間上的延伸變得是無可避免的。因此，若改由整體網路的吞吐量（Network Throughput）觀點來思考，合作式通訊不見得會帶來所期待的效能。在本論文中，我們將以IEEE 802.11媒介接取控制技術為基礎，佐以模擬驗證，利用二維的馬可夫鏈（Two Dimensional Markov Chain）的數學分析來探討合作式通訊於網路吞吐量的表現。在媒介接取控制的考量下，適當地找尋合作式通訊的使用時機。接著，將提出新的通訊協定並整合傳統點對點通訊系統及合作式通訊技術。以完整通道資訊為基礎之合作式通訊協定（Full-CSI based Cooperative MAC Protocol）將會依據完整的通道資訊來決定使用合作式通訊的時機，最適合的節點也會被選取並幫忙資料的傳輸，然而，通道資訊的交換可能會造成網路吞吐量的下降，因此以位元競爭方式之合作式通訊協定（Bitwise Competition based Cooperative MAC Protocol）利用競爭的方式來選擇適合的節點，可以有效的降低控制訊框傳輸的數量，雖然無法獲得所有的通道資訊，從模擬的結果可以發現以位元競爭方式之合作式通訊協定在大部分的通道環境下還是有較佳的網路吞吐量表現。

In recent years, cooperative communication has been developed as a new communication strategy that incorporates a relay node to assist the direct point-to-point transmission. However, owing to the longer transmission time resulting from the cooperative schemes, there is no guarantee to enhance the network throughput in view of the medium access control (MAC) performance. In this thesis, the system throughput of the combined direct/cooperative scheme is evaluated by exploiting the proposed analytical model based on the IEEE 802.11 MAC protocol. In terms of network throughput, whether to adopt the cooperative schemes depends on the tradeoff between the cooperative transmission delay and the channel condition of the direct communication. Two cooperative MAC protocols are proposed to determine the circumstances for activating the cooperative communication based on the channel conditions. The full-CSI based cooperative (FCC) MAC protocol is introduced to choose the feasible transmission scheme and the relay node according to the full channel information. However, the overhead caused by the FCC scheme may degrade the throughput performance as the number of relays is significantly increased. Therefore, the bitwise competition based cooperative (BCC) MAC protocol is utilized to determine a feasible relay node for data transmission. Simulations are also conducted to further validate the effectiveness of the proposed analytical models and cooperative MAC protocols.