OFDM無線網路之合作通訊---子計畫一：運用在支援合作通訊之正交分頻多工系統的跨層機制設計與服務品質(I)

Abstract

合作多樣性（cooperative diversity）目前已成為解決無線網路中頻道衰減現象之一新興技術。藉 由合作通訊（cooperation communication），不同的使用者可以藉由中繼（relay），與其它使用者共組多 重天線。換言之，一使用者除了接收送自己本身的資料外，亦需要在必要時刻，當其它節點的中繼。 透過理論證明，合作通訊已被證實具有提升系統容量（capacity）、降低運行中斷機率（probability of outage）、提昇細胞覆蓋率（cellular coverage）以及服務品質（quality of service）、增加資料速率、降 低使用者的功率、及減輕對通道變化的敏感等優點。 為設計與實作合作通訊，許多複雜的議題需要被討論以及解決，舉凡合作節點有效傳輸率降低、 合作傳輸對整體網路的干擾、合作傳輸指派以及合作節點發生換手、指派公平性、何時該啟動合作傳 輸等。根據相關文獻可以發現合作傳輸早就已為學者的廣泛討論，但當時所的合作傳輸觀念較偏向以 間接路徑（透過中繼傳送）取代直接路徑（由sender 直接傳給receiver）。目前所探討的合作通訊，則 是利用無線傳輸屬廣播之特性，讓receiver 同時可以接收中繼和sender 送過來的資料，同時解碼，增 加資料的正確率，並降低重傳次數。因此，部分的機制需要重新加以設計修改。 本總計劃為期三年，且研究主題是「OFDM 無線網路之合作通訊」，目標是在OFDM 無線網路中， 探討合作通訊於實體層與資料連結層之相關議題。目前正執行第一年之計畫項目。總計劃下分五個子 計劃。子計畫一（即本子計畫）負責堤供服務品質之跨層機制設計。為達到預設目標，此子計畫第二 年的研究主題是針對支援合作通訊之異質OFDM 無線網路，設計合作任務指派協定（cooperation assignment）。第三年的研究主題是針對支援合作通訊之同質及異質OFDM 無線網路，分別設計媒介存 取控制（medium access control）協定。我們將透過模擬方式驗證提出之機制，並以數學模型進一步分 析效能，同時也將在WiFi/WiMAX 實驗平台上實做我們的機制，驗證其可行性與效能。

Cooperative diversity has recently emerged as a new and effective technique to combat fading in wireless networks, and its basic idea is to explore the broadcast nature of the wireless channel. In a cooperative communication system, each wireless user is assumed to transmit data as well as act as a cooperative agent for another user. Cooperation leads to interesting trade-offs in code rates and transmission power. Using transmission power as an example, one may argue on one hand that more power is needed since each user is transmitting for other users, when in cooperative mode. On the other hand, the baseline transmission power for both users will be reduced cause of diversity. However, it』s obvious that not only to a homogeneous wireless network (such as an ad hoc network), but also to a homogeneous wireless network which integrates both WWANs (e.g., WiMAX and 3G) and WLANs (e.g., WLAN), cooperative communication significantly increases total system throughput. To develop and implement cooperative communications, several complicating issues must be addressed, including the loss of rate to the cooperating mobile, overall interference in the network, cooperation assignment and handoff, fairness of the system, and transmit and receive requirement on the mobiles. The topic of our project is 「Cooperative Communication in OFDM Wireless Networks」，and there are five subprojects to study the cross-layer design and QoS, user-cooperative protocol design and analysis, space-time coding and decoding, cooperative multiuser MIMO-OFDM systems, and reconfigurable platform design, respectively. The main objective of this subproject is to support QoS in cooperative OFDM networks. To achieve our design goals, the research topic of second year will investigate cooperative assignment in heterogeneous OFDM networks, which includes IEEE 802.11 WLANs, IEEE 802.16 WiMAX, and IEEE 802.20 FLASH-OFDM systems. In the third year, we will study related issues of MAC protocol design in a cooperative network, and propose our MAC protocol. We will further study power saving function and integrating with the proposed MAC protocol. For the proposed protocol, we will evaluate their performances by means of simulations. In addition, we will implement our proposed protocol on WLAN/WiMAX development platforms.