正交分頻多工存取下載系統在非完全通道資訊下之資源分配及中繼器選取

Abstract

正交分頻多工存取(OFDMA)系統可以利用簡單的通道等化器來有效 的避免頻率選擇性衰減並使用最小（Nyquist）次載波距（subcarrier spacing）來提高頻譜效能。這種多工技術也容許很有彈性的資源分配，以充分利用不同用戶的通道狀況，一則可滿足個別用戶傳送速率的要求，另則可最大 化整個系統的通道容量。此外，由於電磁信號傳播的統計特性，蜂巢式行 動通訊系統之收訊強度無法定量保證，其中難免有部分用戶收訊品質不 佳。一個經濟有效的解決方案便是中繼站的設立，適當的佈建中繼站不但 有助於提升其信號品質，並可擴增基地台的覆蓋範圍與容量。

本論文主要在研究有多座中繼站的OFDMA 下鏈系統之資源分配及中 繼站選取的問題。我們同時考慮傳送（基地台）與接收（用戶）端皆使用 單一天線(SISO)以及兩端皆有多根天線(MIMO)兩種架構，而下傳時雖有多 個中繼站可以挑選，但各通道狀況的資訊卻有不確定性；亦即，基地台只 有通道狀況的估計值以及誤差機率分佈。我們使用在此不確定情況下的通 道容量下限來作為系統效能的評估標準。對上述SISO 與MIMO 兩種架構 下，我們都推導出傳送端與中繼站的最佳功率比來達到最大的通道容量下限。根據這個功率比我們分別提出了幾近最佳的次載波及其能量（功率） 分配的演算法。我們的演算法複雜度不高，電腦模擬結果也顯示它能兼顧 個別用戶的公平性，滿足其傳送速率要求也可以令系統的整體傳送速率達 到最大值。

The Orthogonal Frequency Division Multiple Access (OFDMA) scheme is an efficient anti-fading transmission scheme which renders high spectral efficiency and simple channel equalization. It also allows °exible resource allocation (RA) to meet various user requirements and achieve maximum network capacity. With the help of relays, link quality at cell edge can be improved and both network capacity and the coverage area can therefore be improved. In this thesis, we consider the problem of RA and relay selection for downlink trans- mission in both single-input single-output (SISO) and multiple-input multiple-output (MIMO) OFDMA based cellular networks. We assume the availability of multiple co- operative relay stations but not the perfect channel state information (CSI). Instead, the base station knows only the estimated channel (link) gain and the associated error distribution. We use a tight capacity lower bound (CLB) for a link with imperfect CSI as the performance metric. In SISO networks, we derive the optimal source and relay power allocation ratio that maximizes the CLB of a cascaded source-relay-destination link. Based on this optimal power ratio, we propose a simple suboptimal algorithm that assigns power, subcarriers and cooperative relays to each serving mobile station. We then derive the optimal power ratio for MIMO networks. Using the proposed subcar- rier assignment algorithm for SISO network, we present the optimal and a suboptimal power allocation schemes. To reduce the computation complexity, we derive a near- optimal power ratio, assuming both source-to-relay and relay-to-destination links have the same rank. Simulation results show that our algorithm not only meets the users' rate constraints with very high probabilities but yields an excellent sum rate (CLB) performance.