分散式放大前送多輸入多輸出中繼站網路之設計

Abstract

多進多出系統(MIMO)搭配中繼站之應用可提升系統涵蓋率或提高系統容量。採用分散式平行單天線中繼站網路則額外享有建置彈性、多集性與成本之優勢，然而亦面臨因結構上缺乏中繼站相互連通而衍生的設計議題。本論文針對分散式放大-前送(amplify-forwarding)中繼站型態，首先考慮提升中繼站容量之中繼前送增益(forwarding)設計。在設計中數學結構的限制下，目前並無實用有效之快速設計方式，因此我們採用矩陣低階更新(matrix low-rank updating)的方式，設計遞迴式最佳化解法。此解法雖可快速計算並提供提高系統容量之設計，對於分散式中繼站之特性研究並無實質貢獻，因此我們設計雜訊優勢模型以簡化系統，並據此提除對應之增益設計方法。此設計方法搭配中繼站部分啟用之模式，我們發現此設計方法可類比於單站多天線選擇系統(single-hop MIMO antenna selection)。因此我們可採用多天線選擇系統之已知特性，類比分析我們針對中繼站所提出之設計方法。由模擬結果可知，我們所設計之設計方法可有效提升系統容量，且印證類比於多天線選擇系統之容量分佈特性。

當中繼站的數量增加時，基於中繼站選擇之設計方法將面臨運算量大幅上升之問題。同樣還是由優勢雜訊模型出發，我們設計新的增益最佳化運算並避免中繼站選擇。此演算法更進一步簡化並轉換設計問題，並使用修正之解題空間(solution space)。應用於多用戶系統之低散逸波束設計亦包含於我們所設計之演算法。由模擬結果可知，免除中繼站選擇之演算法，相較於使用中繼站選擇方法，其效能相若或更好。

在第三部分我們討論針對高度散佈(highly dispersive)通道之分頻正交多工系統(OFDM)之通道估計，以及其應用於使用OFDM之中繼站網路。對於OFDM通道估計問題，時域(time-domain)通道估計方法可利用有限的領航訊號(pilot symbols)提供精確之通道估計。然而此類方法須掌握通道多路徑之延遲位置(multipath delay positions)。已知針對延遲位置估計的方法需要特定格式之領航訊號，且在通過慢速衰落(slow fading)通道時可能會降低估計效能。我們所設計之延遲位置估計，採用追求最大程度相合(matching pursuit)之演算法，考慮相鄰OFDM訊框中領航訊號，且不限制領航訊號的位置。經由模擬結果可知，我們所設計之延遲位置估計方法，可大幅提升通道估計之準確率。另外我們討論採用OFDM之放大前送中繼站其對應之傳輸通道模型，經由分析得知可對應於一高度散佈之傳輸通道，因此適用於本文所提出之估計方法。

Relays can potentially enhance the transmission performance of multi-input multi-output (MIMO) systems. A parallel single-antenna relay network has additional advantages in flexibility, diversity, and cost, but also poses significant design problems because the absence of inter-antenna connections over different relays makes the underlying mathematical problems much more difficult to solve. In this thesis, we first consider the design of parallel amplify-and-forward relay networks. More specifically, we focus on the design of relay gains to maximize the system capacity. As no closed-form analytic solution can be found, we first develop an iterative algorithm based on low-rank updating to efficiently find a locally optimal solution. Since iterative optimization provides little insight into the analytical properties of the solution, we attempt analytical solutions based on asymptotic noise conditions and relay selection. It is discovered that the proposed algorithms could be modeled as single-hop MIMO antenna selection systems and could be analyzed in the similar way. We examine the resulting capacity outage diversity orders and confirm the analysis and equivalent modeling with simulation results.

As the algorithms based on relay selection may encounter expensive computation burden as number of relays increases, we develop algorithms based on noise dominant models but avoid computational intensive relay selection. The proposed algorithms are made possible by modified criterion and specifically constrained solution space. Low-leakage beamforming used for multiuser communication is applied in our algorithm. Numerical results demonstrate the proposed algorithms exhibit comparable or better performance than previous selection-based approaches.

In the third part of this thesis, we consider OFDM channel estimation with highly dispersive channel and application in distributed relay network. Time-domain channel estimation techniques have been proposed for OFDM systems for their ability to yield relatively accurate estimates with only a few pilots. Key information needed in such techniques is the multipath delays of the channel. Prior approaches to estimation of multipath delays require regular pilot structures and may not work in slow fading. We propose a group matching pursuit technique for channel estimation. The technique is an extension of the orthogonal matching pursuit technique. It employs the pilots in several OFDM symbols to estimate the multipath delays in a sequential manner, where the pilots can have an arbitrary structure. Simulation results show that the proposed algorithm has superior performance. We then demonstrate that distributed AF relay network with OFDM transmission could be modeled as single-hop OFDM system with equivalently high dispersive delay profile. Thus we apply and examine the proposed channel estimation schemes for the relay network of interest.