在正交分頻多工系統下利用重覆使用重調接收區塊之盲通道估測與盲區塊同步

Abstract

本論論文主要分為兩兩個部分：基於正交分頻多工系統之 一)盲通道估測，以及 二) 盲區塊同步。在第一部分中，我們探討了了基於子空間方法之盲通道估測，不不同於一般的通道估測，盲通道估測並不不需要使用額外的頻寬來來估測通道係數數，因此有較好的頻寬效率率率。現有的子空間通道估測方法中，大部分的運算複雜度度相當高，並且收斂速度度緩慢，難以適用於時變性的通道環境。有鑑於此，本論論文之第一部分著重於探討如何有效率率率地降降低子空間式盲通道估測之運算複雜度度，以及如何提高收斂速度度。我們的方法基於重複使用重調接收區塊，只要適當的調整重複使用接收區塊的次數數，演算法之收斂速度度將會大大的下降降。此外，由於重調方法的使用，使得此子空間式演算法之運算複雜度度大為降降低。最後，藉由電腦模擬結果也展現我們提出的方法優於前人所提出的方法。在第二部分中，我們探討了了盲區塊同步的問題，並提出了了兩兩個演算法。在第一個提出的方法中，我們利利用接收訊號於循環字首與其資料料部分相同的特性，可以估計出時間位置，此法大大降降低了計算的複雜度度。第二個提出的方法中，利利用第一步分所提到的想法，基於重複使用重調接收區塊，在有限的接收資料料下，可有效地估計出時間位置。數數值模擬的結果顯示，所提出之演算法與前人提出的方法比較，有顯著的改善。

Digital signal processing (DSP) techniques have played an important role in channel equalization and estimation in communication systems. While channel equalization and estimation are usually done by pilot-assisted methods in most systems,algorithms for blind channel estimation have also been largely studied due to high bandwidth efficiency. In communication systems employed with cyclic prefixes (CP), subspace-based (SS) methods are among the most popular categories of blind channel estimation schemes. Existing SS methods, however, either require a large amount of received data or possess a high computational complexity. These drawbacks have made many consider blind methods as inapplicable in modern communication systems which feature fast-varying channels. In the first part of this thesis, a new algorithm for blind channel estimation in orthogonal frequency division multiplexing (OFDM) systems that require few received blocks with a reasonable complexity is proposed. The idea is based on combining advantages from two previously reported SS methods, namely, remodulation and repeated use of each received block. The combination of the two separated ideas turns out to be superior to each of them alone in many facets. Simulation results not only confirm the capability of the proposed method to work properly with very few received blocks, but also show that it outperforms all previously reported methods. Another important problem, namely the blind block synchronization, is also studied in the second part of this thesis. Most existing blind estimation methods in linear redundancy precoder systems assume the block boundaries of the received streams are perfectly known to the receiver, but this assumption is usually not true in practice since no extra known samples are transmitted. Two algorithms for blind block synchronization are proposed for CP-based OFDM systems. The first proposed algorithm declares block synchronization when a certain autocorrelation matrix, constructed from the received signal, achieves minimum rank. It has a much lower computational complexity. The second proposed algorithm exploit the concept of remodulation and repeated use of each received block. Simulation results suggest their advantages over all previously reported algorithms, especially when the amount of received data is limited.