DAB系統同步技術之研究

Abstract

近年來，數位訊號處理（Digital Signal Processing, DSP）與超大型積體電路（very large-scale integrated circuit, VLSI）被廣泛地應用在正交分頻多工技術(Orthogonal Frequency Division Multiplexing ,OFDM)於高速無線通訊。 最近數位音訊廣播 （Digital Audio Broadcasting, DAB）發展於歐洲針對地面與衛星多重數位語音節目廣播系統應用於移動式接收器中，已成功地研發成電子消費性產品。然而設計一個無線通訊系統時首先必須了解同步特徵。因此，首要工作為偵測所收到封包之起始，其他部分為估計載波頻率飄移及通道估計。在本論文中，吾人提出數個有效率的方法達成精確估計。在時間偏移部分，吾人使用空白符元時間特性完成訊框估計。對於頻率飄移部分，吾人結合三種頻率偵測器而增加估測範圍且獲得正確的估測結果。特別是吾人更進一部利用保護區的特性來降低載波間的干擾和符元間的干擾。然而在相位偏移，吾人使用通道偵測來補償相位偏移。此外，吾人藉由分集技術來降低因通道衰落所造成的影響，以實現高階調變系統達到提昇整體資料傳輸速率，或者採用較少的反面因素於多重無線環境中來提高整體系統容量。根據Alamouti的架構，吾人提出正弦和餘弦相位移轉函數來獲取週期性延遲分集（Cyclic Delay Diversity, CDD）。在本論最後，吾人藉由電腦模擬於DAB系統中來驗證上述架構在室內與室外無線環境中具有優異與精確的傳輸表現。

Recent advances of digital signal processing (DSP) and very large-scale integrated circuit (VLSI) technologies are applied by the massive implementation of orthogonal frequency division multiplexing (OFDM) techniques in high-speed wireless communication system. Digital audio broadcasting (DAB) is a successful implementation of OFDM recently for the products of consumer electronics, which was developed in Europe for terrestrial and satellite broadcasting of multiple digital audio programs to mobile receivers. However, in designing a wireless communication system, it is essential to have information about the characteristics of the synchronization. Therefore, the first task is to estimate the start timing of incoming packet; the others are frequency offset estimation and channel estimation. In this thesis, we propose useful methods to achieve more accurate estimation. For timing offset, we use the characteristics of null symbol to do frame detection. For frequency offset, we combine the three frequency estimators to gain more accurate estimation results. Especially, we also use guard interval to reduce ICI and ISI, and then compensate for phase shift with channel estimator. In addition, the decreased sensitivity to fading attached by the exploit of diversity techniques may allow the use of higher level modulation schemes to increase the effective data rate, or smaller reverse factors in multi-cell environment to increase system capacity. Base on the Alamouti’s scheme, two methods, denoted as sinusoidal and cosine phase rotation, are proposed here to achieve the cyclic delay diversity (CDD). Finally, we evaluate the performance of our proposed method in DAB systems, and confirm that they have well and accurate operation in a standard indoor and outdoor environment.