Adaptive receivers for OFDM-CDMA Communication Systems in Multipath Fading Channels

正交分頻多工是一項有前景的多載波調制技術，由於它具備下列優點:高效能頻譜使用率、抗選頻衰減以及低成本傳送接收器的硬體實現可行度高。分頻多工及直接序列展頻分碼多工的結合，即OFDM-CDMA，近來在無線通訊的領域引起極大的研究興趣。本篇論文將利用最小均方差(Least-mean-square)及遞迴式最小方差(Recursive-least-square)兩種演算法來建構OFDM-CDMA 通訊系統之適應性接收器。我們將評估兩種不同型式的OFDM-CDMA 架構在多路徑衰減通道中下鏈端(Down-link)之接收性能，即多載波分碼多工以及多載波直接序列展頻分碼多工。然而，分頻多工架構的缺點是由於加上過長的循環式前置(Cyclic prefix)訊號而造成傳輸速率的下降，甚至嚴重地影響到多載波直接序列展頻分碼多工系統的通道使用率。因此我們將針對使用較短或者無循環式前置訊號的多載波直接序列展頻分碼多工系統，提出另一個結合頻域及時域等化的適應性接收器來提升性能。模擬結果將顯示多載波分碼多工以及多載波直接序列展頻分碼多工之間的性能比較，並且驗證針對使用較短或者無循環式前置訊號的多載波直接序列展頻分碼多工系統所提出之接收器的性能。在這篇論文當中，我們將探討在不同長度的循環式前置訊號下，將三種不同適應性偵測的架構，分別是：時域、頻域以及混合時域頻域的技術，運用於多載波直接序列展頻分碼多工的接收端。並針對多載波直接序列展頻分碼多工的三種不同的適應性偵測技巧做下鏈端接收性能以及硬體實現複雜度的比較跟評估。Orthogonal frequency division multiplexing (OFDM) is a promising scheme for multicarrier (MC) transmission, since it has the advantage of high spectral efficiency, robustness to frequency selective fading, and feasibility of low-cost transceiver implementation. The combination of OFDM and direct-sequence code division multiple access (DS-CDMA), referred to as OFDM-CDMA, recently attracted most research interests in wireless communications. In this paper, adaptive receivers for OFDM-CDMA communication systems will be developed based on the least-mean-square (LMS) algorithm and recursive-least-square (RLS) algorithm. We will evaluate the down-link performance for two forms of OFDM-CDMA schemes, namely, MC-CDMA and MC-DS-CDMA, under a multipath fading channel. A drawback of OFDM schemes is the rate reduction due to the cyclic prefix (CP) overhead, which significantly affects the channel utilization in MC-DS-CDMA systems. Thus we will propose another adaptive receiver structure that exploits both frequency-domain and time-domain equalization to improve the performance of MC-DS-CDMA systems with shorter or no CP. Simulation results show performance comparison between MC-CDMA and MC-DS-CDMA systems, which also verify the improved receiver for MC-DS-CDMA systems exploiting shorter or no CP. In this paper, we will employ three different adaptive detection schemes: time-domain, frequency-domain, and time- and frequency- domain hybrid schemes respectively, in the receivers of MC-DS-CDMA communication systems under the two types of conditions : the length of CP is longer than the normalized channel delay spread and no CP is exploited. The comparison of downlink performance and hardware implementation complexity will be made among different adaptive detection schemes for MC-DS-CDMA systemsContents Chinese Abstract i English Abstract iii Acknowledgments v Contents vi List of Tables viii List of Figures viii Chapter 1: Introduction 1 Chapter 2: Introduction to OFDM Schemes 4 Chapter 3: System Model 6 3.1 Multicarrier CDMA Transmission Systems 6 3.2 Multicarrier DS-CDMA Transmission Systems 8 Chapter 4: Proposed Adaptive Receivers for OFDM-CDMA Systems 11 4.1 Adaptive Receivers for Multicarrier CDMA Systems 11 4.1.1 LMS-based Algorithm 13 4.1.2 RLS-based Algorithm 15 4.2 Adaptive Receivers for Multicarrier DS-CDMA Systems 20 4.2.1 Proposed Adaptive Receivers for MC-DS-CDMA Systems Exploiting Longer Cyclic Prefix 21 4.2.2 Proposed Adaptive Receivers for MC-DS-CDMA Exploiting Shorter or No Cyclic Prefix 23 Chapter 5: Simulation Results 32 5.1 Indication of System Parameters 32 5.2 Numerical Results 33 Chapter 6: Conclusion 47 Bibliography 4