高密度分碼多重存取之正交分頻多工系统在多路徑快 速衰退通道中的信號檢測和通道估測

Abstract

正交分頻多工系統以能夠有效的對抗多重路徑落衰落通道且維持良好的傳輸速率的特點，在近幾年來被廣泛的運用在無線訊系統之中。然而，該系統在面對快速衰落通道時，都卜勒效應會對載波間彼此產生干擾，嚴重影響對通道進行結果，造成傳送出去的信號無法確的解調產生錯誤率較差的情形。在先前的研究中，提出了低延遲的迭代通道估測演算法，在解碼端能夠以錯誤更正碼解出來的有利資料提供通道估，來提升整體的精準度。而此通道估測的演算法同樣也能使用在具有空間多樣性的多天線系統，並加入了空頻區塊碼討論。 而本篇論文在前面的研究基礎上，延伸並做更進一步的研究。首先，從原先的單使用者環境將系統擴展到多使用者環境，加入高密度分碼多重存取，相比於低密度分碼多重存取，能夠增加單一時間的系統吞吐量。再者，若能容忍較長的延遲量，在解碼端我們提出了排程檢測法，透過降低每輪解碼的複雜度，改善錯誤率。最後，在該系統下加入及調整原有的通道估測演算法，使其適用在多個使用者環境之中，並提出適合該系統的削弱載波間干擾的方法。

Orthogonal Frequency Division Multiplexing (OFDM) systems have been widely used in wireless communication systems in recent years because of their ability to effectively combat multipath fading channels while maintaining high transmission rates. However, in the presence of fast fading channels, the Doppler effect can introduce interference between subcarriers in OFDM systems, significantly affecting channel estimation and leading to high error rates during demodulation. In previous studies, low-latency iterative channel estimation algorithms have been proposed to exploit the beneficial information provided by error-correcting codes in the decoding stage to improve the overall accuracy. These channel estimation algorithms can also be applied to multi-antenna systems with space diversity, in conjunction with the discussion on space-frequency block coding. In this thesis, we continue the work on OFDM systems in the multipath fading environment based on previous studies. For the first, we consider scaling the system from a single-user environment to a multi-user environment by incorporating High-Density-Spreading Code Division Multiple Access (HDS-CDMA) which enables increased system throughput in a single time slot as compared to the Low-Density-Spreading Code Division Multiple Access (LDS-CDMA). For the second, if longer latency can be tolerated, we propose the scheduling-based detection to help reduce the complexity of each round of decoding and improve the error rate. Finally, we integrate the channel estimation algorithm into this system to observe its performance and propose designed methods to mitigate inter-carrier interference (ICI) in this particular system.