During the last decade, orthogonal frequency division multiplexing (OFDM) has become the core technology of various modern wireless communication systems due to its robustness against multipath fading as well as high spectrum efficiency. Recently, cooperative multiple-input multiple-output (MIMO) that coordinates multiple distributed single-antenna devices has received increasing attention for its feasibility as compared with centralized MIMO. Since the cooperating nodes are distributed in space, their signals thus arrive at the receiver on different timing and are characterized by distinct carrier frequency offsets (CFOs). Consequently, synchronization becomes an even more challenging task for cooperative MIMO systems than centralized MIMO configurations.
In this thesis, we propose two novel subband training sequence designs, along with synchronization schemes, for cooperative MIMO OFDM systems. In Chapter 3, we first propose a training sequence arrangement based on Gaussian pulses. The MIMO training signals are deliberately designed to be separated in both time domain and frequency domain, whereas conventional training signals are only separated either in time domain (time-division-type) or in frequency domain (frequency-division-type) to mitigate mutual interference. The time-domain separation of the proposed MIMO training signals enables joint estimations of multiple timing offsets and CFOs without the aid of bandpass filters, while the frequency-domain separation feature can assist efficient integral CFO acquisition as well as channel estimation. In addition, we provide a guideline for selecting a proper correlation window size to effectively reduce computational complexity as well as to maintain satisfactory performance of the proposed synchronization scheme. On the other hand, the proposed MIMO training signals inherit a high peak-to-average power ratio (PAPR) from Gaussian pulses, and may suffer from considerable distortions after a practical power amplifier (PA). To address this problem, we discuss how to reduce the PAPR of the proposed MIMO training signals, and the impact of PA on the proposed synchronization scheme is evaluated through computer simulations. It is shown that the proposed scheme outperforms two conventional schemes under a practical PA, especially when more cooperating nodes are active. In Chapter 4, we proceed to present another subband MIMO training sequence arrangement based on partial Zadoff-Chu sequences, which not only has much lower PAPR than the Gaussian-pulse-based design, but also preserves both time-domain and frequency-domain training signal separations. Accordingly, the same synchronization scheme developed in Chapter 3 can work with the modified training sequence design directly. Through simulations under a practical PA, the superiority of the modified training sequence design based on partial Zadoff-Chu sequences is verified.

近年來，正交分頻多工（OFDM, orthogonal frequency-division multiplexing）因其對於頻率選擇性通道的高抵抗性及高頻譜使用效率，已經成為眾多無線通訊協定的核心技術。另一方面，由於特殊應用需求或成本考量，有別於集中式多輸出多輸入（centralized MIMO, centralized multiple-input multiple-output）技術，聯合多個單天線裝置以構成虛擬天線陣列的合作式多輸出多輸入（cooperative MIMO）技術正受到產、學、研各界的極大關注。由於參與合作的各節點分散於空間中，其所傳送之訊號將於不同時間點抵達接收端，並受到不同的載波頻率偏移（CFO, carrier frequency offset）所影響。因此，相較於centralized MIMO系統，cooperative MIMO系統的同步技術開發實更具挑戰性。
在此論文中，我們針對cooperative MIMO系統提出兩種新的子頻帶訓練序列設計和同步架構。在第三章中，我們首先提出一種基於高斯脈波的訓練序列架構；有別於傳統訓練訊號只在時間或頻率上作區隔，此新訓練序列架構在時間與頻率上均被隔開，以顯著降低訓練序列間的交互干擾。此外，由於我們所提出的訓練序列架構於時間上有所區隔，不需帶通濾波器之輔助即可用以估測多組時間與頻率偏移，同時亦可利用頻率上之區隔特性完成整數CFO的調整與通道估測。為了降低運算複雜度且維持一定的同步效能，我們亦提供適當選擇同步演算法互相關視窗大小的方法。另一方面，我們提出的訓練訊號繼承了高斯脈波的高功率峰均比（PAPR, peak to average power ratio）的特性，經過實際功率放大器之後可能會遭受相當大的失真。針對此問題，我們對如何降低此訓練序列的PAPR進行討論，並藉由模擬呈現實際功率放大器所造成的影響。整體而言，我們提出的同步架構優於傳統作法，尤其當參與合作的節點數越多時，其效能改善更為顯著。在第四章中，我們提出另一種基於不完全的Zadoff-Chu序列的子頻帶訓練序列，此設計除同時在時域與頻域分隔之外，亦有適中之功率峰均比。因此，在我們於第三章中發展的同步演算法可直接套用在此訓練序列架構上。電腦模擬結果顯示，基於不完全Zadoff-Chu序列的訓練序列設計優於基於高斯脈波之設計。

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