合作式通訊（Cooperative communications）是近年來很熱門的一種傳輸技術，他可以克服在無線傳輸中的通道衰減，以及有效地利用在多使用者環境中存在的空間分歧性（spatial diversity）。將合作式通訊的概念應用在正交分頻多工（orthogonal frequency division multiplexing，簡稱OFDM）系統，則可再利用頻域上面的自由度以得到更好的系統表現。在合作式OFDM系統中，有效率地利用在頻域和空間上的自由度是必須的，然而當子載波（subcarrier）數目很多時，資源分配將會面臨到複雜度很高的困難。因此在此論文中，我們探討並分析在合作式OFDM系統中的資源分配的議題，並提出有效率的資源分配演算法。此外在做資源分配時，我們需要知道通道的資訊，如果沒有精確的通道估計，會使系統的表現被估計誤差所影響，可能導致透過合作所能得到的分歧性增益（diversity gain）消失。在此論文中，我們針對合作式OFDM系統，提出了（1）有效率的子載波分配、功率分配、及合作對象選擇的演算法，及（2）利用回饋通道輔助指符（pilot）位置設計以增強通道估計的表現。

在本篇論文的第一部份中，我們探討合作式OFDM系統中的資源分配的問題，包括子載波分配、功率分配、及合作對象選擇的議題。當使用者及子載波數目很大時，合作對象選擇和子載波分配的設計會因複雜度過高而變得很困難，因此我們提出了一個簡化的可分析模型來探討功率分配的影響，並利用此模型的分析結果提出了有效率的子載波分配和合作對象選擇演算法。在論文的第二部份中，我們探討透過回饋機制（feedback）來輔助指符（pilot）位置設計的研究。傳送端可以利用回饋的通道資訊來調整指符位置，使通道估計誤差變小，並可讓通道情況較好的子載波來傳送資料（data）訊號。我們定義和分析回饋增益（feedback gain），並提出了有效率地利用回饋通道資訊來調整指符位置的演算法。模擬結果顯示此篇論文中提出的資源分配及指符位置設計的演算法可有效率地提升系統表現。

Cooperative communications have emerged as a popular technique to combat fading and to exploit spatial diversity gains inherent in wireless multiuser systems. When combined with orthogonal frequency division multiplexing (OFDM), additional degrees of freedom in frequency can be further exploited to enhance system performance. In cooperative OFDM systems, efficient resource allocation among users and subcarriers is necessary to fully utilize the available degrees of freedom in both frequency and space. However, the dimensions of the problem often pose great challenges in determining the optimal design and, thus, efficient and innovative approaches must be proposed to address these issues. Moreover, to perform these resource allocation policies, accurate channel state information must be obtained at the terminals. Without accurate channel estimates, the system performance will be dominated by channel estimation errors, causing potential diversity gains to diminish. The main contribution of this thesis is to propose (1) an innovative approach to determine efficient subcarrier allocation, power allocation, and partner selection policies for cooperative OFDM systems and (2) a feedback-aided pilot placement scheme to help enhance channel estimation performance.

Specifically, in the first part of the thesis, subcarrier allocation, power allocation, and partner selection algorithms are examined for cooperative multicarrier systems. In general, subcarrier allocation and partner selection problems are known to be intractable for systems with large number of users and subcarriers. In this thesis, a suboptimal but mathematically tractable approach is proposed to address these issues. This is done by considering the power allocation over a relaxed problem formulation, where each subcarrier is allowed to retransmit or relay for all other subcarriers in the cooperative phase over virtual orthogonal channels. Based on the results of the relaxed power allocation problem, an efficient subcarrier allocation algorithm for the original problem can then be derived by allowing subcarriers to forward the data that is allocated the most power. The results of the relaxed problem also yield a necessary and sufficient condition on when cooperation is helpful among two users, and the partner selection scheme is then devised based on
the criterion given in the condition.

In the second part of the thesis, the use of channel feedback to better determine pilot placement in cooperative OFDM systems is examined. With channel feedback from previous symbol periods, pilot locations can be determined to reduce channel estimation errors and to make available favorable subcarriers for data transmission. Based on the feedback gain, which is defined as the ratio between the minimum effective SNR achieved with the optimal feedback-aided pilot placement scheme and that achieved with the
equal-spaced pilot placement scheme, we provide an analytic lower bound on the optimal feedback gain and show its rate of increase with respect to the number of pilot subcarriers. Due to the complexity of finding the optimal placement, a suboptimal but efficient algorithm, called the iterative pilot relocation (IPR) scheme, is proposed. The analysis and the proposed IPR scheme are also extended to systems with subcarrier pairing at the relay and also to systems with multiple relays. The efficacy of the proposed
resource allocation and pilot placement schemes are demonstrated through numerical simulations.

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