在這篇論文中，我們針對了合作式非正交多重接取下行系統的功率分配以及使用者分組問題進行研究。此系統包含一個基地台與K個使用者，而所有使用者會各自配對並分配至N個群組中。在每個群組中，我們加入一個中繼階段來進行使用者的傳輸合作，而通道增益較大的使用者會在中繼階段被視為一個中繼點，並採用解碼轉傳機制來進行訊號傳輸。在這個系統中，我們考慮每個群組中的使用者之間的公平性，並利用最大化接收訊號中最小的接收訊雜比來當作目標函數，以進行兩個使用者與中繼點之間的功率分配。基於這個目標函數，我們將合作式非正交多重接取系統的功率分配及使用者分組之問題公式化，使得所有使用者的接收訊雜比之總和得以最大化。於是，我們在給定的單一群組中，提供一個最佳功率分配之閉合形式解，並使用此功率分配來發展兩種使用者分組的演算法。藉由探索單一群組中的兩個使用者之接收訊雜比行為，我們提出一個有效降低搜尋複雜度之演算法。同時，我們也提供一個基於地理位置資訊的分組演算法來更進一步降低額外的信令訊號。模擬結果顯示所提出之功率分配與使用者分組演算法可以明顯地提升系統性能，並在高訊雜比環境下達到接近竭盡式搜尋法的效能。

In this thesis, we investigate power allocation and user clustering problems for downlink cooperative non-orthogonal multiple access (NOMA) systems with a base station and K users paired into N clusters of two users each. For each cluster, a relaying phase is applied for user cooperation and the user with a stronger channel gain acts as a relay using the decode-and-forward strategy. Considering the users’ fairness in each cluster, we utilize the maximization of the minimum received signal-to-noise ratio (SNR) as the design criterion for power allocation among the two users and the relay. We formulate an optimization problem in terms of power allocation and user clustering for maximizing the sum of received SNRs of all the users in the cooperative NOMA system. To solve the optimization problem, an optimal power allocation solution under a given cluster is first derived in closed form and then used to develop two user clustering algorithms. By exploring the behavior of the received SNRs of the two users in a cluster, an effective algorithm with much lower complexity than exhaustive search is proposed for user clustering. Moreover, a geographic information-based algorithm is provided to reduce the system signaling overhead. Simulation results show that the proposed power allocation and user clustering algorithms yield significant performance gains in terms of the system rate, outage probability, and bit error rate over the scheme with fixed power allocation and exhaustive search for user pairing.

Absrtract................................................i
Contents................................................ii
List of Figures........................................iii
I. Introduction.....................................1
II. System Model and Problem Formulation.............5
A. NOMA Phase.......................................5
B. Relaying Phase...................................6
C. Problem Formulation..............................7
III. Proposed Optimal Power Allocation................8
IV. Proposed User Clustering........................12
A. Strong User Selection...........................12
B. Weak User Assignment............................13
C. Geographic Information-Based User Clustering....16
D. Complexity Comparison...........................17
V. Simulation Results..............................18
VI. Conclusion......................................26
References..............................................27

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