非正交多重接取（non-orthogonal multiple access；NOMA）為一具有潛力的技術，可以處理下一代無線通訊系統大量數據增長的問題。在本論文中，我們考量一個在非完美通道估測情境下運作的多細胞巨量多輸入多輸出（MIMO） NOMA下行系統，其中每個細胞包含一個基地台和多個使用者群組，每個群組由兩個使用者組成，且每個使用者會遭受群組間干擾和細胞間干擾；為了克服這些干擾問題，我們提出強健式預編碼與解碼技術。首先，基於方塊對角化技術設計預編碼器，將多群組MIMO-NOMA通道分解成多個平行的單群組MIMO-NOMA通道，以消除群組間干擾。然後，建構一聯合預編碼與解碼的最佳化問題，以最小化所有使用者端之NOMA訊號的解碼均方誤差（僅考慮雜訊）以及對應的細胞間洩漏訊號總和，進而消除細胞間干擾；此一最佳化問題之解可經由迭代方式不斷更新，以提升效能。接著，推導出一具有低複雜度的使用者功率分配閉合解，以最大化最小的使用者近似容量。最後，基於最小化均方誤差準則設計每一群組中的使用者解碼器，以分離兩個使用者的訊號。電腦模擬結果顯示，結合上述方法之設計不僅可達到良好的平均位元錯誤率，而且可提供NOMA使用者間的位元錯誤率公平性；另外，在正規化通道均方誤差小於0.02的情況下，位元錯誤率的變化並不明顯，這也驗證所提出之設計在非完美通道估測情境下具有強健效能。

Non-orthogonal multiple access (NOMA) is a promising technique that can deal with the massive data growth issues for next-generation wireless communication systems. In this thesis, we consider a multicell downlink massive multiple-input multiple-output (MIMO) NOMA system with imperfect channel estimation. Each cell consists of one base station and multiple clusters, where each cluster contains two users and each user suffers from both intercluster interference and intercell interference. To overcome the interference problems, some robust precoding and decoding techniques are proposed for the system. First, block-diagonalization-based precoders are used to decompose a multiple-cluster MIMO-NOMA channel into multiple parallel single-cluster MIMO-NOMA channels for removing the intercluster interference. Then, a joint precoding and decoding optimization problem is formulated in order to minimize the sum of mean-squared decoding errors (due to noise only) of the desired NOMA signals and the corresponding leakages from one cell to the others for removing the intercell interference. The solution to the optimization problem can be updated iteratively for performance improvement. Subsequently, a low-complexity closed-form solution is derived for power allocation between both users of each cluster to maximize the minimum of an approximated user capacity. Finally, a set of user signal decoders are designed based on the minimum mean-squared error (MSE) criterion to separate the two users’ signals for each cluster. Computer simulation results demonstrate that the proposed design with all the above methods not only achieves excellent performance in terms of the average bit error rate (BER) but also offers BER fairness among NOMA users. Moreover, the BER variations are not significant for normalized channel MSEs smaller than 0.02, and this supports that the proposed design has robust performance under imperfect channel estimation.

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