Design of Computationally-Efficient Flexible MIMO Receivers with Just Acceptable Error Rate

本論文提出一全新概念以節省多輸入多輸出(multiple-input multiple-output)偵測器的平均運算成本。偵測器將依據通道估測結果動態即時選擇偵測演算法，以求達到各種標準要求之恰可接收錯誤率(just acceptable error rate)。例如在通道狀況良好時使用簡單如強制歸零的演算法，反之在通道狀況惡劣時則得使用搜尋式的偵測法。 為了實現此概念，本論文大量分析各種多輸入多輸出偵測演算法之校能，並提出距離晶格(delta lattice)的概念來降低預估演算法校能之複雜度，因為此預估必須實際於接收機上執行，複雜度是一重要考量。根據分析結果，本論文提出一通道可調整多輸入多輸出接收機(channel-adaptive MIMO receiver)，配合偵測器切換機制，可達到上述恰可接收錯誤率之要求。此可調整多輸入多輸出接收機運作範圍同等於最複雜的球面解碼偵測器，但平均功率則遠低於前者。 本論文將此概念延伸至遞迴接收機(iterative receiver)。由於遞迴接收機設計遠比非遞迴系統複雜，本論文先設計出每一迴圈都可達到最佳錯誤率的高效能演算法，稱之為混合列舉演算法(hybrid enumeration)；接著降低複雜度為每一迴圈無法達到最佳錯誤率、但最終可達到最佳收斂錯誤率的演算法，稱之為距離晶格偵測器(delta lattice detector)。之後再將恰可接收錯誤率的概念套至遞回接收機，利用球面解碼偵測器配合兩種不同參數，運作時間限制(run-time constraint)、軟性輸出上限(log-likelihood ratio clipping)，動態調整此兩參數以達到恰可接收錯誤率。模擬結果顯示，恰可接收錯誤率可將不需要、過好的錯誤率轉換成高達90%以上的複雜度節省。In this thesis, we propose a new concept that saves the average computational cost of multiple-input multiple-output (MIMO) detection. The detection methods are adapted on-the-fly according to the estimated channel state information subject to just-acceptable error rate (JAER) performance specified by standards, rather than striving to optimal error rates. For example, it is natural to employ only low-complexity MIMO detection solutions, such as ZF detection or OSIC, under "good" channel scenarios as they achieve acceptable performance with less computational complexity. On the other hand, under "poor" channel conditions, the search-based MIMO detectors seem to be indispensable. To achieve this goal, error rate models for various MIMO detection algorithms are derived. As these analyses are performed on-the-fly, we apply the proposed delta lattice that simplifies their computations. From these models, a channel-adaptive-MIMO (CA-MIMO) receiver with detector-switching strategies for non-iterative receiver is roposed. Compared with the sophisticated sphere decoder (SD) MIMO detector, the average saving of the CA-MIMO at moderate signal-to-noise ratio (SNR) is around 65% to 76%, depending on different modulation alphabets. At high SNR, the CA-MIMO detector almost switches to simple detection methods, where its complexity is several orders lower than the SD. Furthermore, we reduce the complexity of iterative receiver inevitable for low SNR regime. We follow the Divide-and-Conquer principle. An efficient algorithm of soft-input SD is first proposed that achieves optimal error rate at every iteration. Then, the delta lattice mentioned perviously is adopted to realize a iterative MIMO detector, namely the delta lattice detector(DLD). This DLD is regarded as an exemplar to show that in the iterative scenario, the optimal error rate performance is achieved without performing optimal algorithms at every iterations. Last, with the theoretical error rate analysis for iterative MIMO systems, the JAER concept translates the unnecessary error rate performance to more than 90% saving by imposing the LLR clipping trick