單載波區塊傳送的頻域線性等化器，在嚴重的頻率選擇性衰減通道會有很差的效能，非線性的決策回授等化是改善此問題的常見方法之一。在本篇論文中，我們分成兩部分來研究此議題。在第一部分，我們提出混合式雙重決策回授等化器來降低混合式決策回授等化器所產生的錯誤蔓延。其作法是先偵測出低可靠度的資料符元，然後延遲一段時間將後來接收的資料納入考慮才做出決策，以增加準確度。模擬結果顯示，我們所提出的方法之效能比傳統的混合式決策回授等化器較佳。
在第二部分中，我們探討迭代區塊決策回授等化器。硬式迭代區塊決策回授等化器是將前次迭代所產生的符元全部當作回授的輸入，但是會有可靠度低的符元進入回授，導致降低效能，而且濾波器的設計需要額外的複雜度來做參數估測。為了克服這些缺點，我們提出軟式迭代區塊決策回授等化器，其作法是先設置門檻值，不讓可靠度低的符元進入回授，而且參數估測非常簡單。模擬結果顯示提出的軟式迭代區塊決策回授等化器之效能比硬式的較佳。除此之外，在分散性高的通道下，軟式迭代區塊決策回授等化器會比現有的決策回授等化器有更佳的效能。

Single-carrier block transmission (SCBT) with frequency-domain linear equalizer suffers from considerable performance degradation in severely frequency-selective fading channels. Nonlinear decision-feedback equalization (DFE) is an effective method of solving this problem. In this thesis, we discuss this issue in two parts. In the first part, we propose a hybrid dual decision-feedback equalizer (H-DDFE) to mitigate the error propagation effect of the hybrid decision-feedback equalizer (H-DFE). Specifically, low-reliability data symbols are identified first; the decisions of these symbols are deferred for some time to take into account the signals received later to improve the accuracy of the decisions. Simulation results show that the proposed method has a better performance than H-DFE.
In the second part, we discuss the iterative block DFE (IBDFE). For simplicity, IBDFE with hard detection (HD-IBDFE) uses all the hard detected data symbols from the previous iteration as the input of the feedback filter, but the low-reliability data symbols entering the feedback filter results in performance degradation. In addition, the design of filters needs parameters estimation, which incurs additional complexity. To overcome these drawbacks, we propose IBDFE with soft detection (SD-IBDFE), which uses the threshold to keep low-reliability data symbols from being used for decision feedback. Moreover, parameters estimation in SD-IBDFE is simple. Simulation results show that the proposed SD-IBDFE has a better performance than HD-IBDFE, and in highly dispersive channels, outperforms existing DFEs.

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