在嚴重的頻率選擇性衰減通道，單載波區塊傳送的頻域線性等化器之效能不佳，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，我們探討迭代式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硬式決策符元全部當作回授的輸入，不過會有可靠度低的符元進入回授，容易導致效能降低，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，除了文獻中提出軟式迭代區塊決策回授等化器中的無效區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。本論文進一步延伸，提出了數種軟式迭代區塊決策回授等化器，如未定區迭代區塊決策回授等化器、線性區迭代區塊決策回授等化器與條件期望值迭代區塊決策回授等化器。其中未定區迭代區塊決策回授等化器與線性區迭代區塊決策回授等化器作法為:先設置門檻值，讓可靠度低的符元依比例或是線性部分進入回授。條件期望值迭代區塊決策回授等化器的決策回授估計值，則是以最小均方誤差條件期望值的方式去估計。另外，我們亦針對未定區迭代區塊決策回授等化器提出效能更佳且運算複雜度較低之參數估計方法，參數估計方法也能用於硬式迭代區塊決策回授等化器。模擬結果顯示，我們提出的幾種軟式迭代區塊決策回授等化器相較於硬式迭代區塊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), such as iterative block DFE with hard decision (HD-IBDFE), was proposed to tackle this problem. For simplicity, HD-IBDFE always uses the hard decisions from the previous iteration as the input of the feedback filter (FBF), but low-reliability decisions entering FBF limits the performance. In addition, the design of equalizer coefficients needs parameter estimation, which incurs additional complexity. To overcome these drawbacks of HD-IBDFE, we propose IBDFE with various soft decision devices (SD-IBDFE), by extending the idea of IBDFE with null zone (NZ-IBDFE) in the literature, which uses the thresholds to keep low-reliability decisions from being used for decision feedback. We propose several kinds of SD-IBDFE, such as IBDFE with erasure zone (EZ-IBDFE), IBDFE with linear clipper (LC-IBDFE) and IBDFE with conditional mean (CM-IBDFE). When the equalized signal falls between certain thresholds (i.e., falling in the erasure interval), EZ-IBDFE will use the intermediate value of the erasure interval for feedback, while LC-IBDFE will map through a linear function to determine the feedback value. CM-IBDFE uses the minimum mean square error estimate of the symbol (i.e., the conditional mean) as the input of FBF. In the thesis, we also propose a new parameter estimation method for HD-IBDFE and EZ-IBDFE that is simpler and yet more accurate than the conventional one. Simulation results show that the proposed SD-IBDFE has a better performance than HD-IBDFE.

[1] Fumiyuki Adachi, Hiromichi Tomeba, and Kazuki Takeda, “Introduction of frequency-domain signal processing to broadband single-carrier transmissions in a wireless channel,” IEICE Trans. Commun., vol.E92-B, no.9 september 2009.
[2] G. D. Forney Jr., “Maximum likelihood sequence estimation of digital sequence in the presence of intersymbol interference,” IEEE Transactions on Information Theory, vol. 18, no. 3, pp. 363-378, 1972.
[3] D. Falconer, S.L. Ariyavistakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag., vol. 40, no. 4, pp. 58-66, April 2002.
[4] F. Pancaldi, G. M. Vitetta, R. Kalbasi, N. Al-Dhahir, M. Uysal, and H. Mheidat, “Single-carrier frequency domain equalization,” IEEE Signal Process. Mag., vol. 25, no. 5, pp. 37-56, Sep. 2008.
[5] N. Benvenuto and S. Tomasin, “On the comparison between OFDM and single carrier modulation with a DFE using a frequency domain feedforward filter,” IEEE Trans. Commun., vol. 50, no. 6, pp. 947-955, Jun. 2002.
[6] N. Benvenuto and S. Tomasin, “Iterative design and detection of a DFE in the frequency domain,” IEEE Trans. Commun., vol. 53, no. 11, pp. 1867-1875, Nov. 2005.
[7] C. Zhang, Z. Wang, C. Pan, S. Chen, L. Hanzo, “Low-Complexity Iterative Frequency Domain Decision Feedback Equalization,” IEEE Transactions on Information Theory, vol. 60, no.3, pp.1295-1301, Mar. 2011.

[8] J. Y. Pan, “Signal detection of single-carrier block transmission using frequency domain decision feedback equalizer,” Institute of Computer and Communication Engineering National Cheng Kung University Thesis for Master of Science, July 2011.
[9] C. Zhang, Z. Wang, Z. Yang, J. Wang and J. Song, “Frequency domain decision feedback equalization for uplink SC-FDMA,” IEEE Trans. Broadcasting, vol. 56, no. 2, pp. 253-257, Jun. 2010.
[10] Private Communications with J. Y. Pan, 2017.
[11] B. O'Hara and A. Petrick, IEEE 802.11 Handbook: A Designer's Companion, Standards Information Network IEEE Press, Dec. 1999.
[12] J. G. Proakis and M. Salehi, Digital communications, McGraw-Hill, New York, NY, USA, 5th edition, 2008.