現行無線通訊系統中，直接轉換器因有低成本、低功率、體積小等優點而被廣泛使用；然因元件誤差導致實虛部不平衡之問題，以致影響系統性能。實虛部不平衡可分為頻率獨立實虛部不平衡和頻率相依實虛部不平衡，其中頻率相依實虛部不平衡又因實虛部濾波器的頻率響應不相同而嚴重降低寬頻系統的性能。本論文針對傳送與接收端頻率相依實虛部不平衡同時存在之情況，提出估計補償電路參數之方法。作法為將手機自身的傳送端和接收端連接起來，如此可使用自身接收端進行下轉換，並且接收訊號可當作已知的訓練序列，故不需設計額外之回授電路或訓練序列。接著，使用最小平方估計法估計接收端補償電路之參數。啟動補償電路後，即可將接收端視為理想模型，再利用接收訊號以最小平方估計傳送端補償電路之參數。模擬結果顯示此方法對於頻率相依實虛部不平衡的問題有著顯著的改善。

Direct conversion radio has been widely employed in today’s wireless devices because it facilitates a low-cost, low-power and small-sized design. Because of component mismatches, however, it suffers from I/Q imbalance (IQI) which degrades performance. I/Q imbalance can be classified into two categories: frequency-independent IQI (FI IQI) and frequency-dependent IQI (FD IQI). FD IQI includes the effect of mismatches in the frequency responses of the lowpass filters at the I and Q branches, and can seriously limit the performance of broadband communication systems. In the thesis, we address the issue of calibrating FD IQI by proposing a method that estimates the calibration parameters when FD IQI is present at both the transmitter and the receiver. The proposed method operates by connecting the mobile device’s own transmitter and receiver, thus eliminating the needs of both extra, dedicated loopback circuits and specially designed training sequences. We first use least-squares estimation to determine the parameters of the receiver compensator. Once the receiver compensator is activated, the receiver can be regarded as IQI-free. Next, we use least-squares estimation to find the parameters of the transmitter predistorter. The effectiveness of the proposed algorithms is verified by the simulations.

[1] C.-J. Hsu and W.-H. Sheen, “Joint calibration of transmitter and receiver impairments in direct-conversion radio architecture,” IEEE Trans. Wireless Commun., vol. 11, no. 2, pp. 832–841, Feb. 2012.
[2] J. Luo, A. Kortke, and W. Keusgen, “Joint calibration of frequency selective time variant I/Q-imbalance and modulator DC-offset error in broadband direct-conversion transmitters,” in Proc. IEEE ICCCAS, San Jose, CA, Jun. 2009, pp. 255–259.
[3] J. Luo, A. Kortke, W. Keusgen, M. Valkama “A novel adaptive calibration scheme for frequency selective I/Q imbalance in broadband direct-conversion transmitters,” IEEE Trans. on Circuits and Systems II: Express Brief, vol. 60, pp. 61-65, March 2013.
[4] J. Luo, A. Kortke, W. Keusgen, and M. Valkama, “Efficient estimation and pilot-free online re-calibration of I/Q imbalance in broadband direct-conversion transmitters,” IEEE Trans. Veh. Technol., vol. 63, no. 6, pp. 2506–2520, Jul. 2014.
[5] Z. Zhu, X. Huang, and H. Leung, “Joint I/Q mismatch and distortion compensation in direct conversion transmitters,” IEEE Trans. Wireless Commun., vol. 12, no. 6, pp. 2941–2951, Jun. 2013.
[6] Y. Li, et al., “A new method to simultaneously estimate TX/RX IQ imbalance and channel for OFDM systems,” in IEEE ICC, June 2013, pp. 4551–4555.
[7] X. Cheng, Z. Luo, and S. Li, ‘‘Joint estimation for I/Q imbalance and multipath channel in millimeter-wave SC-FDE systems,’’ IEEE Trans. Veh. Technol., vol. 65, no. 9, pp. 6901–6912, Sep. 2016.
[8] F. L. Luo, editor. Digital Front-End in Wireless Communication and Broadcasting. Cambridge University Press, 2011, pp. 475-485.
[9] S. S. Bhattacharyya, et al., editors. Handbook of Signal Processing Systems. 2nd ed., Springer, 2013, pp. 194-215.
[10] U. L. Rohde, Microwave and Wireless Synthesizers: Theory and Design. John Wiley Sons, 1997.
[11] G. Xing, M. Shen, and H. Liu, “Frequency offset and I/Q imbalance compensation for direct-conversion receivers,” IEEE Trans. Wireless Commun., vol. 4, no. 2, pp. 673–680, Mar. 2005.
[12] T. C. Lan, “Estimation of Multipath Channel and I/Q Imbalance in IEEE 802.11ad-based SC-FDE System,” Institute of Computer and Communication Engineering National Cheng Kung University Thesis for Master of Science, July 2017.
[13] U. Madhow, Fundamentals of Digital Communication. New York: Cambridge University Press, 2008.
[14] Adachi, F., H. Tomeba, and K. Takeda, “Introduction of frequency-domain signal processing to broadband single-carrier transmissions in a wireless channel,” IEICE Transactions on Communicationns, Vol. E92-B, No. 9, 2789–2808, 2009.
[15] J. C. Syu, “Estimation of Calibration Parameters for Frequency-Dependent I/Q Imbalance in Direct Conversion,” Institute of Computer and Communication Engineering National Cheng Kung University Thesis for Master of Science, July 2018.