在本論文中，我們考量了兩個正交分頻多工系統中的非理想效應：載波頻率偏移(Carrier Frequency Offset，CFO)與IQ不平衡(IQ imbalance)。此兩種非理想效應都會嚴重的降低系統效能。現有的聯合估測演算法通常需要做迭代運算才能在估測中收斂，或是其計算複雜度相當高。因此，有研究者根據長序文內兩段相同的序列，提出一個以最小平方法(Least Squares method)為基礎的演算法來聯合估測載波頻率偏移與IQ不平衡。不過其演算法在載波頻率偏移小於15 kHz時，對於CFO與IQ imbalance估測具有相當程度的誤差，進而造成位元錯誤率(bit error rate，BER)的提高，因而在本篇論文中，藉著合併長短序文的使用，利用最小平方法與相關器，分別的估測IQ imbalance與CFO，來處理CFO小於15 kHz的情形。模擬證明，當CFO小於15 kHz時，我們的演算法將有效降低位元錯誤率。另外，由於LS-based演算法利用長序文做CFO與IQ不平衡的聯合估測，因此，當CFO超過上下限±156.25 kHz時，對於CFO與IQ不平衡的估測將產生錯誤。在本篇論文中，我們利用短序文實現的聯合估測演算法來處理這個問題，擴大CFO與IQ不平衡的估測範圍。

In this thesis, we consider two non-ideal effects in orthogonal frequency division multiplexing (OFDM) system: carrier frequency offset (CFO) and IQ imbalance. Both CFO and IQ imbalance degrade the system performance seriously. Current joint estimation algorithms need iterations to converge in the estimation, or their computational complexity are considerably high. As a result, researchers proposed a LS-based method to estimate both CFO and IQ imbalance based on the two identical sequences in the preamble. But LS-based method has considerable deviations in estimating CFO and IQ imbalance while actual CFO is below 15 kHz, therefore it will increase the bit error rate. In this thesis, we deal with this problem by using correlator and least squares method to estimate CFO and IQ imbalance separately. Besides, LS-based method uses long preamble to joint estimate CFO and IQ imbalance. Therefore, while CFO is beyond ±156.25 kHz, the joint estimation will be inaccurate. In this thesis we also deal with this problem by short preambles, and the range of joint estimation can be extended.

[1] J. Heiskala and J. Terry, OFDM Wireless LANs: A Theoretical and Practical Guide, Sams, Indianapolis, Indiana, 2002.
[2] F. Horlin and A. Bourdoux, Digital Compensation for Analog Front-Ends: A New Approach to Wireless Transceiver Design, MA: John Wiley, 2008.
[3] IEEE Std 802.11a-1999-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: High-Speed Physical Layer in the 5-GHz Band, 1999.
[4] T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol. 45, no. 12, pp. 1613-1621, Dec. 1997.
[5] ETSI, “Broadband Radio Access Networks (BRAN); HIPERLAN type 2 technical
specification; Physical (PHY) layer,” April 2000.
[6] P. H. Moose, “A technique for orthogonal frequency division multiplexing frequency offset correction,” IEEE Trans. Commun., vol. 42, no. 10, pp. 2908-2914, Oct. 1994.
[7] A. A. Abidi, “Direct-conversion radio transceivers for digital communications,” IEEE J. Solid-State Circuits, vol. 30, no. 12, pp. 1399-1410, Dec. 1995.
[8] J. Tubbax, A. Fort, L. Van der Perre, S. Donnay, M. Engles, M. Moonen, and H. De Man, “Joint compensation of IQ imbalance and frequency offset in OFDM systems,” in Proc. IEEE Globecom, Dec. 2003, pp. 2365-2369.
[9] J. Y. Yu., M. F. Sun, T. Y. Hsu, and C. Y. Lee, “A novel technique for I/Q imbalance and CFO compensation in OFDM systems,” in Proc. IEEE ISCAS, May 2005, pp. 6030-6033.
[10] S. Fouladifard and H. Shafiee, “Frequency offset estimation in OFDM systems in presence of IQ imbalance,” in Proc. IEEE ICC, May 2003, vol. 3, pp. 2071-2075.
[11] F. Horlin, A. Bourdoux, L. Van der Perre, “Low-complexity EM-based joint acquisition of the carrier frequency offset and IQ imbalance,” IEEE Trans. Wireless Commun., vol. 7, no. 6, pp. 2212-2220, June 2008.
[12] 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.
[13] K. Y. Sung, C. C. Chao, “Estimation and compensation of I/Q imbalance in OFDM direct-conversion receivers,” IEEE Trans. Signal Processing, vol. 3, no. 3, pp. 438-453, June 2009.
[14] S. De Rore, E. Lopez-Estraviz, F. Horlin, and L. Van der Perre, “Joint estimation of carrier frequency offset and IQ imbalance for 4G mobile wireless systems,” in Proc. IEEE ICC, June 2006, pp. 2066-2071.
[15] F. Horlin, S. De Rore, E. Lopez-Estraviz, F. Naessens, and L. Van der Perre, “Impact of frequency offsets and IQ imbalance on MC-CDMA reception based on channel tracking,” IEEE J. Select. Areas Commun., vol. 24, no. 6, June 2006.
[16] Sebastien Simoens, Marc de Courville, Francois Bourzeix, Paul de Champs, “New I/Q imbalance modeling and compensation in OFDM systems with frequency offset,” Personal, Indoor and Mobile Radio Communications, vol. 2, pp. 15-18, Sept. 2002.
[17] J. Tubbax, B. Come, L. Van der Perre, L. Deneire, S. Donny, M. Engels, “Compensation of IQ imbalance in OFDM systems,” in Proc. IEEE Int. Conf. Commun., May 2003, pp. 3403-3407.
[18] B. Razavi, “Design consideration for direct-conversion receivers,” IEEE Trans. Circuit Syst. II, vol. 44, no. 6, pp. 428-435, June 1997.
[19] E. Tsui and J. Lin, “Adaptive IQ imbalance correction for OFDM systems with frequency and timing offsets,” in Proc. IEEE Global Telecommun. Conf., Nov. 2004, pp. 4004-4010.
[20] G. T. Gil, I. H. Sohn, J. K. Park, and Y. H. Lee, “Joint ML estimation of carrier frequency, channel, I/Q mismatch, and DC offset in communication receivers,” IEEE Trans. Veh. Technol., vol. 54, no. 1, pp. 338-349, Jan. 2005.
[21] H. Lin, T. Adachi, and K. Yamashita, “Carrier frequency offset and I/Q imbalances compensation in OFDM systems,” in Proc. IEEE Global Telecommun. Conf., Nov. 2007, pp. 2883-2888.
[22] M. Valkama, M. Renfors, and V. Koivunen, “Advanced methods for I/Q imbalance compensation in communication receivers,” IEEE Trans. Signal Process., vol. 49, no. 10, pp. 2335-2344, Oct. 2001.
[23] M. Luise and R. Reggiannini, “Carrier frequency acquisition and tracking for OFDM systems,” IEEE Trans. Commun., vol.44, pp. 1590-1598, Nov. 1996.
[24] C. H. Hsu, C. F. Wu and C. K. Wang, “FPGA prototype for WLAN OFDM baseband with STPE of I/Q mismatch self-calibration algorithm,” in Proc. Asian Sold-state Circ. Conf., pp. 509-512, Nov. 2005.
[25] T. Pollet, M. van Bladel, and M. Moeneclaey, “BER sensitivity of OFDM systems to carrier frequency offset and wiener phase noise,” IEEE Trans. Commun., vol. 43, pp. 191-193, Apr. 1993.
[26] D. Tandur and M. Moonen, “Joint adaptive compensation of transmitter and receiver IQ imbalance under carrier frequency offset in OFDM-based systems,” IEEE Trans. Signal Process., vol. 55, no. 11, pp. 5246-5252, Nov. 2007.
[27] S. L. Su and Y. J. Chiu, “Adaptive IQ imbalance compensation scheme with frequency offset for communication channel,” in Proc. Consumer Commun. Netw. Conf., Jan. 2006, pp. 1038-1042.
[28] H. Lin, Xu Zhu and K. Yamashita, “Pilot-aided low-complexity CFO and I/Q imbalance compensation for OFDM systems,” in Proc. IEEE ICC, May 2008, pp. 713-717.
[29] A. Tarighat, R. Bagheri, and A. H. Sayed, “Compensation schemes and performance analysis of IQ imbalance in OFDM receivers,” IEEE Trans. Signal Process., vol. 53, no. 8, pp. 3257-3268, Aug. 2005.
[30] C. Muschallik, “Influence of RF oscillators on an OFDM signal,” IEEE Trans. Consumer Electron., vol. 41, pp. 592-603, Aug. 1995.
[31] N. T. Hieu, H. G. Ryu, C. X. Wang, and H. H. Chen, “The impact of the I/Q mismatching errors on the BER performance of OFDM communication systems,” in Proc. IEEE Int. Conf. Commun., Jun. 2007, pp. 5423-5427.
[32] J. R. Liang and C. H. Kuo, “Ls-based joint estimation of carrier frequency offset and iq imbalance in ofdm systems,”in 2010 International Symposium on Next-Generation Electronics (ISNE), Nov.2010, pp. 52 –55.
[33] B. Razavi, "Design considerations for direct-conversion receivers", IEEE Trans. Circuits Syst. II, vol. 44, pp.428 -435 1997