取樣時間偏移量(STO)的估測與調整方法是本論文的研究主題，利用四點跟兩點關係值來做檢測的方法將在論文當中做介紹，我們提出了一個新的方法可以使得STO能夠適應性的做調整，在這方法當中，有著對於Roll Off強健性和增強關係量大小值的優點，並且利用線性逼近跟卡門濾波器來近一步改善系統效能，在檢測STO時引入一個條件門檻用來降低誤判的機會,在最後的模擬結果當中，可以證明，我們所以提出新的調整機制，可以有效的降低改善取樣時間偏移量的問題。

This thesis concentrates on the sampling time offset (STO) estimation scheme that employs the correlation of the short symbols specified in the preamble field of 802.11a WLAN frame structure. A simple detection strategy that uses four values and two values of the correlation is introduced. We also present a new algorithm by which the STO can be adaptively adjusted symbol by symbol. The adaptive algorithm inherits robustness against the variation of roll off rate and can improve the magnitude of the desired four points of correlated sequence. To take advantage of the adaptive algorithm, a linear curve fitting that focuses on the vicinity of zero STO is proposed, and then a state space based on the linear curve fitting can be constructed for a smoothing Kalman filter. Additionally, a threshold that can improve the STO detection is proposed to cooperate with the filter. The mean of estimation error resulted from this new scheme has been simulated for AWGN channels and multipath environments. Performance characteristics of the STO estimation using the non-adaptive method and the adaptive mode are evaluated via numerical examples. Our results show that the proposed method can effectively reduce STO.

1.Introduction ................................................. 7

2.Sampling Time Offset Detection .............................. 10
2.1 System Model and Sampling Time Offset Detection ........... 10
2.2 Implementation Issue ...................................... 13
2.3 Estimation Performance .................................... 15

3. Adaptive Sampling Time Offset Synchronization Technique ... 17
3.1 Traditional Timing Recovery Methods ....................... 17
3.2 Adaptive STO Estimation Algorithm ......................... 19

4.Simulation and Discussion ................................... 25

5.Conclusions ................................................. 29

[1]J. V. D. Beek, M. Sandell, and P. O. Borjesson, “ML estimation of time and frequency offset in OFDM systems,” IEEE Trans. Signal Process., vol. 45, no. 7, pp. 1800–1805, Jul. 1997.
[2]M. Sandell, J. V. D. Beek, and P. O. Borjesson, “Timing and frequency synchronization in OFDM systems using the cyclic prefix,” in Proc. Int. Symp. Synchronization, Essen, Germany, pp. 16–19, Dec. 1995.
[3]R. van Bee and R. Prasad, OFDM Wireless Multimedia Communications, Artech House, 2000.
[4]Institute Electrical and Electronics Engineering, “Part 11: Wireless MAC and PHY Specifications: High Speed Physical Layers in the 5GHz Band,” IEEE std 802.11a-1999.
[5]S. Mohamed, Y. Wu, and L. Házy, “OFDM uplink for interactive broadband wireless: analysis and simulation in the presence of carrier, clock and timing errors,” IEEE Trans. on Broadcasting, vol. 47, no. 1, pp.3–19, Mar. 2001.
[6]X. B.Wang, T. T. Tjhung, Y. Wu, and B. Caron, “SER performance evaluation and optimization of OFDM system with residual frequency and timing offsets from imperfect synchronization,” IEEE Trans. on Broadcasting, vol. 49, no. 2, pp. 170–177, June 2003.
[7]M. Gardner, “Interpolation in digital modems-Part I: Fundamentals,” IEEE Trans. Comm. vol.41, NO.3, pp. 501-507, March 1993.
[8]Steendam and M. Moeneclaey, “Sensitivity of orthogonal frequency-division multiplexed systems to carrier and clock synchronization errors,” Signal Processing, vol. 80, pp. 1217-1229, 2000.
[9]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, Feb/March/April 1995
[10]S. Gardner, “Burst modem design techniques, part 1,” Comm. Syst. Design Magazine, vol. 5, pp. 73-81, July 1999.
[11]M. B. Breinholt and M. D. Zoltowski, “An open-loop cyclostationarity based timing recovery algorithm for accelerated timing acquisition in frequency-selective channels,” in Proc. IEEE ICASSP’02, vol. 3, pp. 2293–2296, Apr. 2002.
[12]D. W. Fu, and A. N. Willson, Jr., “A fast synchronizer for burst modems with simultaneous symbol timing and carrier phase estimations,” in Proc. IEEE Int. Symp. on Circuits and Systems, vol.3, pp. 379-382, 28-31 May 2000.
[13]D. Kim, M. J. Narasimha, and D. C. Cox, “Unbiased timing-error estimation in the presence of nonideal interpolation,” IEEE Trans. Commun., vol. 45, no. 6, pp. 647-650, Jun. 1997.
[14]J. H. Grimm, R. N. Kumar, J. Kusuma, and J. V. Krogmeier, “Real time implementation of a symbol timing recovery algorithm for narrow-band wireless modem,” in Proc. IEEE ICASSP’98, vol. 6, pp. 3253-3256, May 1998.
[15]J. Vesma, M. Refors, and J. Rinne, “Comparison of efficient interpolation techniques for symbol timing recovery,” in Proc. IEEE Globalcom’96, London, U.K. pp. 953–957, Nov. 1996,.
[16]F. M. Gardner, “Interpolation in digital modems—Part 1: Fundamentals,” IEEE Trans. Commun., vol. 41, no. 3, pp. 501–507, Mar. 1993.
[17]L. Erup, F. M. Gardner, and R. A. Harrris, “Interpolation in digital modems—Part 2: Implementation and performance,” IEEE Trans. Commun., vol. 41, no. 6, pp. 501–507, Jun. 1993.
[18]A. I. Bo, G. E. Jian-hua, and Wang Yong, “Symbol synchronization technique in COFDM systems” IEEE Trans. on Broadcasting, vol. 50, NP. 1, pp.56-62, March 2004.
[19]W.-P. Zhu, Y. Yan, M. O. Ahmad, and M.N.S. Swamy, “A feedforward timing recovery scheme using two samples per symbol: Algorithm, performance and implementation Issues,” in Proc. IEEE ISCAS’03, vol. 2, Bangkok, Thailand, pp. 25–28, May 2003.
[20]W.-P. Zhu, Y. Yan, M. O. Ahmad, and M.N.S. Swamy, “Feedforward symbol timing recovery technique using two samples per symbol,” in Proc. IEEE Trans. On Circuits And Systems, vol. 52, No. 11, pp.2490-2500, November 2005.
[21]M. Oerder and H. Meyr, “Digital filter and square timing recovery,” IEEE Trans. Commun., vol. 36, no. 5, pp. 605-612, May 1988.
[22]Bo Ai, Zhi-xing Yang, Chang-yong Pan, Jian-hua Ge, Yong Wang, Zhen Lu, “On the synchronization techniques for wireless OFDM systems,” IEEE Trans. on Broadcasting, pp.1-9,2006.
[23]Krishnamuthy V, C R.N, Athaudage, Pawei-hua, “Adaptive OFDM synchronization algorithms based on discrete stochastic approximation”, IEEE Signal Processing, vol. 53, pp.1561-1574, 2005.
[24]B. D. O. Anderson and J. B. Moore, Optimal Filtering. Englewood Cliffs, NJ: Prentice-Hall, 1979.
[25]S. Haykin, Adaptive Filter Theory, 3rd ed. Englewood Cliffs, NJ: Prentice-Hall, 1996.
[26]G. Ferrari, G. Colavolpe and R. Raheli, Detection Algorithms for Wireless Communications With Applications to Wired and Storage Systems ,WILEY 2004.
[27] F. J. Harris and M. Rice, “Multirate digital filters for symbol timing synchronization in software defined radios,” IEEE Journal on Selected Areas in Communications, vol. 19, no. 12, pp. 2346–2357, Dec. 2001.