Performance of Sampling Clock Offset Compensation

In Figure 5.14 – 5.17, we can find the performances of various interpolators are similar when NT≦2. However, the performance of the Cubic B-spline interpolator is poor when NT=3 or 4, because of serious magnitude distortions at the high frequency band. Hence, the Cubic B-spline interpolator is less suitable for IEEE 802.11n systems.

Although the Cubic B-spline is not suitable for IEEE 802.11n system, it has good performance in some other systems, which adopt bit loading process, because of coherent magnitude responses.

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Furthermore, the performances of windowed-sinc interpolator with rectangular window, least-square interpolator, and equiripple interpolator are slightly better than the other interpolators. Table 5.4 shows the computational complexity comparison of various interpolator designs [43]. One can find that the computational complexities of polynomial-based interpolators, Lagrange and Cubic B-spline, are slightly lower than the other interpolators. Unfortunately, the Cubic B-spline interpolator performance is poor when applied to IEEE 802.11n system. However, the Lagrange interpolator has similar performance compared to the three interpolators. For the reason, we suggest that Largange interpolator design is more suitable when applied to IEEE 802.11n standard.

Table 5.4 The computational complexity comparison of various interpolator designs for sampling clock offset compensation. L=4, P=3. [43]

Interpolator type + /− ×constant << or>> ×

Lagrange 11 2 4 3

Cubic B-spline 11 4 3 3

Equiripple General LS.

Windowed- Sinc

15 16 0 3

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(a)

(b)

Figure 5.14 BER performance vs. SNR comparison of the major resampling interpolators for sampling clock offset compensation. NT=1, NR=1.

(a) polynomial-based interpolators. (b) windowed sinc interpolators.

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(a)

(b)

Figure 5.15 BER performance vs. SNR comparison of the major resampling interpolators for sampling clock offset compensation. NT=2, NR=1.

(a) polynomial-based interpolators. (b) windowed sinc interpolators.

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(a)

(b)

Figure 5.16 BER performance vs. SNR comparison of the major resampling interpolators for sampling clock offset compensation. NT=3, NR=1.

(a) polynomial-based interpolators. (b) windowed sinc interpolators.

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(a)

(b)

Figure 5.17 BER performance vs. SNR comparison of the major resampling interpolators for sampling clock offset compensation. NT=4, NR=1.

(a) polynomial-based interpolators. (b) windowed sinc interpolators.

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Chapter 6 Conclusion

In this thesis, we investigate all the required synchronization operations and related design techniques that jointly achieve frame, symbol, carrier, and sampling clock synchronizations for MIMO OFDM systems. Practical designs are applied to IEEE 802.11n receiver. Performances are measured under the multi-path fading channels. From simulation results, the proposed synchronization schemes work well in those conditions.

In the frame detection, we use the received signal power to set the threshold, and detect the beginning of a frame by comparing the auto-correlation outputs to the threshold [44]. In the symbol timing synchronization, we perform the coarse symbol timing estimation first with the double-sliding-window method [45], then adjust the

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symbol timing by the proposed STDFS method to get the more accurate symbol timing estimation than the current techniques. Owing to the features of the IEEE 802.11n preambles which are composed of ten repeat short training symbols in the short training field, two repeat long training symbols, and the guard interval in the long training filed, we utilize the features to enhance the carrier frequency offset estimation by averaging the available auto-correlation outputs, and take advantage of the multi channel diversity of MIMO for better synchronization. In the sampling clock offset estimation, we reduce the computation complexity by only computing the most apart pilots, and take advantage of the multi channel diversity of MIMO for better synchronization. As for the sampling clock compensation, we find that Lagrange interpolation is suitable for IEEE 802.11n standard.

In the future, we will further reduce the computational complexity of the overall synchronization schemes. In the frame synchronization, we will try to reduce the computational complexity by only comparing the real part of the auto-correlation outputs, or by averaging the squares of the auto-correlation real part and the auto-correlation imaginary part. Furthermore, we will try to max-ratio combine the possible pilot pairs to obtain more accurate clock offset estimation, and max-ratio combine the estimated MIMO multi channel parameter samples for better parameter estimation. Up to now, the simulations are conducted by floating-point operations. We will also conduct fixed-point simulations so that we can reflect practical software and hardware realization and implementation designs.

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References

[1] R. W. Chang, “Synthesis of band limited orthogonal signals for multichannel data transmission,” Bell Syst. Tech. J., vol. 45, pp. 1775-1796, Dec. 1966.

[2] G. J. Foschini and M. J. Gans, “On limits of wireless communications in a fading environment when using multiple antennas,” Wireless Pers. Commun., vol. 6, no. 3, pp. 311–335, Mar. 1998.

[3] WWiSE, “WWiSE proposal” http://www.wwise.org/

[4] TGnSync, “TGnSync proposal” http://www.tgnsync.org/home

[5] IEEE Std. 802.11, Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, 1999 edition.

[6] IEEE Std. 802.11a-1999, Wireless LAN Medium Access Control and Physical Layer specifications: High-speed physical layer in the 5 GHz band.

[7] R. V. Nee and R. Prasad, OFDM for Wireless Multimedia Communications. Boston:

Artech House, 2000.

[8] H. Nogami, and T. Nagashima, “A frequency and timing period acquisition technique for OFDM System,” Proc. IEEE Personal, Indoor, Mobile Radio Commun., pp. 1010-1015, 1995.

[9] T. Pollet, M. V. Bladel, and M. Moeneclaey, “BER sensitivity of OFDM systems to carrier frequency and weiner phase noise,” IEEE Trans. Commun., vol. 43, pt. 1, pp.

191-193, Feb.- Apr. 1995.

[10] A. Palin and J. Rinne, “Symbol synchronization in OFDM system for time selective channel conditions,” Proc. Electronics, Circuits and Systems, 1999, Proceedings of ICECS '99, The 6th IEEE International Conference on, vol. 3, pp.

1581 – 1584, Sept. 1999.

[11] B. Yang, K. B. Letaief, R. S. Cheng, and Z. Cao, “Timing recovery for OFDM

79

transmission,” Selected Areas in Communications, IEEE Journal on, vol. 18, pp.

2278 - 2291, 2000.

[12] F. Classen and H. Meyr, “Frequency synchronization algorithms for OFDM systems suitable for communication over frequency selective fading channels,”

Vehicular Technology Conference, 1994 IEEE 44th, vol.3, pp. 1655 - 1659, June 1994.

[13] M. Speth, D. Daecke and H. Meyr, “Minimum overhead burst synchronization for OFDM based broadband transmission,” Global Telecommunications Conference, 1998, GLOBECOM 98, The Bridge to Global Integration, IEEE, Volume: 5 , pp.

2777 – 2782, Nov. 1998.

[14] E. Kuusmik, “Wireless Lan integration into mobile phone,” Master’s Thesis, Department of Signals and Systems, Chalmers University of Technology, Finland, September 2004.

[15] ANSI/IEEE Standard 802.11b, 1999 Edition.

[16] ANSI/IEEE Standard 802.11g, 2003 Edition.

[17] Y. Asai, S. Kurosaki, T. Sugiyama, and M. Umehira, “Precise AFC scheme for performance improvement of SDM-COFDM,” Proc. Vehicular Technology Conference, 2002. Proceedings. VTC 2002-Fall. 2002 IEEE 56th, vol. 3, pp.

1408 – 1412, Sept. 2002.

[18] T. C. W. Schenk and A. van Zelst, “Frequency synchronization for MIMO OFDM wireless LAN systems,” Proc. Vehicular Technology Conference, 2003. VTC 2003-Fall, 2003 IEEE 58th, vol. 2, pp. 781 – 785, Oct. 2003.

[19] S. Johansson, M. Nilsson, and P. Nilsson, “An OFDM timing synchronization ASIC,” Proc. Electronics, Circuits and Systems, 2000. ICECS 2000. The 7th IEEE International Conference on, vol.1, pp. 324 – 327, Dec. 2000.

80

[20] J. Liu and J. Li, “Parameter estimation and error reduction for OFDM-based WLANs,” Mobile Computing, IEEE Transactions on, vol. 3, pp. 152-163, 2004.

[21] A. Fort, J.-W. Weijers, V. Derudder, W. Eberle, and A. Bourdoux, “A performance and complexity comparison of auto-correlation and cross-correlation for OFDM burst synchronization,” Proc. Acoustics, Speech, and Signal Processing, 2003.

Proceedings. (ICASSP '03). 2003 IEEE International Conference on, vol. 2, pp.

341-344, April 2003.

[22] A. N. Mody and G. L. Stuber, “Receiver implementation for a MIMO OFDM system,” Proc. Global Telecommunications Conference, 2002. GLOBECOM '02.

IEEE, vol. 1, pp. 716 – 720, Nov. 2002.

[23] A. N. Mody and G. L. Stuber, “Synchronization for MIMO OFDM systems,” Proc.

Global Telecommunications Conference, 2001. GLOBECOM '01. IEEE, vol. 1, pp. 509 - 513, Nov. 2001.

[24] G. L. Stuber, J. R. Barry, S. W. McLaughlin, Y. Li, M. A. Ingram, and T. G. Pratt,

“Broadband MIMO-OFDM wireless communications,” Proceedings of the IEEE, vol. 92, pp. 271-294, 2004.

[25] K. Taura, M. Tsujishita, M. Takeda, H. Kato, M. Ishida, and Y. Ishida, “A digital audio broadcasting (DAB) receiver,” Consumer Electronics, IEEE Transactions on, vol. 42, pp. 322-327, 1996.

[26] E. Zhou, X. Zhang, H. Zhao, and W. Wang, “Synchronization algorithms for MIMO OFDM systems,” Proc. Wireless Communications and Networking Conference, 2005 IEEE, vol. 1, pp. 18 – 22, March 2005.

[27] N. P. Sands and K. S. Jacobsen, “Pilotless timing recovery for baseband multicarrier modulation,” IEEE Journal on Selected Areas in Communications, Volume: 20, Issue: 5, pp. 1047 – 1054, June 2002.

81

[28] M. Speth, S. Fechtel, G. Fock, and H. Meyr, “Optimum receiver design for OFDM-based broadband transmission .II. A case study,” Communications, IEEE Transactions on, vol. 49, pp. 571-578, 2001.

[29] T. Pollet, P. Spruyt and M. Moeneclaey, “The BER performance of OFDM systems using non-synchronized sampling,” Proc. IEEE Global Telecommunications Conference, pp. 253-257, 1994.

[30] T. Pollet and M. Peeters, “Synchronization with DMT modulation,” IEEE Communications Magazine, Volume: 37 Issue: 4, pp. 80 -86, April 1999

[31] F. M. Gardner, “Interpolation in digital modems-part I: fundamentals,” IEEE Transactions on Communication, vol. 41, pp. 502-508, Mar 1993.

[32] L. Erup, F. M. Gardner, R. A. Harris, “Interpolation in digital modems-part II:

implementation and performance,” IEEE Transactions on Communications, vol.

41, pp. 502-508, Jun 1993.

[33] E. Martos-Naya, J. Lopez-Fernandez, L. D. del Rio, M.C. Aguayo-Torres and J. T.

E. Munoz, “Optimized interpolator filters for timing error correction in DMT systems for xDSL applications,” IEEE Journal on Selected Areas in Communications, Volume: 19, Issue: 12, pp. 2477 –2485, Dec. 2001.

[34] T. I. Laakso, V. Valimaki, Matti Karjalanen and U. K. Laine, “Splitting the unit delay,” IEEE Signal Processing Magazine, pp. 30-60, Jan 1996.

[35] C. W. Farrow, “A continuously variable digital delay element,” Proc. IEEE Int.

Symp. Circuit and System, pp. 2641-2645, Jun 1988.

[36] N. J. Fliege, Multirate Digital Signal Processing: Multirate Systems, Filter banks, Wavelets, Chichester, John Wiley, 1994.

[37] X. Zhao, J. Kivinen, and P. Vainnikainen, “Tapped delay line channel models at 5.3 GHz in indoor environments,” Proc. Vehicular Technology Conference, 2000.

82

IEEE VTS-Fall VTC 2000. 52nd, vol. 1, pp. 1 – 5, Sept. 2000.

[38] Y. Li, N. Seshadri, and S. Ariyavisitakul, “Channel estimation for OFDM systems with transmitter diversity in mobile wireless channels,” Selected Areas in Communications, IEEE Journal on, vol. 17, pp. 461-471, 1999.

[39] Y. Li, “Simplified channel estimation for OFDM systems with multiple transmit antennas,” Wireless Communications, IEEE Transactions on, vol. 1, pp. 67-75, 2002.

[40] S. M. Alamouti, “A simple transmit diversity technique for wireless communications,” Selected Areas in Communications, IEEE Journal on, vol. 16, pp. 1451-1458, 1998.

[41] V. Tarokh, H. Jafarkhani, and A. R. Calderbank, “Space-time block coding for wireless communications: performance results,” Selected Areas in Communications, IEEE Journal on, vol. 17, pp. 451-460, 1999.

[42] V. Valimaki and T. I. Laakso, “Principles of fractional delay filters,” IEEE International Conference on Acoustics, Speech, and Signal Processing, 2000.

ICASSP '00, Proceedings., vol. 6 , pp. 3870 – 3873, June 2000.

[43] M. H. Lai, “Clock synchronization of VDSL system,” Master’s Thesis, Department of Electronics Engineering & Institute of Electronics, National Chiao Tung University, Taiwan, May 2004.

[44] C. H. Cho, “Frame synchronization and digital automatic gain control for OFDM Wireless LAN System,” Master’s Thesis, Department of Electronics Engineering

& Institute of Electronics, National Chiao Tung University, Taiwan, July 2002.

[45] J. Terry and J. Heiskala, OFDM Wireless LANs: a theoretical and practical guide, 2002.

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自 傳

劉佳旻，1980 年 11 月 29 日出生，台灣省台中市人。2003 年自 國立交通大學電子工程學系獲得學士學位，隨即進入國立交通大學電 子研究所攻讀碩士學位，研究興趣為訊號處理與數位通訊，碩士論文 題目為「多重輸入輸出正交分頻多工系統之同步設計研究」。