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Equalization of Multiuser Wireless CDMA Downlink Considering Transmitter Nonlinearity Using Walsh Codes

Abstract

Transmitter nonlinearity has been a major issue in many scenarios: cellular wireless systems have high power RF amplifier (HPA) nonlinearity at the base station; satellite downlinks have nonlinear TWT amplifiers in the satellite transponder and multipath conditions in the ground station; and radio-over-fiber (ROF) systems consist of a nonlinear optical link followed by a wireless channel. In many cases, the nonlinearity is simply ignored if there is no out-of-band emission. This results in poor BER performance. In this paper we propose a new technique to estimate the linear part of the wireless downlink in the presence of a nonlinearity using Walsh codes; Walsh codes are commonly used in CDMA downlinks. Furthermore, we show that equalizer performance is significantly improved by taking into account the presence of the nonlinearity during channel estimation. This is shown by using a regular decision feedback equalizer (DFE) with both wireless and RF amplifier noise. We perform estimation in a multiuser CDMA communication system where all users transmit their signal simultaneously. Correlation analysis is applied to identify the channel impulse response (CIR) and the derivation of key correlation relationships is shown. A difficulty with using Walsh codes in terms of their correlations (compared to PN sequences) is then presented, as well as a discussion on how to overcome it. Numerical evaluations show a good estimation of the linear system with 54 users in the downlink and a signal-to-noise ratio (SNR) of 25 dB. Bit error rate (BER) simulations of the proposed identification and equalization algorithms show a BER of achieved at an SNR of dB.

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References

  1. Fernando XN, Pinter SZ: Radio over fiber for broadband wireless access. PHOTONS: Technical Review of the Canadian Institute for Photonic Innovations 2004,2(1):24-26.

    Google Scholar 

  2. Billings SA, Fakhouri SY: Non-linear system identification using the Hammerstein model. International Journal of Systems Science 1979,10(5):567-578. 10.1080/00207727908941603

    Article  MathSciNet  MATH  Google Scholar 

  3. Al-Raweshidy H, Komaki S: Radio Over Fiber Technologies for Mobile Communications Networks. 1st edition. Artech House, Norwood, Mass, USA; 2002.

    Google Scholar 

  4. Oliphant MW: Radio interfaces make the difference in 3G cellular systems. IEEE Spectrum 2000,37(10):53-58. 10.1109/6.873918

    Article  Google Scholar 

  5. Bai E-W: Frequency domain identification of Hammerstein models. Proceedings of the 41st IEEE Conference on Decision and Control, December 2002, Las Vegas, Nev, USA 1: 1011-1016.

    Article  Google Scholar 

  6. Gómez JC, Baeyens E: Subspace identification of multivariable Hammerstein and Wiener models. Proceedings of the 15th IFAC World Congress, July 2002, Barcelona, Spain

    Google Scholar 

  7. Gómez JC, Baeyens E: Identification of block-oriented nonlinear systems using orthonormal bases. Journal of Process Control 2004,14(6):685-697. 10.1016/j.jprocont.2003.09.010

    Article  Google Scholar 

  8. Proakis JG: Digital Communications. 4th edition. McGraw-Hill, New York, NY, USA; 2001.

    MATH  Google Scholar 

  9. Fernando XN, Sesay AB: Adaptive asymmetric linearization of radio over fiber links for wireless access. IEEE Transactions on Vehicular Technology 2002,51(6):1576-1586. 10.1109/TVT.2002.804841

    Article  Google Scholar 

  10. Saleh AAM: Frequency-independent and frequency-dependent nonlinear models of TWT amplifiers. IEEE Transactions on Communications 1981,29(11):1715-1720. 10.1109/TCOM.1981.1094911

    Article  Google Scholar 

  11. Fernando XN: Signal processing for optical fiber based wireless access, Ph.D. dissertation. 2001.

    Google Scholar 

  12. Billings SA, Fakhouri SY: Identification of a class of nonlinear systems using correlation analysis. Proceedings of the IEE 1978,125(7):691-697.

    MathSciNet  Google Scholar 

  13. Billings SA, Fakhouri SY: Identification of nonlinear systems using correlation analysis and pseudorandom inputs. International Journal of Systems Science 1980,11(3):261-279. 10.1080/00207728008967012

    Article  MATH  Google Scholar 

  14. Raziq P, Nakagawa M: Semiconductor laser's nonlinearity compensation for DS-CDMA optical transmission system by post nonlinearity recovery block. IEICE Transactions on Communications 1996,E79-B(3):424-431.

    Google Scholar 

  15. Fernando XN, Sesay AB: Fibre-wireless channel estimation using correlation properties of PN sequences. Canadian Journal of Electrical and Computer Engineering 2001,26(2):43-47.

    Google Scholar 

  16. Lang T, Chen X-H: Comparison of correlation parameters of binary codes for DS/CDMA systems. Proceedings of the IEEE International Conference on Communications Science (ICCS '94), November 1994, Singapore 3: 1059-1063.

    Google Scholar 

  17. Fernando XN, Sesay AB: A Hammerstein-type equalizer for concatenated fiber-wireless uplink. IEEE Transactions on Vehicular Technology 2005,54(6):1980-1991. 10.1109/TVT.2005.858170

    Article  Google Scholar 

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Correspondence to Stephen Z. Pinter.

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Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Pinter, S.Z., Fernando, X.N. Equalization of Multiuser Wireless CDMA Downlink Considering Transmitter Nonlinearity Using Walsh Codes. J Wireless Com Network 2007, 049525 (2007). https://doi.org/10.1155/2007/49525

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