- Research Article
- Open Access
Equalization of Multiuser Wireless CDMA Downlink Considering Transmitter Nonlinearity Using Walsh Codes
EURASIP Journal on Wireless Communications and Networking volume 2007, Article number: 049525 (2007)
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.
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.
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
Al-Raweshidy H, Komaki S: Radio Over Fiber Technologies for Mobile Communications Networks. 1st edition. Artech House, Norwood, Mass, USA; 2002.
Oliphant MW: Radio interfaces make the difference in 3G cellular systems. IEEE Spectrum 2000,37(10):53-58. 10.1109/6.873918
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.
Gómez JC, Baeyens E: Subspace identification of multivariable Hammerstein and Wiener models. Proceedings of the 15th IFAC World Congress, July 2002, Barcelona, Spain
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
Proakis JG: Digital Communications. 4th edition. McGraw-Hill, New York, NY, USA; 2001.
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
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
Fernando XN: Signal processing for optical fiber based wireless access, Ph.D. dissertation. 2001.
Billings SA, Fakhouri SY: Identification of a class of nonlinear systems using correlation analysis. Proceedings of the IEE 1978,125(7):691-697.
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
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.
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.
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.
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
About this article
Cite this article
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
- Channel Impulse Response
- Decision Feedback Equalizer
- Transmitter Nonlinearity
- Walsh Code
- Cellular Wireless System