BER Performance for Downlink MC-CDMA Systems over Rician Fading Channels
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BER Performance for Downlink MC-CDMA Systems over Rician Fading Channels Zhihua Hou Positioning & Wireless Technology Centre (PWTC), School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Drive, Singapore 637553 Email: [email protected]
Vimal K. Dubey School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 Email: [email protected] Received 30 July 2003; Revised 1 April 2004 We consider downlink multicarrier code-division multiple-access (MC-CDMA) systems using binary phase-shift keying (BPSK) modulation scheme and maximal ratio combining (MRC) in frequency-selective Rician fading channels. A time-domain method to obtain bit error rate (BER) by calculating moment generating function (MGF) of the decision variable for a tapped-delayline channel model is proposed. This method does not require any assumption regarding the statistical or spectral distribution of multiple access interference (MAI), and it is also not necessary to assume that the fading encountered by the subcarriers is independent of each other. The analytical formula is also verified by simulations. Keywords and phrases: frequency-selective Rician fading channels, MC-CDMA, moment generating function, performance analysis, BER for downlink MC-CDMA systems over Rician fading channels.
1.
INTRODUCTION
MC-CDMA systems, based on the combination of code-division multiple-access (CDMA) and orthogonal frequency-division multiplexing (OFDM) techniques, were proposed in 1993 [1]. The multicarrier CDMA schemes can be categorized into two groups: MC-CDMA and MCDS-CDMA [2]. Due to the attractive features like efficient frequency diversity and high bandwidth efficiency [3], MC-CDMA has received greater attention. Furthermore, MC-CDMA outperforms direct sequence CDMA (DSCDMA) and MC-DS-CDMA in terms of BER performance over the downlink. Hence, MC-CDMA appears to be a suitable candidate for supporting multimedia services in mobile radio communications for the downlink. Most of the previous papers [1, 4, 5], which investigated the performance of the MC-CDMA systems, assumed that the fading in different subcarriers is independent of each other, so that the variance of the interference can be approximated by using the central limit theorem. Nevertheless, the assumption is not guaranteed in practice, as the fading of the subcarriers is usually correlated due to insufficient frequency separation between the subcarriers. Also, the assumption of independent fading characteristic implies a frequency-selective fading channel at each subcarrier, since it
requires sufficient independent paths uniformly distributed over the symbol duration [3], which contradicts the assumption of flat fading at each subcarrier. An exact error floor without taking into account the noise term is obtained in [6] under the assumption of exponential multipath intensity profile. In [7], a closed-form BER expression for a synchronous MC-CDMA in the uplink has been obtained assuming independent fading am
