Performance of Asynchronous MC-CDMA Systems with Maximal Ratio Combining in Frequency-Selective Fading Channels
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Performance of Asynchronous MC-CDMA Systems with Maximal Ratio Combining in Frequency-Selective Fading Channels Keli Zhang School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798 Email: [email protected]
Yong Liang Guan School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798 Email: [email protected] Received 28 February 2003; Revised 29 August 2003 The bit error rate (BER) performance of the asynchronous uplink channel of multicarrier code division multiple access (MCCDMA) systems with maximal ratio combining (MRC) is analyzed. The study takes into account the effects of channel path correlations in generalized frequency-selective fading channels. Closed-form BER expressions are developed for correlated Nakagami fading channels with arbitrary fading parameters. For channels with correlated Rician fading paths, the BER formula developed is in one-dimensional integration form with finite integration limits, which is also easy to evaluate. The accuracy of the derived BER formulas are verified by computer simulations. The derived BER formulas are also useful in terms of computing other system performance measures such as error floor and user capacity. Keywords and phrases: MC-CDMA, MRC, asynchronous transmission, correlations, fading channels.
1. INTRODUCTION Multicarrier code division multiple access (MC-CDMA) is a technique that combines direct sequence (DS) CDMA with orthogonal frequency division multiplexing (OFDM) modulation. It is one of the candidate technologies considered for the 4th-generation wireless communication systems [1]. MC-CDMA transmits every data symbol on multiple narrowband subcarriers and utilizes cyclic prefix to absorb and remove intersymbol interference (ISI) arising from frequency-selective fading. As it is unlikely for all subcarriers to experience deep fade simultaneously, frequency diversity is achieved when the subcarriers are appropriately combined at the receiver. In [2, 3], it is shown that MC-CDMA outperforms the conventional DS-CDMA and two other forms of CDMA with OFDM modulation, namely MC-DS-CDMA and multitone CDMA. Several combining techniques have been proposed for MC-CDMA systems. Maximal ratio combining (MRC) offers maximum improvement in the presence of spectrally white Gaussian noise [4, 5]. It is shown to achieve better performance for MC-CDMA uplink than equal gain combining (EGC) in [3], and the resultant system has lower error floor than DS-CDMA and MC-DS-CDMA.
The bit error rate (BER) performance of MC-CDMA systems is not easy to analyze as the receiver operations involve coherent combining of a large number of independent or correlated fading subcarriers with possibly different fading statistics. Signal analysis is further complicated by the presence of multiuser interference (MUI) in the received signal. In the literature, simulations are often used to study the performance of MC-CDMA systems [2, 6, 7, 8]. For the downlink performance of MC-C
