International Journal of Electrical and Electronics Research ISSN 2348-6988 (online) Vol. 8, Issue 4, pp: (43-47), Month: October - December 2020, Available at: www.researchpublish.com
BER PERFORMANCE OF MULTI-USER MC-DS-CDMA USING SPREADING CODES IN FADING CHANNELS * TATA BALAJI, ** J.RAVINDRABABU * E.C.E Department, P.V.P. Siddhartha Institute of Technology, Vijayawada. Andhra Pradesh. INDIA. ** E.C.E Department, P.V.P. Siddhartha Institute of Technology, Vijayawada. Andhra Pradesh. INDIA *jrb0009@gmail.com, **balu170882@gmail.com
Abstract: This paper examines the Bit Error Rate (BER) performance of Multi-user Direct Sequence Multi-Carrier Code Division Multiple Access (DS/MC-CDMA) using spreading codes in fading channels. Multi-Carrier Code Division Multiple Access (MC-CDMA) system operating over frequency selective, slowly fading channels is considered and its performance is studied analytically and by computer simulations. We consider the downlink of a cellular radio system where for each user a BPSK modulation is used. The spreading codes are arranged in a way that reduces the effect of the multipath fading channel and restores some of the orthogonality losses between users. The obtained results show considerable performance improvement compared to conventional OFDM and to MCCDMA that uses single spreading codes. Moreover, the spreading codes chosen by the method are in a consistent sequence. Using this code selection method, the system capacity of MU-MC-CDMA can be effectively increased without increasing too much system complexity. Simulation results show that the presented method can work well for fading channels. Keywords: BPSK, DS/MC-CDMA, Walsh codes, Gold codes, PN Sequence.
I. INTRODUCTION Multi-Carrier Code Division Multiple Access (MC-CDMA) is a modulation scheme that combines the advantages of Orthogonal Frequency Division Multiple (OFDM) and Code Division Multiple Access (CDMA) to provide robustness against frequency selectivity in wireless channels. In MCCDMA, the total system bandwidth is divided into a number of sub-bands, where each sub-band may use direct-sequence (DS) spreading and each sub-band signal is transmitted using a subcarrier frequency. MC-CDMA systems using the OFDM technique resolve the frequency selectivity in multipath fading channels and have good spectral properties. The addition of a spread spectrum component to the OFDM introduces a frequency diversity gain that can combat deep multipath fading situations[1]. Future wireless communication systems must be able to accommodate a large number of users and simultaneously to provide the high data rates at the required quality of service. MC-CDMA is taking the advantage of two advanced technological concepts of wireless communications such as orthogonal frequency division multiplex (OFDM) and the code division multiple access (CDMA), what results especially in high spectral efficiency, the multiple access capability, robustness in the case of frequency selective channels, simple one-tap equalization, narrow-band interference rejection and high flexibility of the MC-CDMA[1]. OFDM is currently being used in several new radio broadcast systems including the proposal for high definition digital television (HDTV) and digital audio broadcasting (DAB). However, little research has been done into the use of OFDM as a transmission method for mobile telecommunications systems. In CDMA, all users transmit in the same broad frequency band using specialized codes as a basis of channelization. Both the base station and the mobile station know these codes, which are used to modulate the data sent.
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