INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS Int. J. Commun. Syst. (2011) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/dac.1293 Effect of multipath fading and multiple access interference on broadband code division multiple access systems Md. Anwar Hossain 1, * ,† and Mohammed Tarique 2 1 Department of Electrical and Electronic Engineering, University of the Ryukyus, Okinawa, Japan 2 Department of Electrical and Electronic Engineering, American International University-Bangladesh, Dhaka, Bangladesh SUMMARY In this paper, the performance of uncoded uplink transmissions in a broadband code division multiple access (CDMA) system using tapped delay line (TDL) antenna array under multipath conditions is investigated. An expression for broadband CDMA system is derived to calculate the optimal weights of TDL antenna array. Using the derived broadband expression, the performance of TDL antenna array is investigated and compared with that of the already existing narrowband systems. Simulation results show that TDL antenna array can reduce multipath fading and multiple access interference (MAI). It is shown that if the signal-to- noise ratio (SNR) level is maintained at 20 dB and two-ray channel model is used, two antennas with four taps each can support 30 users with a BER of 10 3 . If three-ray channel model is used, the same antenna arrangement is not enough to support 30 users. To support these 30 users with BER of 10 3 , the number of taps needs to be increased to eight. Finally, the number of taps needs to be further increased to 16 if four-ray channel model is used. Not only that, the SNR should also be increased to 25 dB to achieve the same BER. Copyright © 2011 John Wiley & Sons, Ltd. Received 22 March 2010; Revised 15 December 2010; Accepted 28 April 2011 KEY WORDS: antenna array; beamforming; multipath fading; CDMA; multiple access interference (MAI); tapped delay line (TDL) 1. INTRODUCTION Fourth-generation mobile communication systems are being designed to support a very high-speed wireless service of 100 Mbps to 1 Gbps. For such high-speed data transmissions, the channel becomes severely frequency selective because of time dispersion. A promising wireless multiple access technique that can overcome the frequency-selective fading as well as can take advantage of this to improve the transmission quality is code division multiple access (CDMA). The ulti- mate object of this technique is to achieve and maintain the required QoS for the systems. Two broadband CDMA techniques are suggested for such systems namely direct sequence CDMA (DS-CDMA) and multicarrier CDMA (MC-CDMA). MC-CDMA is considered advantageous over DS-CDMA because it has less intersymbol interference (ISI). But it was shown in the literatures that DS-CDMA can achieve a comparable performance if a proper frequency domain equalization is adopted. Nowadays, a special case of MC-CDMA called orthogonal frequency division multi- plexing (OFDM) is also recognized as a multicarrier solution to combat multipath fading effects. An alternative approach based on more traditional single-carrier modulation methods is shown to have similar performance compared with OFDM provided a frequency domain equalization is used [1–4]. Because multiple access interference (MAI) and ISI limit the capacity of the conventional *Correspondence to: Md. Anwar Hossain, Department of Electrical and Electronic Engineering, University of the Ryukyus, Okinawa, Japan. † E-mail: anwar.ee113@gmail.com Copyright © 2011 John Wiley & Sons, Ltd.