Spreading Code Assignment Strategies for MIMO-CDMA Systems Operating in Frequency-Selective Channels
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Research Article Spreading Code Assignment Strategies for MIMO-CDMA Systems Operating in Frequency-Selective Channels Claude D’Amours1 and Adel Omar Dahmane2 1 School
of Information Technology and Engineering, University of Ottawa, 550 Cumberland Avenue, Ottawa, ON, Canada K1N 6N5 of Electrical and Computer Engineering, Universit´e du Quebec a` Trois-Rivi`eres, 3351 boul. des Forges, Trois-Rivi`eres, QC, Canada G9A 5H7
2 Department
Correspondence should be addressed to Claude D’Amours, [email protected] Received 13 August 2009; Revised 10 November 2009; Accepted 25 December 2009 Recommended by Ibrahim Develi Code Division Multiple Access (CDMA) and multiple input multiple output- (MIMO-) CDMA systems suffer from multiple access interference (MAI) which limits the spectral efficiency of these systems. By making these systems more power efficient, we can increase the overall spectral efficiency. This can be achieved through the use of improved modulation and coding techniques. Conventional MIMO-CDMA systems use fixed spreading code assignments. By strategically selecting the spreading codes as a function of the data to be transmitted, we can achieve coding gain and introduce additional degrees of freedom in the decision variables at the output of the matched filters. In this paper, we examine the bit error rate performance of parity bit-selected spreading and permutation spreading under different wireless channel conditions. A suboptimal detection technique based on maximum likelihood detection is proposed for these systems operating in frequency selective channels. Simulation results demonstrate that these code assignment techniques provide an improvement in performance in terms of bit error rate (BER) while providing increased spectral efficiency compared to the conventional system. Moreover, the proposed strategies are more robust to channel estimation errors as well as spatial correlation. Copyright © 2009 C. D’Amours and A. O. Dahmane. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1. Introduction The object of much research in wireless communications is to enable high and variable data rates to users to support the growing number of applications that involve the transfer of data [1]. Code Division Multiple Access (CDMA) systems employ spread spectrum (SS) technology and were developed for second and third generation (2G, 3G) wireless communications. For example, IS-95 and Wideband CDMA (WCDMA) systems are based on direct sequence SS techniques. Multiple access interference (MAI) is present in CDMA systems due to the nonzero cross-correlation between the different users’ spreading codes [2]. The MAI that each user’s signal creates in all other users’ signals results in increased bit error rate (BER). The overall system capacity is determined by the number of simultaneous transmitters that can be supported before the BER increases to an unacceptable
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