Adaptive Iterative Soft-Input Soft-Output Parallel Decision-Feedback Detectors for Asynchronous Coded DS-CDMA Systems
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Adaptive Iterative Soft-Input Soft-Output Parallel Decision-Feedback Detectors for Asynchronous Coded DS-CDMA Systems Wei Zhang School of Information Technology and Engineering (SITE), University of Ottawa, 800 King Edward Avenue, Ottawa, ON, Canada K1N 6N5 Email: [email protected]
Claude D’Amours School of Information Technology and Engineering (SITE), University of Ottawa, 800 King Edward Avenue, Ottawa, ON, Canada K1N 6N5 Email: [email protected]
˘ Abbas Yongac¸oglu School of Information Technology and Engineering (SITE), University of Ottawa, 800 King Edward Avenue, Ottawa, Ontario, Canada K1N 6N5 Email: [email protected] Received 29 April 2004; Revised 4 October 2004 The optimum and many suboptimum iterative soft-input soft-output (SISO) multiuser detectors require a priori information about the multiuser system, such as the users’ transmitted signature waveforms, relative delays, as well as the channel impulse response. In this paper, we employ adaptive algorithms in the SISO multiuser detector in order to avoid the need for this a priori information. First, we derive the optimum SISO parallel decision-feedback detector for asynchronous coded DS-CDMA systems. Then, we propose two adaptive versions of this SISO detector, which are based on the normalized least mean square (NLMS) and recursive least squares (RLS) algorithms. Our SISO adaptive detectors effectively exploit the a priori information of coded symbols, whose soft inputs are obtained from a bank of single-user decoders. Furthermore, we consider how to select practical finite feedforward and feedback filter lengths to obtain a good tradeoff between the performance and computational complexity of the receiver. Keywords and phrases: soft-input soft-output multiuser detection, adaptive multiuser detection, parallel decision-feedback detection, adaptive soft-input soft-output parallel decision-feedback detection, asynchronous coded CDMA systems.
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INTRODUCTION
Iterative soft-input soft-output (SISO) multiuser receivers for coded multiuser systems have received widespread attention since they can provide near single-user performance in a system with multiple-access interference (MAI) by iteratively combining multiuser detection and single-user decoding. The optimum SISO multiuser detector employs either the cross-entropy minimization [1] or the maximum a posteriori (MAP) algorithm [2]. The computational complexity of these techniques is exponentially proportional to the 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.
number of users which can be prohibitive for large systems. Therefore, much work has been done on reduced-complexity suboptimum SISO multiuser detectors. SISO multiuser detection based on the reduced-complexity MAP algorithms which are applied to the trellis of the multiple-access channel is proposed in [3, 4]. The simplest SISO multiuser detector is the so
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