An Iterative Multiuser Detector for Turbo-Coded DS-CDMA Systems
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An Iterative Multiuser Detector for Turbo-Coded DS-CDMA Systems Emmanuel Oluremi Bejide Department of Electrical Engineering, University of Cape Town, Private Bag, Rondebosch 7701, South Africa Email: [email protected]
Fambirai Takawira School of Electrical, Electronic & Computer Engineering, University of KwaZulu-Natal, Durban 4041, South Africa Email: [email protected] Received 1 October 2003; Revised 9 October 2004 We propose an iterative multiuser detector for turbo-coded synchronous and asynchronous direct-sequence CDMA (DS-CDMA) systems. The receiver is derived from the maximum a posteriori (MAP) estimation of the single user’s transmitted data, conditioned on information about the estimate of the multiple-access interference (MAI) and the received signal from the channel. This multiple-access interference is reconstructed by making hard decisions on the users’ detected bits at the preceding iteration. The complexity of the proposed receiver increases linearly with the number of users. The proposed detection scheme is compared with a previously developed one. The multiuser detector proposed in this paper has a better performance when the transmitted powers of all active users are equal in the additive white Gaussian noise (AWGN) channel. Also, the detector is found to be resilient against the near-far effect. Keywords and phrases: iterative decoding, multiuser detection, wireless communication, code-division multiple access, turbo codes.
1.
INTRODUCTION
A significant amount of work has been done on the development of multiuser detectors (MUD) for CDMA since the publication of the novel work of Verdu´ [1]. The main focus of work on MUD development has been the search for suboptimal detectors because the optimum receiver of [1] has an implementation complexity that increases exponentially with the number of users. Suboptimal detectors that have been reported in the literature can be classified as linear or nonlinear detectors [2]. In linear multiuser detection, linear filters are used in processing the received signal in order to extract the signal of the user of interest and suppress the multiple-access interference. Nonlinear multiuser detection involves the subtraction of the estimate of the multiple-access interference from the received signal [2, 3]. Realizing that error correction coding alone cannot remove the effects of the multiple-access interference effectively, a lot of emphasis is now being placed on designing multiuser detectors for channel-coded CDMA systems. A pioneering work in this respect is the work of Giallorenzi and Wilson [4] where the optimum detector of [1] is combined with convolutional decoding. The complexity of the receiver
of [4] increases exponentially with the product of the number of users and the constraint length of the convolutional encoder. Some suboptimal implementations of the receiver of [4] were proposed in [5]. The advent of turbo codes [6] and the generalization of the “turbo principle” in many aspects of digital communication [7] have inspired the development of many “i
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