Further Study on MOE-Based Multiuser Detection in Unknown Multipath
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Further Study on MOE-Based Multiuser Detection in Unknown Multipath Zhengyuan Xu Department of Electrical Engineering, University of California, Riverside, CA 92521, USA Email: [email protected] Received 9 January 2002 The minimum output energy (MOE) receiver has been developed for multiuser detection when multipath distortion is present. Its performance has been shown to be very close to the minimum mean square error (MMSE) receiver at high signal-to-noise ratio. However, due to the additive noise, the constraint vector required to construct the MOE receiver is a biased estimate of the channel vector. Thus, the MOE receiver exhibits degraded performance. To mitigate the noise effect, the constraint cost function is modified to obtain a modified MOE (MMOE) receiver in this paper, leading to a significantly improved channel estimate and detection performance. It is also revealed that the MMOE method converges to the well-known subspace method under certain conditions. In addition to the additive noise, imperfect estimation of the output data covariance matrix also causes performance loss and it is studied in details based on perturbation theory. Keywords and phrases: minimum output energy, constrained optimization, perturbation analysis.
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INTRODUCTION
The rapidly growing demands for integrated wideband/ broadband services have created considerable research interest in developing new wireless communication technologies [1, 2]. Among several options for the implementation of the air interface of future wideband wireless systems, direct sequence (DS) code division multiple access (CDMA) spread spectrum has emerged as a leading technology due to its intriguing features. It allows many users to simultaneously share the finite amount of available spectrum with minimal coordination, thus increase the system capacity. It also exhibits exceptional robustness to jamming, interception and multipath fading. Additionally, it provides much flexibility in system design and implementation by its unique spreading mechanism. This technology will continue to thrive in multiuser systems and will further establish its dominance in the future wideband wireless networks. In a DS/CDMA system, multiuser interference (MUI) is a typical obstacle to be obviated in detection of input signals. Substantial efforts have focused on multiuser detection [3, 4, 5, 6]. Although the optimal receiver is well recognized to be the maximum likelihood sequence estimator (MLSE) [7], linear detectors receive considerable attention. Their low complexity, ease of implementation, and acceptable performance make them so attractive in many applications. Among those linear detectors, blind solutions are particularly suitable for a bandwidth constrained system. Without multipath fading, a blind minimum mean square error (MMSE)
detector can be implemented under certain constraints [8]. If multipath propagation occurs, a direct blind MMSE receiver has been reported [9] which requires a noise subspace from the eigenvalue decomposition (EVD) of a data covariance matrix. S
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