Research On Multiuser Detection For Wide-band CDMA Mobile Communication System

Multiple access interference (MAI) caused by code non-orthogonality and inter-symbol interference (ISI) due to the time dispersion of wireless channels are the main difficulties in wide-band code division multiple access (CDMA) systems. Multiuser detection(MUD) that suppresses the MAI as well as the ISI remains an interesting topic in the past decades. The current research of MUD focuses on two directions. One is toward practical applications. Commonly complicated models regarding practical issues are applied; such as the turbo multiuser detection, which considers channel decoding and multiuser detection jointly. The other direction focuses on new algorithm development and performance analysis. Commonly simple system models, for instant a baseband system with short spreading code, are applied. Researches in this direction generally assume the user sequences have identical independent distribution and focus on improvement in BER, stability and computation. Our researches focus on the latter. According to different channel models, CDMA systems can be divided into additive white gaussian noise (AWGN) channel and dispersive channel, time-variant and time-invariant channel. In this paper, we mainly study multiuser detection in slow time-variant dispersive channels. The contributions are in five folds.1. Constrained optimization(CO) based methods play an important role in MUD. A CO method generally includes a cost function and a constraint to confine the searching space. In this thesis, a new algorithm based on linearly constrained(LC) MMSE criterion is proposed.. The key novelty is that the cost function and the constraint are developed based on the same criterion, which guarantees that the desired optimum can be located in the confined searching space. An iterative implementation is proposed and its convergence is also discussed.2. Blind multiuser detection can be implemented by batch algorithm and adaptive algorithm. Adaptive algorithm is more feasible to trace the channel variation. In the thesis we also propose a batch algorithm and an adaptive algorithm for the LCMMSE detector. Their performances are evaluated by computer simulations, which show the proposed LCMMSE detector outperforms some typical existing detectors.3. Constant Modulus (CM) is an important criterion in MUD. Since the cost functions of CM based Algorithms are nonconvex and there exists local minima. It is...