An efficient algorithm is proposed to identify the active users and extracting their respective timing information in asynchronous direct sequence CDMA (DS-CDMA) communication system over Rayleigh fading channel. The joint identification and timing estimation algorithm is derived by performing discrete Fourier transform (DFT) on the observation vector and exploiting the uniqueness and nullity characteristics of the root-MUSIC test polynomial. The root-MUSIC based algorithm is shown to be asymptotically near-far resistant. Compared to the maximum a posteriori (MAP) or maximum likelihood (ML) based multiuser timing estimator, the complexity is greatly reduced by separating the multi-dimensional optimization problem into several polynomial rooting problems. Moreover, we characterize the dependence of system performance with respect to signature sequence length, number of active users, window size, desired user's signal-to-noise ratio (SNR) and crosscorrelation property of the code structure. The analytical results reveal that under the uncorrelated Rayleigh fading model, the root-MUSIC timing estimator tends to achieve the Cramer-Rao lower bound (CRLB) at interesting signature sequence length and desired user's SNR.

A new timing estimation algorithm for asynchronous DS/CDMA multiuser communication system is proposed in this paper. The algorithm is based on the Minimum Variance Distortionless Response (MVDR) beamforming technique that minimizes the beamformer's output power with the constraint that only the signal with exact timing is distortionlessly passed. Exploiting the characteristics that the MVDR beamformer's output power is severely degraded according to erroneous timing estimation, we develop an efficient algorithm to estimate each user's timing by scanning the beamformer's output power variation. Compared to the maximum a posteriori (MAP) or maximum likelihood (ML) based multiuser timing estimator, the complexity is extensively reduced by separating the multi-dimensional optimization problem into several one-dimensional optimization problems. Furthermore, the algorithm is computationally feasible than the subspace-based timing estimator since no eigendecomposition (EVD) is required. Moreover, the proposed algorithm is near-far resistant since the MVDR beamformer is inherently energy independent to the interferers.

This paper aims to provide a robust multiuser detection structure that adaptively tracks signature waveform distortion for CDMA multipath signals. In practical wireless environment, multipath fading leads to signature waveform distortion that severely degrades the performance of the linear multiuser detectors (LMDs) designed by exploiting the original signature waveform. In what follows, an iterative algorithm is proposed to track the signature waveform perturbation. The rationale of adaptive processing is based on the subspace method and the Minimum Variance Distortionless Response (MVDR) beamforming concept. Performance evaluation reveals that the proposed adaptive multiuser detection structure reduces the impact of signature waveform perturbation on the performance of the LMDs to a great extent. Moreover, the proposed iterative algorithm is near-far resistant since both the subspace method and the MVDR beamforming technique are energy independent to the interferers.