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.

A convenient signaling scheme, termed orthogonal on-off BPSK (O3BPSK), along with a simple one-shot linear decorrelating detector (LDD) has been proposed by Zheng and Barton as a technique for near-far resistant detection in the asynchronous DS/CDMA systems. The temporally adjacent bits from different users in the received signals are decoupled by using the on-off signaling, and the data rate is maintained with no increase in transmission rate by adopting an orthogonal structure. The system performance of this signaling scheme in terms of bit error rate (BER) has been analyzed over an AWGN channel by Zheng and Barton. In this paper, we further study the system performance over Nakagami-m fading channel. A closed form for the BER of such a scheme is successfully derived. Numerical results show that the O3BPSK signal scheme along with the LDD receiver still offers a good near-far resistant property over Nakagami-m channel.