In this letter, a decision-directed MOE detector with excellent robustness against signature waveform mismatch is proposed for DS-CDMA systems. Both the theoretic analysis and computer simulation results demonstrate that the proposed detector can provide better SINR performance than that of conventional detectors.

In this paper, the design of signature waveforms for asynchronous CDMA systems equipped with a correlation receiver is first considered. Optimal signature waveforms that minimize the average multiple access interference (MAI) at the output of a correlation receiver are found, while satisfying the constraint on available transmission bandwidth. Comparison to signature waveforms previously obtained for synchronous systems is also made to justify the superior performance of the designed signature waveforms in asynchronous systems. Furthermore, for direct-sequence CDMA (DS-CDMA) systems with random signature sequences, the use of multiple chip waveforms is also proposed as a means of suppressing MAI. Bandwidth constrained multiple chip waveforms that maximize the signal-to-interference ratio (SIR) at the output of each correlation receiver are found. Numerical results show that by using double chip waveforms instead of a single chip waveform, it is possible to reduce the MAI by 10% for a fixed transmission bandwidth (or equivalently, to save about 10% of transmission bandwidth for a given SIR requirement). The advantage of using double chip waveforms is also demonstrated in terms of the bit error rate (BER), whose calculation is based on our extension to Holtzman's approximation in.

Synchronous Gaussian code-division multiple access (CDMA) systems employing group-orthogonal signature waveforms are proposed and analyzed. All users in the system are divided into groups of users. The signature waveforms are constructed such that all the signature waveforms in one group are orthogonal to all the signature waveforms used in all other groups. This construction of signature waveforms ensures that there is no inter-group interference (i.e., among users in different groups), but at the expense of having intra-group interference (i.e., among users in the same group). However, by choosing a small size for each group, the intra-group interference can be effectively handled by a low-complexity, optimal (or suboptimal) multiuser detector. It is shown that a significant improvement in the system capacity can be achieved by the proposed technique over the conventional one that uses signature waveforms constructed from Welch-bound-equality (WBE) sequences. In particular, it is demonstrated that, while the conventional system's error performance is very sensitive to even small amount of overload, the proposed system with an appropriate design of signature waveforms can achieve a much higher overload (up to 300% as shown in the paper) with an excellent error performance.

This paper studies the optimization of signature waveforms and power allocation for synchronous code-division multiple access (CDMA) systems under the root-mean-square (RMS) bandwidth constraint. The optimization is considered for two types of receivers, namely the conventional matched filter (MF) receiver and the minimum mean-square error (MMSE) receiver. For both cases, the optimization criterion is to maximize the average signal to interference ratios (SIRs) at the receivers' outputs. For a given RMS bandwidth constraint and an arbitrary power allocation scheme, a procedure to obtain the optimal signature waveforms is provided. Based on this procedure, it is then shown that the optimal power allocation is achieved when all the received powers are equal. With the optimal power allocation, solutions for the optimal signature waveforms are presented and discussed in detail. It is also demonstrated that, compared to the previously obtained Welch-bound-equality (WBE) signature waveforms, the proposed signature waveforms can significantly improve the user performance.

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.