In this dissertation, direct-sequence code-division multiple access (DS-CDMA) systemswith multiuser detectors used at receiver are investigated and two kinds of multiuser detectors are developed for DS-CDMA systems. In the investigation of DS-CDMA systems using multiuser detectors at receiver, a study on the performance of the system is presented, where heterogeneous traffic with different transmission rates and quality of service (QoS) requirements is supported. The effects of some realistic factors, such as imperfect power control and the existence of multiple cells, on the system performance are studied. In addition, algorithms are proposed to deal with the forward link power allocation problem based on the measurements of random characteristics of the received signals. This power allocation problem is formulated as a constrained optimization problem. To make the problem easy to solve, an additional appropriate constraint is proposed. Two methods are developed to identify the feasible region of this constrained optimization problem.The first proposed multiuser detector is an adaptive minimum mean-squared-error(MMSE) detector. Particularly, it is desirable for the cases where communication channelshave severe near-far problem, and thus the convergence rates of adaptive MMSE detectorsfor users with different power are quite different. To improve the convergence ratesof adaptive MMSE detectors for weak power users, the interference effects of the strongpower user signals are subtracted from the received signal successively. The method to estimatethe parameters required in the proposed detector is also developed. It is shown thatthe proposed detector achieves fast convergence rates in various near-far scenarios. Otherstudies conducted include the transient mean-squared-error (MSE) analysis to explain thedifferent convergence rates of adaptive MMSE detectors for users with different power, and the bit-error-rate (BER) performance analysis for the proposed detector. The second proposed multiuser detector is a set of semi-blind linear parallel interferencecancellation (PIC) detectors for the reverse link of multiple-cell systems, where only information about intra-cell users is available. To decrease the interference from inter-cellusers whose information is unavailable to the receiver, the inter-cell user signal subspace is identified first by making use of the available information about intra-cell users. The eigen vectors and eigen values of this signal subspace are then used in the traditional linear PIC structure, in place of the unknown inter-cell users' signature codes and signal amplitudes. Based on this idea, three detection schemes are proposed. In addition, an efficient adaptation implementation method is developed, and the performance of the proposed detectorsis studied. The proposed detectors are shown to be suitable for practical implementations and have satisfactory performance.