Abstract
Future wireless personal and mobile communication systems are expected to provide high-capacity flexible services. Direct-sequence code-division multiple-access (DS-CDMA) has emerged as a promising candidate to meet these challenges. It is well known that performance of DS-CDMA systems is interference-limited, where factors lead to such an obstruction can be classified into two categories. One is the narrowband interference (NBI) resulting from external overlaid narrowband services and the other is the wideband interferences, which include the multiple access interference (MAI) incurred from other co-channel CDMA users and the near-far effect originated with an unbalanced received power arrangement among the users. In this thesis, advanced receiver techniques capable of suppressing these interferences are considered through NBI suppression, multiuser detection, forward error correction (FEC), and power control. NBI suppression techniques play an important role in separating a DS-CDMA multiplex and the external narrowband jamming and hence heavily affect the performance of a CDMA system especially when the NBI is strong. Most of the existing methods on this issue either perform unsatisfactorily or involve high complexity. We present a new approach for NBI suppression in a DS-CDMA system. The proposed scheme is an adaptive nonlinear predictor with a novel feedback compensation for the predicted result. This scheme achieves comparable performance to the nearly optimal approach but involves much lower complexity. Multiuser detection techniques are essential to guarantee acceptable performance in DS-CDMA systems where signals conveying the desired information are received in the presence of MAI. Multiuser detection using multistage parallel interference cancellation (PIC) is particularly attractive owing to its simple structure and potential interference cancellation capability. We propose a low-complexity multistage turbo partial PIC (TPPIC) detector for DS-CDMA systems. At each stage of TPPIC, extrinsic information is extracted and then used as the a priori information for the next stage, just as the manner of powerful turbo processing. We also explore the properties of TPPIC with respect to some essential parameters and consider the incorporation of adaptive linear filtering techniques for further performance enhancement. The proposed TPPIC detector exhibits an impressive performance improvement over many existing multiuser detection schemes. Soft-information exchange between different receiver sub-modules has been demonstrated to give good performance. This motivated us to consider joint multiuser detection and FEC decoding in a DS-CDMA system. We focus on the joint design of the TPPIC-based multiuser detection and the turbo decoding algorithm, since turbo codes have been chosen as one of the coding schemes for the forthcoming third-generation wireless communication system. We propose a low-complexity iterative multiuser receiver based on TPPIC for a turbo-coded DS-CDMA system. This structure is further elaborated for performance reinforcement by incorporating the adaptive interference suppression techniques. The performance of the proposed iterative receiver is significantly better than that of the traditional non-iterative receivers and is close to that of some well-known iterative schemes of much higher complexity. Power control is aimed to mitigate the deleterious near-far effect and refers to a resource allocation technique that balances the received powers of the users so that no user creates excessive interference. To effectively overcome the near-far effect, we implement power control with various multiuser detection algorithms discussed in this thesis and present a new method for estimating the required parameter of power control. Extensive numerical results reveal that the proposed power control strategy is effective to attain a given quality of service in a near-far environment.