In this dissertation, three multiuser detectors are developed for different application scenarios in direct-sequence code-division multiple access systems. The first detector is an overlapping widow decorrelating detector aimed at asynchronous reverse links. In companion with the design of this detector, a study on the decay property of the ideal decorrelating impulse response is presented, resulting in a quantitative description of the decay rate as a function of the Cholesky factors of the cross-correlation matrix of user signature signals. This result can serve as a guide for determining window length of decorrelating or minimum mean-squared error multiuser detection in asynchronous multiuser systems. Based on this result, a signal-adapted window-length determination algorithm is developed for the proposed detector. Several supporting utilities for efficient implementation of the proposed detector are also described. The second detector is a linear multiuser detector that is also aimed at the reverse links. Particularly, it is desirable for cases where the number of users is small and, thus, significant performance gain over the existing linear multiuser detectors is possible. Unlike in the decorrelating and MMSE detectors, minimizing the bit-error rate is taken as the optimization objective in the proposed detector. To avoid undesired local minima of the highly nonlinear BER cost function, a set of convex constraints is proposed for the optimization problem. It is shown that this constrained optimization problem has a unique solution once the decorrelating detector exists. It is also shown that the proposed detector achieves the best performance among linear detectors for most realistic situations. In addition, a Newton barrier method is developed for efficiently calculating the coefficient vector of the proposed detector (i.e., the solution of the constrained optimization problem). The third detector is an adaptive detector that is aimed at the forward link where information about interfering users is often unavailable. The proposed detector consists of a bank of blind adaptive filters, one for each resolvable path, followed by a channel estimator and a coherent diversity combiner. To allow blind adaptation, the impulse response of each filter is decomposed into two orthogonal parts: one part is fixed as the decorrelating coefficient vector for the path in the absence of interfering users and the other is free to be adapted according to the mean-squared error criterion. Assuming perfect adaptation, the performance of the proposed detector is shown to be between those of the decorrelating detector and the minimum mean-squared error detector. Other studies conducted include the effects of fading on the performance of the proposed detector and the behavior of the proposed blind adaptation algorithm.

Future broadband wireless communication systems are expected to be able to offer new and powerful services enabling fast transmission rates of several tens of Mbit/s. This is an ambitious challenge especially for mobile communication systems since these systems should be able to cope with severely time dispersive channels, associated to the signal multipath propagation. Moreover, these systems should have high spectral and power efficiencies, as well as high capacity and flexibility. Spread spectrum techniques, particularly coded division multiple access (CDMA) techniques allow high capacity and flexibility, continuous transmission requiring low-peak power requirements for the amplifiers, as well as some robustness against fading and time-dispersion effects associated with the multipath propagation. When employed in prefix assisted (PA) block transmission schemes combined with frequency-domain receiver implementations they become especially interesting for broadband wireless systems. In Frequency-Domain Multiuser Detection for CDMA Systems the use of PA block transmission is considered in the context of both DS (Direct Sequence) and MC (Multicarrier) CDMA schemes. The main goal is the study of frequency-domain multiuser detection techniques with iterative signal detection/decoding techniques, also in combination with estimation and cancelation of nonlinear distortion effects. The receiver structures are suitable to scenarios with high interference levels and strongly time-dispersive channels.

This book covers the basic aspects of Code Division Multiple Access or CDMA. It begins with an introduction to the basic ideas behind fixed and random access systems in order to demonstrate the difference between CDMA and the more widely understood TDMA, FDMA or CSMA. Secondly, a review of basic spread spectrum techniques are presented which are used in CDMA systems including direct sequence, frequency-hopping and time-hopping approaches. The basic concept of CDMA is presented, followed by the four basic principles of CDMA systems that impact their performance: interference averaging, universal frequency reuse, soft handoff, and statistical multiplexing. The focus of the discussion will then shift to applications. The most common application of CDMA currently is cellular systems. A detailed discussion on cellular voice systems based on CDMA, specifically IS-95, is presented. The capacity of such systems will be examined as well as performance enhancement techniques such as coding and spatial filtering. Also discussed are Third Generation CDMA cellular systems and how they differ from Second Generation systems. A second application of CDMA that is covered is spread spectrum packet radio networks. Finally, there is an examination of multi-user detection and interference cancellation and how such techniques impact CDMA networks. This book should be of interest and value to engineers, advanced students, and researchers in communications.

With various added services in future wireless communication systems, the demand for fast and highly reliable data transmissions is continuously increasing. Multiple access interference (MAI) in direct-sequence code division multiple access (DS-CDMA) and constrained capacity in single-input single-output (SISO) are the major obstacles to achieve this purpose. For overcoming these limitations, the use of multiuser detection (MUD) and multiple-input multiple-output (MIMO) is necessary. In the area of error control coding, the Chase algorithm, which takes advantage of the different reliabilities of the received symbols and focuses on correcting the error in the weakest symbols, can achieve a near maximum-likelihood (ML) performance and with significant complexity reduction. In this thesis, we discuss the application of the Chase algorithm in multiuser detectors for the SISO-CDMA and MIMO-CDMA systems. In the SISO-CDMA systems, we develop a parallel interference cancellation (PIC) scheme using the Chase algorithm after the matched filters (MFs). This scheme is called Chase-PIC. Instead of optimizing the weight of each user as most of PIC detectors do, this detector provides a new optimizing approach right after the MFs. Based on the analysis of the complexity and performance over the flat and frequency-selective Rayleigh fading channels, we also propose a new Chase based multistage PIC that uses the normalized least-mean-square (NLMS) algorithm at the earlier stages and the Chase algorithm at the later stages. This proposed scheme can achieve a better performance than NLMS-PIC but with less complexity. In the MIMO-CDMA systems, we proposed a new Chase based multiuser detector over the flat Rayleigh fading channels. This detector focuses on correcting the error of the weakest detected symbol in the layered space-time multiuser detector (LAST-MUD). Compared to another Chase based detector, B-Chase, the proposed scheme has less complexity but with the tendency of achieving better performance at high signal-to-noise ratio (SNR).

Describes a technique called a Detector/Estimator (DE) which increases the flexibility of multiuser detection for direct sequence-code division multiple-access (DS/CDMA). Proposes a reduced complexity multiuser detector called a multistage multiuser detector (MMD) which removes identifiable multiple-access interference from a frequency hopping/code division multiple-access (FH/CDMA) channel.