Multiuser wireless communication over time and frequency selective channels

This thesis makes contributions in the design of code division multiple access (CDMA) systems for multiuser wireless communication over dispersive channels. The techniques developed address fundamental problems affecting reliable communication, such as channel dispersion effects, multiaccess interference (MAI), and system complexity.;The first contribution develops a framework for decentralized multiuser detection in direct sequence (DS)-CDMA systems. The framework is based on the notion of canonical multipath-Doppler coordinates that are defined by a fixed basis derived from a fundamental characterization of multipath propagation effects. It provides an integrated design approach for combating time-varying multipath dispersion, suppressing MAI and tailoring receiver complexity to a desired level of performance.;The second contribution is a signal model and a corresponding receiver structure for multicarrier (MC)-CDMA systems in the presence of imperfections, such as frequency offset, phase noise and fast fading. Such imperfections are commonly encountered in practice and destroy the orthogonality between subcarriers, thereby degrading performance. In the proposed receiver, the information transmitted on a particular subcarrier is decoded by processing a small subset of subcarriers surrounding the desired subcarrier. The receiver restores performance loss due to imperfections and exploits temporal channel variations for diversity.;The third contribution is a new signaling scheme for time- and frequency-selective channels that is a hybrid between MC- and DS-CDMA systems: time-frequency (TF)-CDMA. This signaling scheme approximately diagonalizes the doubly dispersive channel; that is, each basis function serves as an approximate eigenfunction of the channel. We also derive a near-optimal choice of basis parameters for given multipath and Doppler spreads of the channel.;The fourth contribution is a unified framework for describing and analyzing DS-CDMA, MC-CDMA and TF-CDMA systems. In particular, relations between the codes used in the three systems are derived which result in near-identical performance of the systems in a variety of channel conditions.