Blind channel estimation and single-user detection for multi-carrier and spread-spectrum systems with transmit diversity.

摘要

The application of transmit diversity techniques such as Space-Time Block Coding (STBC) to the downlink of multiuser wireless communications systems has received considerable attention. The main advantage of such an approach is its ability to provide diversity gains through the use of multiple antennas only at the transmitting side without significantly increasing the complexity at the receiving end. Among the many multiple access techniques proposed, Multi-Carrier (MC) and Direct Sequence (DS) Code Division Multiple Access (CDMA) techniques are known as the most promising candidates for future broadband mobile communication networks. In MC and DS-CDMA systems or combination thereof, employing transmit diversity, the spatial diversity gains can only be realized if the underlying channels are accurately acquired at the receiver. Furthermore, such systems suffer from high computational complexity which should be properly addressed for practical implementations.;Motivated by these observations, in the first part of this dissertation, we introduce the chip- level ST block coding scheme for DS-CDMA systems. For this scheme, we address the problems of single-user detection as well as blind channel estimation, and we show that chip-level coding does not suffer from antenna order ambiguity. Moreover, we demonstrate that chip-level schemes exhibit low decoding delay and allow for the design of adaptive single-user detectors with improved short data-record performance characteristics compared to their symbol-level counterparts.;In the second part of this dissertation, we present a novel transmission scheme for the downlink of MC-CDMA systems with transmit diversity that is based on chip-level Space-Frequency (SF) block coding. For this scheme, we investigate the problem of blind channel estimation when the received signal processing is done (i) pre-Fast Fourier Transform (FFT); and (ii) post-FFT. We propose two blind channel estimation algorithms based on subspace and Minimum Variance Distortionless Response (MVDR) principles. Moreover, we present an analytical performance analysis of the proposed algorithms by investigating the bias as well as the finite data record mean-square error of the channel estimates. Our analysis shows that SFBC MC-CDMA systems do not suffer from antenna order ambiguity. In addition, to benchmark the accuracy of our estimation algorithms, we derive the corresponding Cramer-Rao bounds (CRB) based on a novel approach that assumes the knowledge of only the spreading code of the desired user. Our approach has the advantage of providing lower bounds which are tighter than the CRBs with known signatures.;We also study the problem of single user detection for downlink transmissions to address the issue of multiuser interference. In the case of the post-FFT approach, we take advantage of the SFBC-induced signal structure to derive linear single-user detectors with improved performance in short data-record situations. Finally, in order to address the issue of computational complexity, we exploit the structure of the covariance matrix of the received signal to simplify the computations involved in estimating the channel, and forming the detector.