Adaptive estimation and equalization of fading channels and code-timing acquisition of DS-CDMA signals.

摘要

This thesis covers two topics of great interest in wireless communications and signal processing: aspects of adaptive fading channel estimation and equalization, and code-timing acquisition of DS-CDMA signals. First, several aspects of fading channel estimation and equalization are explored. Adaptive receiver design requires accurate adaptive channel estimation and/or effective equalization. Typical adaptive algorithms suffer from conflicting requirements of large step-size to maximize convergence rate and small step-size to minimize misadjustment. An adaptive channel estimation algorithm is then proposed, which exploits knowledge of link parameter information and can provide improved performance. The resulting algorithm is a type of variable step-size LMS algorithm. Its stability is studied as well. Another aspect that we look into is the tracking capability of LMS for fading channels. By approximating fading variation as an autoregressive (AR) process, it is shown that there exist a fading rate above which LMS is ineffective. This thesis also analyzes the performance of direct and indirect equalization of fading channels, the results indicate that indirect techniques generally work better. Second, code-timing acquisition of DS-CDMA signals is investigated. Future CDMA systems may be required to operate with low processing gain in order to accommodate high rate users. The resulting increase in channel dispersion will have a detrimental impact on code-timing acquisition. A multiuser and a single-user Maximum-Likelihood code-timing acquisition algorithms are then proposed for multipath channels. By exploiting the multipath diversity and the correlations between signals from different paths, the estimators are shown to be robust to near-far power ratio and channel dispersion.