An Investigation of Carrier Recovery Techniques for PSK Modulated Signals in CDMA and Mulipath Mobile Environments
Nicoloso, Steven P.
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The challenge of carrier recovery for digitally phase modulated, suppressed-carrier signals has been the target of much attention in mobile radio system design for many years. Code division multiple access (CDMA) cellular systems are now being deployed in mobile environments, and the performance of carrier recovery for CDMA is much less well understood. CDMA systems typically utilize lower bit energies, relying instead upon coding gain for reliable performance. As an additional challenge, the multiple access interference (MAI) inherent to direct-sequence spread-spectrum (DSSS) signalling further hampers many carrier recovery techniques. This thesis first surveys general carrier recovery strategies, conventional as well as new, and then applies them to the particular task of recovering a suppressed carrier in mobile CDMA systems. First some of the theory surrounding conventional, closed-loop, carrier recovery techniques and several newer open-loop estimation structures, based on the maximum likelihood (ML) principle is presented. A novel decision-feedback ML phase estimator is proposed. The statistical behavior of this new structure as well as the related squaring estimator are examined and are shown to be equivalent to the stochastic performance of classical closed-loop techniques. Candidate receivers based upon closed- and open-loop carrier recovery structures are proposed and then examined via simulation. Both forward and reverse cellular links are examined under a variety of both of single-path and multipath conditions. The notion of exploiting cyclostationary MAI in CDMA systems by applying adaptive receiver techniques is also examined, especially as this scheme relies to some extent upon coherent carrier recovery for its promising performance enhancements. This work concludes with recommendations of the best carrier recovery strategies for the variety of environments examined and suggestions for future research are made.
- Masters Theses