| In this dissertation, issues arising in code acquisition of two different CDMA applications, i.e., overlay and transmit diversity, have been investigated. In a CDMA overlay system with transmitter filtering, the spreading sequence is altered to form a filtered sequence with spillover. A reasonably approximated system model is formulated in this scenario. The MUSIC estimator, the LSML estimator and the MMSE estimator are first developed in AWGN channels based on this system model. It is shown that, due to the effect of notch filtering, the signal subspace dimension is increased to 3K in the MUSIC estimator and the interference subspace dimension is increased to 3( K − 1) in the LSML estimator and the MMSE estimator, where K is the number of the CDMA user. Furthermore, three estimators have been extended to flat fading channels. It is shown that the performance of the LSML estimator and the MMSE estimator suffer a loss in fast fading channels. The eigenanalysis method and phase precompensation have been proposed to improve the performance of the LSML estimator and the MMSE estimator, respectively.; Next, code acquisition performance in DS-CDMA systems employing two possible transmit diversity schemes, i.e., orthogonal transmit diversity simulcast (OTD-S) and space time selective spreading transmit diversity (STSTD), is studied. Two timing estimators have been proposed. One is a training based single user maximum likelihood (SUML) estimator. The other is the blind MUSIC estimator. Both the analysis and simulation results show that the presence of multiple transmit antennas did improve the performance of the code timing estimators, although for reasonably heavily loaded systems in which channel state information was not available, the MUSIC based estimator gave better performance in a single antenna system than in either of the dual antenna transmit diversity schemes. It was also found that the training based single user estimation schemes outperformed the blind MUSIC schemes, even in the presence of heavy multiple access interference. The sensitivity of the timing estimators to carrier frequency offset was studied via simulation. A simple frequency offset estimator was proposed and shown to complement the SUML timing estimator quite well in the sense that together the joint estimators allowed for timing estimation in the presence of fairly substantial frequency offsets. |
