| Pilot-based, blind and semi-blind channel estimation and equalization are investigated for three different scenarios. Improved algorithms are developed and analyzed, exploiting the properties of the scenarios, either by proper parametrization, or by taking into account the structure imposed by the particular scenario.; In the first scenario, pilot-based channel parameter estimation problem is addressed in a multiuser Direct-Sequence Code-Division Multiple-Access (DS-CDMA) framework, which is the emerging physical layer signaling for wireless communication standards. The focus in this scenario is on sparse channels, where the delay spread is large with respect to the number of strong paths. As data rates increase, the channel experienced by the DS-CDMA signal will become more sparse in nature. Rather than employing the conventional discrete time tapped delay line model, a different parametrization is investigated, where each path is represented by its continuous delay and complex gain. In addition, realistic bandlimited pulse shaping is assumed. Use of spreading codes optimized to estimate the continuous delays is proposed. In the second scenario, semi-blind channel estimation is studied for multiuser DS-CDMA systems where the pilot signal and data conveying signal are transmitted in parallel. This is one of the signaling modes proposed for the third generation Wideband CDMA (WCDMA) cellular standards. It is illustrated that performance gain using semi-blind estimation is larger in parallel transmission case than the gain in serial transmission case. However the relatively low complexity methods which have closed form solutions are exclusively for serial transmission case. Low complexity approximations to the Gaussian maximum-likelihood method are proposed, where the pilot and the data are code multiplexed and transmitted in parallel. Approximations are justified by the analytical and simulation results. In the third scenario, the focus is on blind receivers for multiple antenna systems using space time coding. In particular unitary space-time coding is considered. A linear equalizer which mitigates the inter-symbol interference is studied rather than channel estimation for this scenario. The proposed equalizer exploits the structure in the unitary code in order to cancel the inter-symbol interference. |
