Topics in signal processing for multi-user, multi-channel digital communications
Abstract
This thesis is in two sections, one on blind channel identification based on second-order statistics, the other on downlink equalization for Code Division Multiple Access (CDMA) systems. Both sections are applicable to multi-user communications systems in which multiple channels are obtained through oversampling at an integer multiple of the chip or symbol rate, and/or spatial diversity through multiple antennas. In the blind channel identification section, we present two novel algorithms for multiuser channel identification based on space-time covariance matrix estimates of the received signal at an antenna array. One algorithm uses two covariance matrices (Rxx[0] and Rxx[1]) to estimate the channel via an “eigenvector matching” approach. The other method is a “bilinear approach” in which products of channel parameters are first estimated from the covariance of the received data. The channel parameters are then obtained as the dominant eigenvectors of the outer-product estimate. Necessary and sufficient identifiability conditions are presented. Simulations show that the technique is applicable in some situations where typical identifiability conditions fail. Comparisons are made, when possible, to a “subspace” method incorporating knowledge of the basis functions. In the downlink equalization section, we derive and compare several linear equalizers for the CDMA downlink: minimum mean-square error (MMSE), zero-forcing (ZF), and the traditional matched filter (also known as the RAKE receiver). The downlink specific structure involves first inverting the multipath channel to restore the synchronous multi-user signal transmitted from the base-station at the chip-rate, and then correlating with the product of the desired user's channel code times the base-station specific scrambling code once per symbol to decode the symbols. The optimal linear symbol-level MMSE equalizer is derived and slightly out-performs the chip-level but at greater computational cost. An MMSE soft hand-off receiver is derived and simulated. Performance analysis and simulations illustrate that MMSE out-performs RAKE in many practical operating conditions, while the ZF equalizer is unsuitable due to noise gain.
Degree
Ph.D.
Advisors
Zoltowski, Purdue University.
Subject Area
Electrical engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.