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Beamforming Techniques
for Multiuser Detection in
CDMA Systems

Michail K. Tsatsanis and Georgios B. Giannakis
University of Virginia
Department of Electrical Engineering
Charlottesville, VA 22903-2442
email: michail@virginia.edu

Existing optimal receivers for Code-Division Multiple Access (CDMA) systems are resistant to near-far effects, but their complexity grows exponentially with the number of users. This presentation introduces a novel viewpoint to CDMA modeling which incorporates multipath effects and allows alternative linear solutions. We develop a multirate/multichannel description which provides a link between the CDMA setup and corresponding array processing problems. Based on this formulation, novel optimal receiver designs are proposed. Conditions are derived under which linear decorrelating receivers exist in a multipath environment, i.e., receivers that completely suppress multiuser interference. Minimum mean square error (MSE) Wiener receivers have been proposed for this problem, but are sensitive to the accurate estimation of the users' and additive noise powers. In this work, the receiver is designed such that the MSE is minimized subject to the zero multiuser interference constraint. This approach is analogous to MVDR beamformers, where the interference is minimized subject to a constrained response at the desired direction. The constrained optimization problem is solved using Lagrange multipliers and the solution is independent of the additive noise power.



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