Uppsala universitet
The Multivariable Decision Feedback Equalizer
Multiuser Detection and Interference Rejection

Claes Tidestav

PhD Thesis, Uppsala University, ISBN 91-506-1371-5, December 1999.

The thesis available in Pdf.

Paper copies of the thesis can be obtained from Ylva Johansson, Signals and Systems Group, Uppsala University, Box 534, SE-75121 Uppsala, Sweden.

During the last three decades, decision feedback equalizers (DFEs) have been used in digital communications to suppress intersymbol interference. The DFE is a nonlinear symbol-by symbol detector which uses two linear filters and a feedback from old decided symbols. It provides a good compromise between performance and complexity, obtaining a much better performance than a linear equalizer, at a much lower complexity than for the optimum detector, the maximum likelihood sequence estimator (MLSE).

Multivariable DFEs, which use several sensor signals to simultaneously produce estimates of several messages, may be used to suppress not only intersymbol interference in one channel, but also interchannel interference. The present thesis investigates their design, their performance and their use in several applications.

More details on CDMA applications
More details on multi-antenna receiver applications .

The multivariable decision feedback equalizer is investigated as a tool for multiuser detection and interference rejection. Three different DFE structures are introduced.

The first DFE has a non-causal feedforward filter and a causal feedback filter. We show how its parameters can be tuned to give a minimum mean-square error.

The second DFE is derived under the constraint of realizability. The explicit structure and design equations for an optimum realizable minimum mean-square error DFE are obtained. The zero-forcing criterion is also considered, and conditions for the existence of a zero-forcing solution are derived.

We then consider a DFE where both feedforward and feedback filters are FIR filters of predetermined degrees. We discuss the tuning procedure for obtaining the parameters of a minimum mean-square error DFE and present the conditions for the existence of a zero-forcing solution.

Two specific applications are considered next. In the first scenario, an antenna array is used at the receiver in a cellular system to accomplish spatial division multiple access. We compare two DFEs, operating as multiuser detectors and interference cancellers, respectively, and it is demonstrated that the difference in performance is small when few users are active in the system. We also show that the parameter estimation problem is more complicated for interference rejection.

The second application is multiuser detection in DS-CDMA. A family of minimum mean-square error detectors with different amounts of decision feedback is designed, based on a possibly rapidly time-varying linear model. The linear model includes effects of the multipath channel, pulse shaping filters and the spreading. We also show that near-far resistance of the minimum mean-square error detectors can be guaranteed if and only if the parameters of the detector can be tuned so that the zero-forcing condition is fulfilled.

Decision feedback equalizer, multiuser detection, interference rejection, antenna arrays, DS-CDMA, near-far resistance.

Related publications:
IEEE-Com October 1999 on channel reuse within cells.
Submitted paper on the general realizable multivariable DFE.
ICASSP'99 conference paper on the general realizable multivariable DFE.
ICUPC'98 conference paper on iterative "bootstrap" improvement of equalizers.
ICUPC'96 conference paper on designing equalizers based on explicit channel models of DS-CDMA systems.
PIMRC'95 conference paper on multiuser detection using a multivariable DFE.
Master thesis (1993) by Claes Tidestav on reuse within cells using DFEs.

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