  This paper presents a novel adaptive reduced-rank multi-input-multi-output
(MIMO) decision feedback equalization structure based on joint iterative
optimization of adaptive estimators. The novel reduced-rank equalization
structure consists of a joint iterative optimization of two equalization
stages, namely, a projection matrix that performs dimensionality reduction and
a reduced-rank estimator that retrieves the desired transmitted symbol. The
proposed reduced-rank structure is followed by a decision feedback scheme that
is responsible for cancelling the inter-antenna interference caused by the
associated data streams. We describe least squares (LS) expressions for the
design of the projection matrix and the reduced-rank estimator along with
computationally efficient recursive least squares (RLS) adaptive estimation
algorithms. Simulations for a MIMO equalization application show that the
proposed scheme outperforms the state-of-the-art reduced-rank and the
conventional estimation algorithms at about the same complexity.
