  This paper investigates the achievable sum rate of multiple-input
multiple-output (MIMO) wireless systems employing linear minimum mean-squared
error (MMSE) receivers. We present a new analytic framework which unveils an
interesting connection between the achievable sum rate with MMSE receivers and
the ergodic mutual information achieved with optimal receivers. This simple but
powerful result enables the vast prior literature on ergodic MIMO mutual
information to be directly applied to the analysis of MMSE receivers. The
framework is particularized to various Rayleigh and Rician channel scenarios to
yield new exact closed-form expressions for the achievable sum rate, as well as
simplified expressions in the asymptotic regimes of high and low signal to
noise ratios. These expressions lead to the discovery of key insights into the
performance of MIMO MMSE receivers under practical channel conditions.
