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In practical wireless multiple-input multiple-output (MIMO) communications system, the channel estimation must be imperfect. Unfortunately, the channel estimation errors have not been taken into account by existing soft-output MIMO minimum mean square error (MMSE) vertical Bell lab space time (V- BLAST) detectors when calculating the log-likelihood ratio of each coded bit, i.e., the soft information. As a result, the system performance can be significantly degraded. In this paper, we propose a novel soft-output MIMO MMSE V-BLAST detector based on random vector theorem, which takes the channel estimation error into account in the computation of the MMSE filter and LLR of each coded bit. Furthermore, a simplified implementation of the proposed detector is given so that the variance of channel fading coefficient is not needed. When compared with existing MIMO MMSE V-BLAST detectors, our simulation results show that the proposed novel detector can obtain significant performance gain at the cost of negligible increase of complexity.