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The low-rank minimum mean-squared error (MMSE) detector for direct sequence (DS) spread-spectrum code-division multiple-access (CDMA) is proposed. The low-rank linear MMSE detector lies in a subspace of the signal space. We demonstrate that when the dimension of the signal space is lower than the processing gain and the MMSE detector is obtained using the estimated covariance matrix, the low-rank MMSE detector can perform better than the full rank MMSE detector. A subspace tracking algorithm that tracks both the rank and the signal subspace is used to implement the low-rank MMSE detector adaptively.