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In this letter, the performance of non-orthogonal multiple access (NOMA) is compared with conventional orthogonal multiple access (OMA) over multiple-input multiple-output (MIMO) channels. It is proved analytically that for a simple scenario of two users, MIMO-NOMA dominates MIMO-OMA in terms of both sum rate and ergodic sum rate. Furthermore, for a more practical scenario of multiple users, with two users paired into a cluster and sharing a common transmit beamforming vector, the conclusion still holds. Numerical simulations are conducted, which corroborate the analytical findings.