In the process of maximising the utilisation of spatial and radio resources, interference is the major factor limiting the system capacity. In this Letter, a novel multi-carrier signal alignment (MCSA) scheme based on diagonal-block channel matrix is first proposed, to mitigate the interference in downlink multi-input multi-output non-orthogonal multiple access (MIMO-NOMA) cellular networks. The maximum degree of freedom (DoF) can be achieved by the iterative design of MCSA. Simulation results show the effectiveness and superiority of the proposed MCSA scheme in improving the sum-rate and DoF, by fully using multiple antennas and multiple carriers. [ABSTRACT FROM AUTHOR]