The non-orthogonal multiple access (NOMA) technique or more generally the non-orthogonal principle is a crucial component for the next generation cellular system. For two-user downlink NOMA, the constellations of near and far users are superposed for transmission, and maximum likelihood (ML) or successive interference cancellation (SIC) receiver is used for reception. In this paper, we aim to further enhance the linklevel performance for NOMA with ML receiver. A method of NOMA constellation rotation is proposed where the constellations are respectively rotated before superposition so as to exploit signal space diversity. The symbol error rate (SER) upper bounds for both near and far users are derived and used for rotation angle optimization. Within this framework, we show the best we can do in terms of enhancing the performance for a particular user (either near or far). Simulation results have verified the effectiveness of our proposed method. It can be observed that there is performance gain for one user while almost no negative impact on the other user.