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Non-orthogonal multiple access (NOMA) is expected to be a promising technique for future wireless networks due to its superior spectral efficiency. In this paper, the sum rate optimization problem for multiple-input multiple-output (MIMO) NOMA systems is studied with the total transmit power constraint and the minimum rate constraint of weak user. We first derive a channel state information (CSI) condition in which MIMO NOMA systems can achieve full rate transmission, i.e. the transmission rate of the weak user equals to the channel capacity of weak user. Based on the CSI condition, we propose an optimal power allocation scheme for MIMO NOMA systems, which can achieve the capacity region of MIMO Broadcast channel as dirty paper coding. A low complexity suboptimal scheme is proposed as well for all CSI channel conditions. Numerical results show that the proposed NOMA schemes significantly outperform the traditional time division based single user MIMO scheme and the multi-user MIMO scheme.
Sun et al. (Sun,) studied this question.