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In this paper, we study optimal power allocation for multiple-input multiple-output (MIMO) nonorthogonal multiple access (NOMA) systems when a layered transmission scheme is employed. An approach to maximize the sum rate of MIMO-NOMA with layered transmissions is proposed once we show that the sum rate is concave in allocated powers to multiple layers of users. We also derive a closed-form expression for the average sum rate when statistical channel state information (CSI) is available at a transmitter, which allows us to allocate powers to multiple layers for the maximization of the average sum rate. We find lower- and upper-bounds on the average sum rate, from which it is shown that the scaling property of MIMO-NOMA with layered transmissions also holds as conventional MIMO does (i.e., the average sum rate grows linearly with the number of antennas).
Jinho Choi (Fri,) studied this question.