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We consider the system model with one base station ${\sf B}$ and $N$ downlink users ${\sf U}_1, \ldots, {\sf U}_N$; all terminals are equipped with a single antenna. In the traditional orthogonal multiple access methodology, $\sf B$ serves only one users at any time slot. In NOMA, $\sf B$ simultaneously serve all users by using the entire bandwidth to transmit data via a superposition  coding technique at the transmitter side and SIC techniques at the users. More specifically, the transmit signal of $\sf B$ is $\sqrt{P}\sum_{n = 1}^N \sqrt{\alpha_n} x_n$, where $P$ is the transmit power, $\alpha_n$ is the power allocation coefficient and $x_n$ is the message for ${\sf U}_n$. The received signal at ${\sf U}_n$ is   \begin{align}  y_n = h_n \sqrt{P}\sum_{m = 1}^N \sqrt{\alpha_m} x_m + v_n, v_n.  \nonumber  \end{align}  When the channels are ordered as $\left|h_1\right|^2 \leq \left|h_2\right|^2 \leq \cdots \left|h_N\right|^2$