On the Power Allocation for a Practical Multiuser Superposition Scheme in NOMA Systems

In this letter, we study nonorthogonal multiple access (NOMA) systems with a linear multiuser superposition transmission (MUST) scheme. To perform the power allocation or the determination of the transmission power ratio for linear MUST with practical modulation schemes of finite constellation sizes, we consider the mutual information. An approach based on Monte Carlo simulations is devised to obtain the mutual information. While the sum rate maximization (based on the capacity) has a trivial solution, numerical examples show that there is a nontrivial solution to the power allocation problem for maximizing total mutual information. Thus, for certain given conditions (e.g., the total transmission power and modulation schemes), we are able to decide the optimal power (and rate) allocation for the linear MUST scheme to maximize the total mutual information.