Abstract
This paper presents a new power allocation method for two-user downlink non-orthogonal multiple access (NOMA) systems with imperfect channel estimation. Given channel estimates and the corresponding mean-squared error (MSE) information, capacity lower bounds are used to determine an upper bound of the system outage probability (SOP) based on the transmission rate requirements of both users. Subsequently, a closed-form power allocation solution is derived for the two users such that the SOP upper bound is minimized under a total power constraint. The solution requires only a few numerical operations to calculate the power allocation factor according to the MSE information of channel estimation. Its complexity is lower than existing closed-form or iterative solutions. Simulation results demonstrate that the proposed method achieves pretty good performance in terms of the SOP upper bound and the bit error rate for different transmission rate requirements.