Low-Complexity Power Allocation for Outage Balancing in NOMA Systems with Imperfect CSIT

This paper investigates the outage performance of a two-user downlink non-orthogonal multiple access (NOMA) system with imperfect channel state information at the transmitter (CSIT). A general closed-form outage probability expression is derived under consideration of imperfect successive interference cancellation (SIC) for both NOMA users based on instantaneous channel estimates and the corresponding mean-squared errors. With the derived expression, a low-complexity power allocation formula is proposed for outage balancing, i.e., minimizing the maximum user outage probability, in order to provide robustness against channel uncertainty. As compared with a previous iterative power allocation method for the same purpose, the proposed one involves much lower complexity and achieves a significantly better balance between the outage performance for the two users when there exist SIC errors.