Resource Allocation for NOMA Networks under Alternative Outage Constraints

In non-orthogonal multiple access (NOMA) systems, the outage is considered to happen when a user cannot correctly decode the messages for the users with higher decoding order and hence the successive interference cancellation (SIC) is failed in traditional definition. However, in this case, the user may still correctly decode its message by treating the uncancelled signal as interference and the outage is avoided. By considering this behavior, a more accurate alternative outage probability can be defined. In this paper, we investigate user scheduling and power allocation for a downlink NOMA system with imperfect SIC by employing the alternative outage probability as then performance metric. The coupling of user scheduling and power allocation makes the problem complicated. Therefore, we propose a two-phase algorithm, in which the user scheduling is first optimized through a matching theory based algorithm, and then power allocation is performed with the aid of the concave-convex procedure (CCCP) method. Simulation results show that the proposed low-complexity algorithm can achieve near-optimal performance and the algorithm based on the alternative outage probability outperforms the traditional one when the decoding is significantly affected by imperfect SIC.