Two-Stage Power Allocation for Cooperative NOMA in D2D Communications with Imperfect CSI

In this paper, a novel cooperative non-orthogonal multiple access (NOMA) in device-to-device (D2D) communications scheme is proposed. We investigate the two-stage power allocation problem to maximize the sum rate in this system, and considering the fairness factor simultaneously. Since perfect knowledge of channel state information (CSI) may be difficult to acquire at the base station (BS), we innovatively consider the imperfect CSI in our system. It should be noted that our formulated problem is a mixed fraction non-linear programming problem, thus we propose the convex approximation method to decouple the problem in two sub-problems: 1) power allocation for the D2D users (DUE) from BS; 2) power allocation for far DUEs from the DUE relay. And we use Lagrangian multiplier method to get the optimal power allocation coefficients for two-stage data transmission. Moreover, the theoretical analysis and simulation results are presented to evaluate our proposed scheme for the cooperative NOMA in D2D communications.