Two-Step User Pairing for OFDM-Based Cooperative NOMA Systems

In this letter, we propose a two-step user-paring scheme for an orthogonal frequency-division multiplexing (OFDM)-based cooperative non-orthogonal multiple-access (NOMA) system where far users cannot communicate with the base station directly. The first step is for allocating near users, and the second step is for allocating far users. To evaluate the performance of the proposed scheme, we derive the closed-form expression for the upper bound of the outage probability, and then obtain the achievable diversity gain. Numerical results show that the proposed scheme can achieve the optimal diversity gain confirmed by exhaustive search.