Non-Orthogonal Multiple Access Based on Superposition and Three-Index Assignment in a Rayleigh Fading Channel

The throughput in wireless cells using orthogonal multiple-access schemes like OFDMA is limited due to the fact that a significant part of the approximately mutually orthogonal radio resources are for fairness reasons assigned to users with high pathloss channels and therefore used with only low spectral efficiency. In this paper we study the performance of a multipleaccess scheme for the uplink of a wireless system based on nonorthogonal superposition in a fading channel with single antennas both at the base station and at the terminals. Signals from at least two users are superimposed on each of the mutually orthogonal radio resources. At the receiver the superimposed signals are separated by non-linear multi-user detection based on e.g., successive interference cancellation. We propose a scheduling scheme based on solving a so-called three-index assignment problem for assigning pairs of users to resources in the case where not all resources have equal performance. By pairing users with different path losses and assigning them in an optimal way to appropriate resources significant gains in cell throughput, cell-edge throughput and fairness are possible compared to the orthogonal (single-user per resource) case. The system performance with the proposed scheduling scheme is significantly better than with random scheduling.