Repetition-Based NOMA Transmission and Its Outage Probability Analysis

In this paper, we discuss a non-orthogonal multiple access (NOMA) scheme to exploit a high diversity gain using repetition, namely repetition-based NOMA. Unlike conventional power-domain NOMA, all the users can have the same transmit power, but different number of repetitions. Thanks to a high diversity gain, a low outage probability can be achieved without instantaneous channel state information (CSI) feedback for power allocation. A closed-form expression for an upper-bound on the outage probability is derived so that the values of key parameters can be decided to maintain the outage probability below a target value. We also consider the average error probability for finite-length codes. Simulation results are compared with the derived bounds and it is shown that the bounds are reasonably tight and can be used to decide key parameters (e.g., code rates) to guarantee target error probabilities.

I. Introduction

Since non-orthogonal multiple access (NOMA) has a higher spectral efficiency than orthogonal multiple access (OMA), it has been extensively studied [1], [2], although there are a number of challenges (e.g., optimal user clustering [3] and beamforming [4], [5]). In [6], [7], the notion of NOMA can be employed in uncoordinated transmissions such as random access for uplink transmissions in order to improve the throughput, which is important for massive machine-type communication (MTC) that provides the connectivity for various Internet-of-Things (IoT) applications [8].