Quasi-Degradation Probability of Two-User NOMA over Rician Fading Channels

TL;DR

This paper analyzes the probability that NOMA performs as well as the optimal capacity in two-user MISO channels with Rician fading, providing a closed-form approximation and insights into NOMA's effectiveness in practical LOS scenarios.

Contribution

It introduces a novel analysis of quasi-degradation probability for NOMA over Rician channels, extending previous Rayleigh-based results with a closed-form approximation.

Findings

QD probability depends on the angle between user channels.

The derived approximation accurately predicts NOMA performance.

Insights into NOMA's optimality in Rician fading environments.

Abstract

Non-orthogonal multiple access (NOMA) has a great potential to offer a higher spectral efficiency of multi-user wireless networks than orthogonal multiples access (OMA). Previous work has established the condition, referred to quasi-degradation (QD) probability, under which NOMA has no performance loss compared to the capacity-achieving dirty paper coding for the two-user case. Existing results assume Rayleigh fading channels without line-of-sight (LOS). In many practical scenarios, the channel LOS component is critical to the link quality where the channel gain follows a Rician distribution instead of a Rayleigh distribution. In this work, we analyze the QD probability over multi-input and single-output (MISO) channels subject to Rician fading. The QD probability heavily depends on the angle between two user channels, which involves a matrix quadratic form in random vectors and a stochastic matrix. With the deterministic LOS component, the distribution of the matrix quadratic form is non-central that dramatically complicates the derivation of the QD probability. To remedy this difficulty, a series of approximations is proposed that yields a closed-form expression for the QD probability over MISO Rician channels. Numerical results are presented to assess the analysis accuracy and get insights into the optimality of NOMA over Rician fading channels.