Achievable Diversity Order of HARQ-Aided Downlink NOMA Systems

TL;DR

This paper characterizes the asymptotic outage probability scaling law, known as diversity order, of HARQ-aided downlink NOMA systems, revealing how different HARQ types and system parameters affect reliability and performance.

Contribution

It provides closed-form expressions for the diversity order of three HARQ types in downlink NOMA, including novel bounds for HARQ-CC and HARQ-IR, and analyzes their dependence on transmission rate and channel conditions.

Findings

HARQ-IR achieves the highest diversity order among the studied schemes.

Full diversity is attainable only at sufficiently low transmission rates.

Diversity order decreases as transmission rate increases.

Abstract

The combination between non-orthogonal multiple access (NOMA) and hybrid automatic repeat request (HARQ) is capable of realizing ultra-reliability, high throughput and many concurrent connections particularly for emerging communication systems. This paper focuses on characterizing the asymptotic scaling law of the outage probability of HARQ-aided NOMA systems with respect to the transmit power, i.e., diversity order. The analysis of diversity order is carried out for three basic types of HARQ-aided downlink NOMA systems, including Type I HARQ, HARQ with chase combining (HARQ-CC) and HARQ with incremental redundancy (HARQ-IR). The diversity orders of three HARQ-aided downlink NOMA systems are derived in closed-form, where an integration domain partition trick is developed to obtain the bounds of the outage probability specially for HARQ-CC and HARQ-IR-aided NOMA systems. The analytical results show that the diversity order is a decreasing step function of transmission rate, and full time diversity can only be achieved under a sufficiently low transmission rate. It is also revealed that HARQ-IR-aided NOMA systems have the largest diversity order, followed by HARQ-CC-aided and then Type I HARQ-aided NOMA systems. Additionally, the users' diversity orders follow a descending order according to their respective average channel gains. Furthermore, we expand discussions on the cases of power-efficient transmissions and imperfect channel state information (CSI). Monte Carlo simulations finally confirm our analysis.