On the Power Allocation Limits for Downlink Multi-user NOMA with QoS

TL;DR

This paper analyzes the fundamental power allocation limits for downlink multi-user NOMA systems with QoS constraints, showing conditions for outage probability and comparing NOMA's performance to OMA.

Contribution

It derives power allocation coefficient limits ensuring successful SIC and decoding, and proves NOMA can match OMA outage performance with fixed power schemes.

Findings

Power allocation coefficients depend only on target rate and number of users.

NOMA with fixed power can match OMA outage performance.

A power allocation strategy can improve NOMA outage performance over OMA.

Abstract

The fundamental power allocation requirements for NOMA systems with minimum quality of service (QoS) requirements are investigated. For any minimum QoS rate $R_0$, the limits on the power allocation coefficients for each user are derived, such that any power allocation coefficient outside of these limits creates an outage with probability equal to 1. The power allocation coefficients that facilitate each user's success of performing successive interference cancellation (SIC) and decoding its own signal are derived, and are found to depend only on the target rate $R_0$ and the number of total users $K$. It is then proven that using these power allocation coefficients create the same outage event as if using orthogonal multiple access (OMA), which proves that the outage performance of NOMA with a fixed-power scheme can matched that of OMA for all users simultaneously. Simulations confirm the theoretical results, and also demonstrate that a power allocation strategy exists that can improve the outage performance of NOMA over OMA, even with a fixed-power strategy.