# Fundamentals of Power Allocation Strategies for Downlink Multi-user NOMA with Target Rates

## TL;DR

This paper investigates power allocation in downlink multi-user NOMA systems with SIC, establishing fundamental limits related to target rates, and demonstrates that NOMA can outperform OMA in outage probability without needing instantaneous channel gains.

## Contribution

It introduces the concept of well-behaved power allocation strategies based solely on target rates and proves their existence and optimality in improving outage performance over OMA.

## Key findings

- NOMA can outperform OMA in outage probability without channel gain knowledge.
- Well-behaved power allocation strategies are sufficient for performance gains.
- The proposed SIC decoding order is energy efficient.

## Abstract

For downlink multi-user non-orthogonal multiple access (NOMA) systems with successive interference cancellation (SIC) receivers, and a base-station not possessing the instantaneous channel gains, the fundamental relationship between the target rates and power allocation is investigated. It is proven that the total interference from signals not removed by SIC has a fundamental upper-limit which is a function of the target rates, and the outage probability is one when exceeding this limit. The concept of well-behaved power allocation strategies is defined, and its properties are proven to be derived solely based on the target rates. The existence of power allocation strategies that enable NOMA to outperform OMA in per-user outage probability is proven, and are always well-behaved for the case when the outage probability performance of NOMA and OMA are equal for all users. The proposed SIC decoding order is then shown to the most energy efficient. The derivation of well-behaved power allocation strategies that have improved outage probability performance over OMA for each user is outlined. Simulations validate the theoretical results, demonstrating that NOMA systems can always outperform OMA systems in outage probability performance, without relying on the exact channel gains.