# NOMA in the Uplink: Delay Analysis with Imperfect CSI and Finite-Length Coding

## TL;DR

This paper analyzes the performance of uplink NOMA versus OMA under URLLC requirements, considering ideal and practical conditions, and finds that NOMA often underperforms due to finite-length coding and imperfect CSI.

## Contribution

It provides a comprehensive delay analysis of uplink NOMA with imperfect CSI and finite-length coding, highlighting limitations compared to OMA in URLLC scenarios.

## Key findings

- Joint decoding performs well under delay constraints.
- NOMA with SIC often underperforms OMA in practical URLLC settings.
- Finite-length coding and imperfect CSI significantly impact NOMA performance.

## Abstract

We study whether using non-orthogonal multiple access (NOMA) in the uplink of a mobile network can improve the performance over orthogonal multiple access (OMA) when the system requires ultra-reliable low-latency communications (URLLC). To answer this question, we first consider an ideal system model with perfect channel state information (CSI) at the transmitter and long codewords, where we determine the optimal decoding orders when the decoder uses successive interference cancellation (SIC) and derive closed-form expressions for the optimal rate when joint decoding is used. While joint decoding performs well even under tight delay constraints, NOMA with SIC decoding often performs worse than OMA. For low-latency systems, we must also consider the impact of finite-length channel coding, as well as rate adaptation based imperfect CSI. We derive closed-form approximations for the corresponding outage or error probabilities and find that those effects create a larger performance penalty for NOMA than for OMA. Thus, NOMA with SIC decoding may often be unsuitable for URLLC.