# Non-Orthogonal Contention-Based Access for URLLC Devices with Frequency Diversity

## TL;DR

This paper explores non-orthogonal frequency domain access schemes for URLLC, analyzing two models of coded multichannel random access to improve reliability and diversity in low-latency uplink transmissions.

## Contribution

It introduces and compares two models of contention-based multichannel access with coded diversity, highlighting the benefits of deterministic coding in low arrival rate scenarios.

## Key findings

- Deterministic coding outperforms random coding at low arrival rates.
- Using multiple subchannels enhances diversity and reliability.
- Collision and interference models impact the effectiveness of MRC combining.

## Abstract

We study coded multichannel random access schemes for ultra-reliable low-latency uplink transmissions. We concentrate on non-orthogonal access in the frequency domain, where users transmit over multiple orthogonal subchannels and inter-user collisions limit the available diversity. Two different models for contention-based random access over Rayleigh fading resources are investigated. First, a collision model is considered, in which the packet is replicated onto $K$ available resources, $K' \leq K$ of which are received without collision, and treated as diversity branches by a maximum-ratio combining (MRC) receiver. The resulting diversity degree $K'$ depends on the arrival process and coding strategy. In the second model, the slots subject to collisions are also used for MRC, such that the number of diversity branches $K$ is constant, but the resulting combined signal is affected by multiple access interference. In both models, the performance of random and deterministic repetition coding is compared. The results show that the deterministic coding approach can lead to a significantly superior performance when the arrival rate of the intermittent URLLC transmissions is low.