# Conservative Link Adaptation for Ultra Reliable Low Latency Communications

## TL;DR

This paper proposes a conservative link adaptation algorithm for URLLC in 5G, balancing high reliability and resource efficiency by accounting for worst-case channel degradation.

## Contribution

It introduces a novel conservative link adaptation method that estimates and compensates for the worst channel conditions to improve reliability and resource use in URLLC.

## Key findings

- The proposed algorithm reduces packet loss ratio effectively.
- It optimizes channel resource consumption.
- The method is robust in highly-variant channels.

## Abstract

Ultra reliable low latency communications (URLLC) is one of the most promising and demanding services in 5G systems. This service requires very low latency of less than $1-10$ ms and very high transmission reliability: the acceptable packet loss ratio is about $10^{-5}$. To satisfy such strict requirements, many issues shall be solved. This paper focuses on the link adaptation problem, i.e., the selection of a modulation and coding scheme (MCS) for transmission based on the received channel quality indicator (CQI) reports. On the one hand, link adaptation should select a robust MCS to provide high reliability. On the other hand, it should select the highest possible MCS to reduce channel resource consumption. The paper shows that even for one URLLC user, link adaptation is still a challenging problem, especially in highly-variant channels. To solve this problem, a conservative link adaptation algorithm is designed. The algorithm estimates the strongest channel degradation at the time moment of the actual packet transmission and selects an MCS taking into account the worst degradation. The obtained results show that the proposed algorithm is efficient in terms of both the packet loss ratio and the channel resource consumption.